Introduction to FreeS/WAN

Table of Contents

Introduction FreeS/WAN quick start guide FreeS/WAN FAQ Installing FreeS/WAN Configuration FreeS/WAN manual pages FreeS/WAN and firewalls Linux FreeS/WAN Troubleshooting Guide Linux FreeS/WAN Compatibility Guide Interoperation with other IPsec implementations Performance of FreeS/WAN Testing FreeS/WAN Kernel configuration for FreeS/WAN Other configuration possibilities Linux FreeS/WAN background FreeS/WAN script examples History and politics of cryptography The IPsec protocols Mailing lists and newsgroups Web links Glossary for the Linux FreeS/WAN project Bibliography for the Linux FreeS/WAN project

IPsec RFCs and related documents Distribution Roadmap: What's Where in Linux FreeS/WAN


This section gives an overview of:

This section is intended to cover only the essentials, things you should know before trying to use FreeS/WAN.

For more detailed background information, see the history and politics and IPsec protocols sections.

IPsec, Security for the Internet Protocol

FreeS/WAN is a Linux implementation of the IPsec (IP security) protocols. IPsec provides encryption and authentication services at the IP (Internet Protocol) level of the network protocol stack.

Working at this level, IPsec can protect any traffic carried over IP, unlike other encryption which generally protects only a particular higher-level protocol -- PGP for mail, SSH for remote login, SSL for web work, and so on. This approach has both considerable advantages and some limitations. For discussion, see our IPsec section

IPsec can be used on any machine which does IP networking. Dedicated IPsec gateway machines can be installed wherever required to protect traffic. IPsec can also run on routers, on firewall machines, on various application servers, and on end-user desktop or laptop machines.

Three protocols are used

Our implementation has three main parts:

IPsec is optional for the current (version 4) Internet Protocol. FreeS/WAN adds IPsec to the Linux IPv4 network stack. Implementations of IP version 6 are required to include IPsec. Work toward integrating FreeS/WAN into the Linux IPv6 stack has started.

For more information on IPsec, see our IPsec protocols section, our collection of IPsec links or the RFCs which are the official definitions of these protocols.

Interoperating with other IPsec implementations

IPsec is designed to let different implementations work together. We provide:

The VPN Consortium fosters cooperation among implementers and interoperability among implementations. Their web site has much more information.

Applications of IPsec

Because IPsec operates at the network layer, it is remarkably flexible and can be used to secure nearly any type of Internet traffic. Two applications, however, are extremely widespread:

There is enough opportunity in these applications that vendors are flocking to them. IPsec is being built into routers, into firewall products, and into major operating systems, primarily to support these applications. See our list of implementations for details.

We support both of those applications, and various less common IPsec applications as well, but we also add one of our own:

This is an extension we are adding to the protocols. FreeS/WAN is the first prototype implementation, though we hope other IPsec implementations will adopt the technique once we demonstrate it. See project goals below for why we think this is important.

A somewhat more detailed description of each of these applications is below. Our setup section will show you how to build each of them.

Using secure tunnels to create a VPN

A VPN, or Virtual Private Network lets two networks communicate securely when the only connection between them is over a third network which they do not trust.

The method is to put a security gateway machine between each of the communicating networks and the untrusted network. The gateway machines encrypt packets entering the untrusted net and decrypt packets leaving it, creating a secure tunnel through it.

If the cryptography is strong, the implementation is careful, and the administration of the gateways is competent, then one can reasonably trust the security of the tunnel. The two networks then behave like a single large private network, some of whose links are encrypted tunnels through untrusted nets.

Actual VPNs are often more complex. One organisation may have fifty branch offices, plus some suppliers and clients, with whom it needs to communicate securely. Another might have 5,000 stores, or 50,000 point-of-sale devices. The untrusted network need not be the Internet. All the same issues arise on a corporate or institutional network whenever two departments want to communicate privately with each other.

Administratively, the nice thing about many VPN setups is that large parts of them are static. You know the IP addresses of most of the machines involved. More important, you know they will not change on you. This simplifies some of the admin work. For cases where the addresses do change, see the next section.

Road Warriors

The prototypical "Road Warrior" is a traveller connecting to home base from a laptop machine. Administratively, most of the same problems arise for a telecommuter connecting from home to the office, especially if the telecommuter does not have a static IP address.

For purposes of this document:

These require somewhat different setup than VPN gateways with static addresses and with client systems behind them, but are basically not problematic.

There are some difficulties which appear for some road warrior connections:

In most situations, however, FreeS/WAN supports road warrior connections just fine.

Opportunistic encryption

One of the reasons we are working on FreeS/WAN is that it gives us the opportunity to add what we call opportuntistic encryption. This means that any two FreeS/WAN gateways will be able to encrypt their traffic, even if the two gateway administrators have had no prior contact and neither system has any preset information about the other.

Both systems pick up the authentication information they need from the DNS (domain name service), the service they already use to look up IP addresses. Of course the administrators must put that information in the DNS, and must set up their gateways with opportunistic encryption enabled. Once that is done, everything is automatic. The gateways look for opportunities to encrypt, and encrypt whatever they can. Whether they also accept unencrypted communication is a policy decision the administrator can make.

This technique can give two large payoffs:

Opportunistic encryption is not (yet?) a standard part of the IPsec protocols, but an extension we are proposing and demonstrating. For details of our design, see links below.

Only one current product we know of implements a form of opportunistic encryption. Secure sendmail will automatically encrypt server-to-server mail transfers whenever possible.

The need to authenticate gateways

A complication, which applies to any type of connection -- VPN, Road Warrior or opportunistic -- is that a secure connection cannot be created magically. There must be some mechanism which enables the gateways to reliably identify each other. Without this, they cannot sensibly trust each other and cannot create a genuinely secure link.

Any link they do create without some form of authentication will be vulnerable to a man-in-the-middle attack. If Alice and Bob are the people creating the connection, a villian who can re-route or intercept the packets can pose as Alice while talking to Bob and pose as Bob while talking to Alice. Alice and Bob then both talk to the man in the middle, thinking they are talking to each other, and the villain gets everything sent on the bogus "secure" connection.

There are two ways to build links securely, both of which exclude the man-in-the middle:

Automatic keying is much more secure, since if an enemy gets one key only messages between the previous re-keying and the next are exposed. It is therefore the usual mode of operation for most IPsec deployment, and the mode we use in our setup examples. FreeS/WAN does support manual keying for special circumstanes. See this section.

For automatic keying, the two systems must authenticate each other during the negotiations. There is a choice of methods for this:

Public key techniques are much preferable, for reasons discussed later, and will be used in all our setup examples. FreeS/WAN does also support auto-keying with shared secret authentication. See this section.

The FreeS/WAN project

For complete information on the project, see our web site,

In summary, we are implementing the IPsec protocols for Linux and extending them to do opportunistic encryption.

Project goals

Our overall goal in FreeS/WAN is to make the Internet more secure and more private.

Our IPsec implementation supports VPNs and Road Warriors of course. Those are important applications. Many users will want FreeS/WAN to build corporate VPNs or to provide secure remote access.

However, our goals in building it go beyond that. We are trying to help build security into the fabric of the Internet so that anyone who choses to communicate securely can do so, as easily as they can do anything else on the net.

More detailed objectives are:

If we can get opportunistic encryption implemented and widely deployed, then it becomes impossible for even huge well-funded agencies to monitor the net.

See also our section on history and politics of cryptography, which includes our project leader's rationale for starting the project.

Project team

Two of the team are from the US and can therefore contribute no code: The rest of the team are Canadians, working in Canada. ( Why Canada?) The project is funded by civil libertarians who consider our goals worthwhile. Most of the team are paid for this work.

People outside this core team have made substantial contributions. See

Additional contributions are welcome. See the FAQ for details.

Products containing FreeS/WAN

Unfortunately the export laws of some countries restrict the distribution of strong cryptography. FreeS/WAN is therefore not in the standard Linux kernel and not in all CD or web distributions.

FreeS/WAN is, however, quite widely used. Products we know of that use it are listed below. We would appreciate hearing, via the mailing lists, of any we don't know of.

Full Linux distributions

FreeS/WAN is included in various general-purpose Linux distributions, mostly from countries (shown in brackets) with more sensible laws:

For distributions which do not include FreeS/WAN and are not Redhat (which we develop and test on), there is additional information in our compatibility section.

The server edition of Corel Linux (Canada) also had FreeS/WAN, but Corel have dropped that product line.

Office server distributions

FreeS/WAN is also included in several distributions aimed at the market for turnkey business servers:


Several distributions intended for firewall and router applications include FreeS/WAN:

There are also several sets of scripts available for managing a firewall which is also acting as a FreeS/WAN IPsec gateway. See this list.

Firewall and VPN products

Several vendors use FreeS/WAN as the IPsec component of a turnkey firewall or VPN product.

Software-only products:

Products that include the hardware:, makers of the Netwinder Linux machines (ARM or Crusoe based), had a product that used FreeS/WAN. The company is in receivership so the future of the Netwinder is at best unclear. PKIX patches for FreeS/WAN developed at Rebel are listed in our web links document.

RPM sets

For some distributions which do not include FreeS/WAN, it may be possible to install using RPM (Redhat Package Manager), rather than going through our more complex procedure.

Some caution is required on this. The RPMs are specific to a Linux distribution and an attempt to use them on another distribution is likely to cause problems.

RPMs for FreeS/WAN 1.91 and Red Hat 7.1 are available for download from Steamballoon. Check there for later versions.

As of version 1.93, the FreeS/WAN distribution incorporates some of the Steamballoon work, providing a facility for building your own RPMs. Details are in our installation document.

Information sources

This HowTo, in multiple formats

FreeS/WAN documentation up to version 1.5 was available only in HTML. Now we ship two formats:

and provide a Makefile to generate other formats if required:

The Makefile assumes the htmldoc tool is available. You can download it from Easy Software. You may need to get source code and change some of the limits in #define MAX_<whatever> statements near the end of its file. Otherwise it core dumps when those limits are exceeded on large files such as our glossary.html.

All formats should be available at the following websites:

The distribution tarball has only the two HTML formats.

Note: If you need the latest doc version, for example to see if anyone has managed to set up interoperation between FreeS/WAN and whatever, then you should download the current snapshot. What is on the web is documentation as of the last release. Snapshots have all changes I've checked in to date.

Other documents in the distribution

Text files in the main distribution directory are README, INSTALL, CREDITS, CHANGES, BUGS and COPYING.

FreeS/WAN commands and library routines are documented in standard Unix manual pages, accessible via the man(1) command. We also provide them in HTML, accessible from this index. In the event of disagreement between this HowTo and the man pages, the man pages are more likely correct since they are written by the implementers. Please report any such inconsistency on the mailing list.

The gmp (GNU multi-precision arithmetic) and Libdes (encryption) libraries which we use each have their own documentation. You can find it in those library directories.

Background material

Throughout this documentation, I write as if the reader had at least a general familiarity with Linux, with Internet Protocol networking, and with the basic ideas of system and network security. Of course that will certainly not be true for all readers, and quite likely not even for a majority.

However, I must limit amount of detail on these topics in the main text. If I tried to explain everything here, the result would be completely unreadable.

If one or more of those areas is unknown territory for you, there are plenty of other resources you could look at:

the Linux Documentation Project or a local Linux User Group and these links
IP networks
Rusty Russell's Networking Concepts HowTo and these links
Schneier's book Secrets and Lies and these links

Also, I do make an effort to provide some background material in these documents. All the basic ideas behind IPsec and FreeS/WAN are explained here. Explanations that do not fit in the main text, or that not everyone will need, are often in the glossary, which is the largest single file in this document set. All files are heavily sprinkled with links to each other and to the glossary. If some passage makes no sense to you, try the links.

For other reference material, see the bibliography and our collection of web links .

Of course, no doubt I get this (and other things) wrong sometimes. Feedback via the mailing lists is welcome.

Archives of the project mailing list

Until quite recently, there was only one FreeS/WAN mailing list, and archives of it were: The two archives use completely different search engines. You might want to try both.

More recently we have expanded to five lists, each with its own archive.

More information on mailing lists.

User-written HowTo information

Various user-written HowTo documents are available. The ones covering FreeS/WAN-to-FreeS/WAN connections are:

User-wriiten HowTo material may be especially helpful if you need to interoperate with another IPsec implementation. We have neither the equipment nor the manpower to test such configurations. Users seem to be doing an admirable job of filling the gaps.

Check what version of FreeS/WAN user-written documents cover. The software is under active development and the current version may be significantly different from what an older document describes.

Papers on FreeS/WAN

Two design documents show current team thinking on new developments:

Both documents are works in progress and are frequently revised. For the latest version, see the design mailing list . Comments should go to that list.

There is now an Internet Draft on Opportunistic Encryption by Michael Richardson, Hugh Redelmeier and Henry Spencer. This is a first step toward getting the protocol standardised so there can be multiple implementations of it. Discussion of it takes place on the IETF IPsec Working Group mailing list.

A number of papers giving further background on FreeS/WAN, or exploring its future or its applications, are also available:

Several of these provoked interesting discussions on the mailing lists, worth searching for in the archives.

There are also several papers in languages other than English, see our web links.

License and copyright information

All code and documentation written for this project is distributed under either the GNU General Public License (GPL) or the GNU Library General Public License. For details see the COPYING file in the distribution.

Not all code in the distribution is ours, however. See the CREDITS file for details. In particular, note that the Libdes library and the version of MD5 that we use each have their own license.

Distribution sites

FreeS/WAN is available from a number of sites:

The "munitions" archive of Linux crypto software

There is also an archive of Linux crypto software called "munitions", with its own mirrors in a number of countries. It includes FreeS/WAN, though not always the latest version. Some of its sites are:

Any of those will have a list of other "munitions" mirrors. There is also a CD available.

Links to other sections

For more detailed background information, see:

To begin working with FreeS/WAN, go to:

FreeS/WAN quick start guide

This is a quick guide to and then setting up some common configurations: This should cover everything you need to set up

More complex requirements are covered elsewhere:

However, please read this quick start section first , before tackling the others.

Easy installation

There are two easy ways to install FreeS/WAN:

If your distribution does not include FreeS/WAN and no RPMs are available, see our installation from source document.

Enabling FreeS/WAN

Once you have FreeS/WAN on the system, ensure that it is enabled:
Set your boot loader to get the system booting with the new kernel.
On many systems, you do this by editing lilo.conf(5) and running lilo(8). If this is unfamilar territory, see the LILO mini-HowTo.
Enable ipsec in your boot scripts.
Typically, this is done with chkconfig(8). If this is unfamilar territory, see the Power-up to Bash Prompt HowTo.

That's it. FreeS/WAN is installed.

Creating an RSA key

The next step is to generate an RSA key for use in authenticating IPsec negotiations.

RSA is a public key cryptographic technique. Keys are created as matched pairs. Each pair includes:

For FreeS/WAN, both keys for your system are in the ipsec.secrets(5) file. Maintaining security of this file is essential since it holds your private key.

To generate your key pair, give these commands as root:

        ipsec newhostkey > /etc/ipsec.secrets
        chmod 600 /etc/ipsec.secrets

Key generation may take some time, even on a fast system. Also, it needs a lot of random numbers so you may need to switch consoles and do something like typing a lot of text or running du / > /dev/null. These give random(4) some inputs to work with.

The RSA keys we generate are suitable only for authentication, not for encryption. IPsec uses them only for authentication. See our IPsec section for details.

Setting up opportunistic encryption

Opportunistic encryption makes setup and adminstration of IPsec simple.

For opportunistic encryption, you do not need to communicate with the administrator of a site before establishing secure communications to that site. In particular, you do not have to send them your keys or collect and authenticate theirs. All you have to do is set up your end correctly and from there on, everything is automatic.

Opportunistic client

In the general case, an IPsec connection involves two gateways securing communication between two client subnets, and either end may initiate an IPsec connection.

In this section, we treat the simplest case:

The next section covers opportunistic gateways, where these restrictions do not apply.

There are two steps:

Once this is done, your system will automatically encrypt whenever it can.

ipsec.conf(5) for opportunistic client

The ipsec.conf(5) file for this setup is just:
# general IPsec setup
config setup
        # Use the default interface
        # Use auto= parameters in conn descriptions to control startup actions.

# defaults for subsequent connection descriptions
conn %default
        # How to authenticate gateways
        # Try to start all connections by default

# description for opportunistic connections
conn us-to-anyone
        also=our_stuff             # our system details, stored below
        left=%opportunistic        # anyone we can authenticate via DNS
        rekey=no                   # let unused connections die

# description of our system
# included in other connection descriptions via also= lines
# must come after the lines that use it
conn our_stuff
        # all connections should use our default route
        # also controls the source address on IPsec packets
        # our identity for IPsec negotiations
        # must match what is in DNS and ipsec.secrets(5)

The last line above is the only one that you need to edit for your system. All the rest is identical for any standalone machine doing opportunistic encryption.

There is no need to provide any keys in this file. Your private key is in ipsec.secrets(5) and, for opportunistic encryption, the public keys for remote gateways are all looked up in DNS.

Client DNS key

You need to put your system's RSA public key in a DNS record so that systems you communicate with can find it.

What follows here is a simple technique suitable for systems which always initiate any opportunistic encryption they are involved in. If you need to let others inititiate -- for example, if you run services on your machine and want remote clients to be able to acess them securely -- then you need the more complex techniques described in the gateways section.

Find a helpful DNS administrator
You need the co-operation of a DNS administrator somewhere for this, to place a KEY record so that you can use a name in some domain he or she controls. This need not be either the domain you get your IP address from or the domain that points to your system.

For example, a reverse lookup on the IP address for a home gateway might give, and a forward lookup for might point to that gateway. You could use either of these names as your ID for IPsec purposes, if the admins at either or co-operate.

If not, you can use any domain whose DNS adminstrator is willing to help out. You do not need an A record (address record, associating your chosen name with an address) in that domain, only a KEY record.

Generate a KEY record

You can generate a DNS KEY record containing your system's public key with the command:

     ipsec showhostkey
The result should look like this (with the key data trimmed down for clarity):

  ; RSA 2048 bits   Sat Apr 15 13:53:22 2000   IN   KEY   0x4200 4 1 AQOF8tZ2...+buFuFn/

The name here is taken from ipsec.secrets(5). If it is not what you want, edit that file to correct it, then run ipsec showhostkey again.

The name must also match what you used for rightid= in ipsec.conf(5).

Give this record to the DNS administrator, for insertion into the zone file of the domain.

Opportunistic gateway

An IPsec gateway is a system that does IPsec for a group of client machines located behind it. Typically, gateways are installed at or near organisational boundaries. Often the same machine is used as a firewall and as an IPsec gateway, but these functions can also be separated.

There are two steps in the setup. Setting up your ipsec.conf(5) for this is quite simple. Setting up the DNS entries to support it is a bit more complex.

ipsec.conf(5) for an opportunistic gateway

You need only make a few additions to in the ipsec.conf(5) file to expand from a standalone system (which protects only its own traffic) to a gateway (which protects traffic for other systems):

A few other things in ipsec.conf(5) will be changed as well in our example:

With those changes, the ipsec.conf(5) file is:

# general IPsec setup
config setup
        # Use the default interface
        # Use auto= parameters in conn descriptions to control startup actions.

# defaults for subsequent connection descriptions
conn %default
        # How to authenticate gateways
        # Default is to load all connection descriptions
        # but not try to start the connection
        # Some conns may over-ride this with auto=start

# opportunistic connections to our gateway
conn us-to-anyone
        also=gate_stuff             # our system details, stored below
        right=%opportunistic        # anyone we can authenticate via DNS
        rekey=no                    # let unused connections die

# opportunistic connections for client systems
# our gateway will build opportunistic tunnels on behalf of any
# machine in the specified subnet
conn subnet-to-anyone
        also=gate_stuff             # our system details, stored below
        also=public_subnet          # subnet description, below
        right=%opportunistic        # anyone we can authenticate via DNS
        rekey=no                    # let unused connections die

# description of our gateway system
# included in other connection descriptions via also= lines
# must come after the lines that use it
conn gate_stuff
        # all connections should use our default route
        # also controls the source address on IPsec packets
        # our identity for IPsec negotiations
        # must match what is in DNS and ipsec.secrets(5)

# description of the subnet this gateway encrypts for
# numbers used here are arbitrary, just for example
conn public_subnet
Supporting additional subnets
If required, a gateway can easily provide this service for more than one subnet. You just add a connection description and a subnet description for each. For example, leaving everything else above unchanged, you could add these sections:
# opportunistic connections for additional systems
conn second-to-anyone
        also=gate_stuff             # our system details, stored below
        also=second_subnet          # subnet description, below
        right=%opportunistic        # anyone we can authenticate via DNS
        rekey=no                    # let unused connections die

# description of a second subnet this gateway encrypts for
# numbers used here are arbitrary, just for example
conn second_subnet

There is one small thing to be careful of here. An also= line must appear in the file before the conn it references, so the first section above must appear before conn gate_stuff.

The subnets used in these descriptions need not correspond to physical subnets. This is discussed in more detail in our advanced configuration document.

DNS entries for an opportunistic gateway

DNS setup for an opportunistic gateway involves several types of record:

If you are not familar with DNS, see our DNS background section.
Gateway record in forward map
The gateway should have a KEY record in its forward map. It will look like this (except that we shorten the key here):
  ; RSA 2048 bits   Sat Apr 15 13:53:22 2000   IN   KEY   0x4200 4 1 AQOF8tZ2...+buFuFn/
This can be generated with ipsec showhostkey.
Gateway record in reverse map
To accept incoming connections, you also need a KEY record in the reverse map. The initiator will not always know your gateway's name. It must be possible to look up the key knowing only the IP address.

The record you need looks like this:

  ; RSA 2048 bits   Sat Apr 15 13:53:22 2000   IN   KEY   0x4200 4 1 AQOF8tZ2...+buFuFn/

Generate a record with ipsec showhostkey, and then edit it to insert the IP address.

As always, IP addresses in the reverse map are written backwards. In the above example, the gateway IP address is

Authorisation records in reverse map
For the gateway to provide an opportunistic encryption service for other systems, it must be possible for the initiator of an IPsec connection to: This is done by adding a KEY record to the reverse map for the endpoint. The record looks like this:
	; RSA 2048 bits   Sat Apr 15 13:53:22 2000
	IN TXT  "X-IPsec-Server(10)= AQOF8tZ2...+buFuFn/"
This record must be generated on the gateway so it can get the key from ipsec.secrets(5). The command is:
     ipsec showhostkey --txt
You must supply the gateway IP address on the command line.

One of these records is required in the reverse map for each system using this gateway for opportunistic IPsec.

"Road Warrior" remote access

A common requirement is for pre-configured connections between a specfic network and some set of remote machines. For example, an office network will often need to provide remote access services for:

We refer to the remote machines as "Road Warriors". For purposes of IPsec, anyone with a dynamic IP address is a road warrior.

Of course, if both the warrior and the gateway at the office are set up for opportunistic encryption, then you may not need the pre-configured connection. Here we assume that you do need it. For example:

This section has three sub-sections:

On either end, the opportunistic setup is unaffected by this. You leave it in place so both systems can continue to do opportunistic encryption with everyone but each other.

Information exchange

To set up an explicitly configured connection, you need some information about the system on the other end.

The gateway adminstrator needs to know some things about each Road Warrior:

To get this information, in a format suitable for insertion directly into the gateway's ipsec.conf(5) file, issue this command on the Warrior machine:

	ipsec showhostkey --right
The output should look like this (with the key shortened for easy reading):

The Road Warrior needs to know:

which can be generated by runing ipsec showhostkey --left on the gateway. Each Warrior must also know:

This information should be provided in a convenient format, ready for insertion in the Warrior's ipsec.conf(5) file. For example:


The gateway administrator typically needs to generate this only once. The same file can be given to all Warriors.

Of course it is also possible to provide different versions (in particular, access to differnet subnets) to different groups of Warriors. See our advanced configuration document.

Setup on the Road Warrior machine

To set up a Road Warrior machine, we start from the opportunistic client setup shown above. We need not change anything there, only add a connection description for the pre-configured tunnel.

Connection description for a pre-configured tunnel

We insert the new connection description before the conn our_stuff section, so that it can use an also= line referring to that section.

# pre-configured link to office network
conn us-to-office
        also=our_stuff             # our system details, stored below
        # information obtained from office system admin
        # goes to the right of the = signs in these lines
        # values shown here are just for example
        left=               # gateway IP address
        leftsubnet=      # the office network
        # real keys are much longer than shown here

Everything else remains as it was when we had only opportunistic connections.

We could easily add more connections as required, perhaps one each for his office, her office, the kid's school, ... The file would grow longer, but nothing already in the file would need to change.

NAT for hidden clients

In many cases, the gateway for a small home network can use exactly the same Road Warrior ipsec.conf(5) file that a traveller's laptop would.

If your gateway uses NAT to allow machines to access the Internet without having their own routable IP addresses, then from the point of view of anyone else on the Internet:

For purposes of IPsec across the Internet, your gateway can be treated as a standalone machine. Consequently, your ipsec.conf(5) file needs no changes to support the NAT. It can be identical to what you would use on a standalone system, as shown above.

Firewall rules do need to be a little more complex to support the NAT. See the firewall section below for details.

For a more detailed discussion of NAT, see our background section.

Road Warrior support on an office gateway

Adding road warrior support so people can connect remotely to your office network is straightforward.

We start from the opportunistic gateway setup shown above.

Putting connection descriptions in separate files

You could put a complete connection description for each Warrior in your ipsec.conf(5) file, but this makes for a rather unmanageable file if you have many Warriors.

Instead, we suggest you give each warrior its own file, choosing some directory and naming convention that suits your system and style.

For this example, we use the directory /etc/ipsec.road and use filenames based on IPsec ID, so the Warrior using ID gets a file named xy.conf.

Using such files, you need add only one line to ipsec.conf(5). With our naming convention, the line is:

      include /etc/ipsec.road/*.conf

FreeS/WAN will then read all those files and behave as if they were part of the ipsec.conf(5) file.

This needs to come before the conn gate_stuff section, so that the Warriors' connection descriptions can use also=gate_stuff. A convenient place for the line is right after the conn %default section.

Each of the Road Warrior files then contains a connection description for that Warrior. For example:

# connection description for Road Warrior "xy"
conn gate-xy
	# use the gateway description in ipsec.conf(5)
	# allow connection attempt from any address
	# attempt fails if caller cannot authenticate
	# authentication information

With this technique, it becomes fairly simple to administer a gateway that supports many Road Warriors. For example:

To add a new user, simply add a suitable file.

To disable an account -- for example if a key is compromised -- first remove the file, then take any existing connection down with:

	ipsec auto --down connection
and delete it from Pluto's internal database with:
	ipsec auto --delete connection

If you have many users, it would be worthwhile to write scripts to automate such tasks.

Network-to-network VPN

Often it is useful to have explicitly configured IPsec tunnels between different offices of an organisation, or between organisations that have joint projects.

Of course, if both offices are set up for opportunistic encryption and the security policies in place allow you to use that, explicitly configured tunnels become unnecessary. However, this will not always be the case.

Gateway setup for net-to-net

Adding up a network-to-network tunnel does not require any change to the opportunistic or Road warrior parts of your ipsec.conf(5). You can keep those parts exactly as shown above.

Of course, a network-to-network tunnel requires its own connection description, so you have to add that. There are two ways to do this.

identical connection description on the two ends
needs to specify more detail so the machine can figure out which end it is on
slightly different descriptions on the two ends
needs less detail, but you need to manage two descriptions
Choose whichever is more convenient to administer in your environment.

A connection description that works on either end

Here is a network-to-network tunnel description from our examples file:
# sample tunnel
# The network here looks like:
#   leftsubnet====left----leftnexthop......rightnexthop----right====rightsubnet
# If left and right are on the same Ethernet, omit leftnexthop and rightnexthop.
conn sample
        # left security gateway (public-network address)
        # next hop to reach right
        # subnet behind left (omit if there is no subnet)
        # right s.g., subnet behind it, and next hop to reach left
Due to an unfortunate interaction between FreeS/WAN and the kernel routing code, you must specify leftnexthop (the router which left sends packets to in order to get them delivered to right) and rightnexthop (vice versa).

The *nexthop parameters will be eliminated in a future release, but perhaps not soon. We know they should go, but getting them out is not a simple problem. For now, live with them.

This description can be generated on either machine and simply inserted in the ipsec.conf(5) file on the other. No change is required or desired.

Using slightly different descriptions

Provided both machines do IPsec over the interface that is their default route to the Internet (a common case, but by no means the only one) you can simplify the description somewhat.

When using left=%defaultroute, you do not need to specify leftnexthop. left does not need to know rightnexthop either, so on left the connection description can be:

conn sample
        # left security gateway (public-network address)
         # subnet behind left (omit if there is no subnet)
        # right s.g., subnet behind it
On right it is:
conn sample
        # left security gateway (public-network address)
        # subnet behind left (omit if there is no subnet)
        # right s.g., subnet behind it


FreeS/WAN often runs on a firewall machine and, even if it is on a separate machine, it must interact with firewalls to get its job done.

This section describes simple firewall setups, suitable for getting a FreeS/WAN machine running. A separate FreeS/WAN and firewalls document provides more detail.

Firewalling for a standalone system

Firewall rules on a standalone system doing IPsec -- opportunistic, "road warrior" remote access, or both -- can be very simple.

The first step is to allow IPsec packets (IKE on UDP port 500 plus ESP, protocol 50) in and out of your gateway. A script to set up iptables(8) rules for this is:

# edit this line to match the interface you use as default route
# ppp0 is correct for many modem, DSL or cable connections
# but perhaps not for you
# allow IPsec
# IKE negotiations
iptables -A INPUT  -p udp -i $world --sport 500 --dport 500 -j ACCEPT
iptables -A OUTPUT -p udp -o $world --sport 500 --dport 500 -j ACCEPT
# ESP encrypton and authentication
iptables -A INPUT  -p 50 -i $world -j ACCEPT
iptables -A OUTPUT -p 50 -o $world -j ACCEPT

Optionally, you could restrict this, allowing these packets only to and from a list of known gateways.

A second firewalling step -- access controls built into the IPsec protocols -- is automatically applied:

Pluto -- the FreeS/WAN keying daemon -- deals with the IKE packets.
Pluto authenticates its partners during the IKE negotiation, and drops negotiation if authentication fails.
KLIPS -- the FreeS/WAN kernel component -- handles the ESP packets.
KLIPS drops outgoing packets
if they are routed to IPsec, but no tunnel has been negotiated for them
KLIPS drops incoming unencrypted packets
if source and destination addresses match a tunnel; the packets should have been encrypted
KLIPS drops incoming encrypted packets
if source and destination address do not match the negotiated parameters of the tunnel that delivers them
if packet-level authentication fails
These errors are logged. See our troubleshooting document for details.

Optionally, you can add a third step using whatever additional firewall rules are required for your situation. These rules can recognise packets emerging from IPsec. They are marked as arriving on an interface such as ipsec0, rather than eth0, ppp0 or whatever. For example, in an iptables(8) rule set, you would use:

-i ipsec+
to specify packets arriving on any ipsec device
-o ipsec+
to specify packets leaving via any ipsec device
It is therefore straightforward to apply whatever filtering you like to these packets.

Firewalling for gateways

On a gateway, the IPsec-related firewall rules applied for input and output on the Internet side are exactly as shown above. A gateway exchanges exactly the same things -- UDP 500 packets and IPsec packets -- with other gateways that a standalone system does, so it can use exactly the same firewall rules as a standalone system would.

However, on a gateway there are additional things to do:

You need additional rules to handle these things. For example, adding some rules to the set shown above we get:

# edit this line to match the interface you use as default route
# ppp0 is correct for many modem, DSL or cable connections
# but perhaps not for you
# edit these lines to describe your internal subnet and interface
# allow IPsec
# IKE negotiations
iptables -A INPUT  -p udp -i $world --sport 500 --dport 500 -j ACCEPT
iptables -A OUTPUT -p udp -o $world --sport 500 --dport 500 -j ACCEPT
# ESP encrypton and authentication
iptables -A INPUT  -p 50 -i $world -j ACCEPT
iptables -A OUTPUT -p 50 -o $world -j ACCEPT
# packet forwarding for an IPsec gateway
# handle packets emerging from IPsec
# ipsec+ means any of ipsec0, ipsec1, ...
iptables -A FORWARD -d $localnet -i ipsec+ -j ACCEPT
# simple rule for outbound packets
# let local net send anything
# IPsec will encrypt some of it
iptables -A FORWARD -s $localnet -i $internal -j ACCEPT 

On a production gateway, you would no doubt need tighter rules than the above. For details, see:


This is a collection of questions and answers, mostly taken from the FreeS/WAN mailing list. See the project web site for more information. All the FreeS/WAN documentation is online there.

Contributions to the FAQ are welcome. Please send them to the project mailing list.

Index of FAQ questions

What is FreeS/WAN?

FreeS/WAN is a Linux implementation of the IPsec protocols, providing security services at the IP (Internet Protocol) level of the network.

For more detail, see our introduction document or the FreeS/WAN project web site.

How do I report a problem or seek help?

See our troubleshooting document. It may guide you to a solution. If not, see its problem reporting section.

Basically, what it says is give us the output from ipsec barf from both gateways. Without full information, we cannot diagnose a problem. However, ipsec barf produces a lot of output. If at all possible, please make barfs accessible via the web or FTP rather than sending enormous mail messages.

Use the mailing list for problem reports, rather than mailing developers directly. This gives you access to more expertise, including users who may have encountered and solved the same problems. In particular, for problems involving interoperation with another IPsec implementation, the users often know more than the developers.

Using the list may also be important in relation to various cryptography export laws. A US citizen who provides technical assistance to foreign cryptographic work might be charged under the arms export regulations. Such a charge would be easier to defend if the discussion took place on a public mailing list than if it were done in private mail.

Generic questions

This is too complicated. Isn't there an easier way?

There are a number of Linux distributions or firewall products which include FreeS/WAN. See this list. Using one of these, chosen to match your requirements and budget, may save you considerable time and effort. (If you don't know your requirements, start by reading Schneier's Secrets and Lies. That gives the best overview of security issues I have seen.)

If you want the help of a contractor, or to hire staff with FreeS/WAN expertise, you could:

For companies offerring support, see the next question.

Can I get commercial support for this product?

Many of the distributions or firewall products which include FreeS/WAN (see this list) come with commercial support or have it available as an option.

Various companies specialize in commercial support of open source software. Our project leader was a founder of the first such company, Cygnus Support. It has since been bought by Redhat. Another such firm is Linuxcare.

Can I modify FreeS/WAN to ...?

You are free to modify FreeS/WAN in any way. See the discussion of licensing in our introduction document.

Can I contribute to the project?

In general, we welcome contributions from the community. Various contributed patches, either to fix bugs or to add features, have been incorporated into our distribution. Other patches, not yet included in the distribution, are listed in our web links section.

Users have also contributed heavily to documentation, both by creating their own HowTos and by posting things on the mailing lists which I have quoted in these HTML docs.

There are, however, some caveats.

FreeS/WAN is being implemented in Canada, by Canadians, largely to ensure that is it is entirely free of export restrictions. See this discussion. We cannot accept code contributions from US residents or citizens, not even one-line bugs fixes. The reasons for this were recently discussed extensively on the mailing list, in a thread starting here.

Not all contributions are of interest to us. The project has a set of fairly ambitious and quite specific goals, described in our introduction. Contributions that lead toward these goals are likely to be welcomed enthusiastically. Other contributions may be seen as lower priority, or even as a distraction.

Is there detailed design documentation?

There are: The only formal design documents are a few papers in the last category above. All the other categories, however, have things to say about design as well.

Will FreeS/WAN work in my environment?

Is a ... fast enough to handle FreeS/WAN with ... connections?

A quick summary:
FreeS/WAN can run happily on limited machines.
A 486 can handle a T1, ADSL or cable link, though the machine may be breathing hard.
Most requirements can be met
A current (mid-2001) technology PC handling 20 Megabits per second of IPsec processing will still have some cycles available for other tasks.
There are definite limits at the high end
A current high end CPU might handle a loaded T3 at 45 Mbit/second. However, it will not even come close to handling a fully loaded 100 Mbit/second Ethernet link, let alone 155 Mbit/second OC3.
Beyond that, the picture is not clear. Much depends on details of your network, your traffic, and other loads on both machine and net.

See our FreeS/WAN performance document for more detail.

Can FreeS/WAN talk to ...?

The IPsec protocols are designed to support interoperation. In theory, any two IPsec implementations should be able to talk to each other.

In practice, it is considerably more complex. We have a whole interop document devoted to it.

Can different FreeS/WAN versions talk to each other?

Linux FreeS/WAN can interoperate with many IPsec implementations, including earlier versions of Linux FreeS/WAN itself.

In a few cases, there are some complications. See our interoperation document for details.

Will FreeS/WAN run on my version of Linux?

We build and test on Redhat distributions, but FreeS/WAN runs just fine on several other distributions, sometimes with minor fiddles to adapt to the local environment. Details are in our compatibility document. Also, some distributions or products come with FreeS/WAN included.

FreeS/WAN is intended to run on all CPUs Linux supports . As of June 2000, we know of it being used in production on x86, ARM, Alpha and MIPS. It has also had successful tests on PPC and SPARC, though we don't know of actual use there. Details are in our compatibility document.

FreeS/WAN has been tested on multiprocessor Intel Linux and worked there. Note, however, that we do not test this often and have never tested on multiprocessor machines of other architectures.

Will FreeS/WAN run on the latest kernel version?

Sometimes yes, but quite often, no.

Kernel versions supported are given in the README file of each FreeS/WAN release. Typically, they are whatever production kernels were current at the time of our release (or shortly before; we might release for kernel n just as Linus releases n+1). Often FreeS/WAN will work on slightly later kernels as well, but of course this cannot be guaranteed.

For example, FreeS/WAN 1.91 was released for kernels 2.2.19 or 2.4.5, the current kernels at the time. It also worked on 2.4.6, 2.4.7 and 2.4.8, but 2.4.9 had changes that caused compilation errors if it was patched with FreeS/WAN 1.91.

When such changes appear, we put a fix in the FreeS/WAN snapshots, and distribute it with our next release. However, this is not a high priority for us, and it may take anything from a few days to several weeks for such a problem to find its way to the top of our kernel programmer's To-Do list. In the meanwhile, you have two choices:

We don't even try to keep up with kernel changes outside the main 2.2 and 2.4 branches, such as the 2.4.x-ac patched versions from Alan Cox or (when they appear) the 2.5 series of development kernels. We'd rather work on developing the FreeS/WAN code than on chasing these moving targets. We are, however, happy to get patches for problems discovered there.

See also the Choosing a kernel section of our installation document.

Will FreeS/WAN work on ... network hardware?

IPsec is designed to work over any network that IP works over, and FreeS/WAN is intended to work over any network interface hardware that Linux supports.

If you have working IP on some unusual interface -- perhaps Arcnet, Token Ring, ATM or Gigabit Ethernet -- then IPsec should "just work".

That said, practice is sometimes less tractable than theory. Our testing is done almost entirely on:

If you have some other interface, especially an uncommon one, it is entirely possible you will get bitten by a FreeS/WAN bug which our testing did not turn up. It is also possible you will be bitten by a bug in the driver that shows up only with our loads. Another FAQ section describes the commonest of this class of problem.

If IP works on your interface and FreeS/WAN doesn't, seek help on the mailing lists.

Does FreeS/WAN support ...

For a discussion of which parts of the IPsec specifications FreeS/WAN does and does not implement, see our compatibility document.

For information on some often-requested features, see below.

Does FreeS/WAN support site-to-site VPN applications

Yes, FreeS/WAN can be used to build site-to-site Virtual Private Networks.

This application is discussed in our introduction and an example given in our FreeS/WAN configuration document.

Does FreeS/WAN support remote users connecting to a LAN

Yes, FreeS/WAN can be used to connect remote users. In the documentation, we refer to them as "Road Warriors".

This application is discussed in our introduction and an example given in our FreeS/WAN configuration document.

Road warriors using Windows or Macintosh may need an IPsec client program for their machines.

Does FreeS/WAN support wireless networks

Yes, it is a common practice to use IPsec over wireless networks because their built-in encryption, WEP, is insecure.

There is some discussion in our configuration document.

Does FreeS/WAN support X.509 or other PKI certificates?

FreeS/WAN, as distributed, does not currently support use of X.509 or other PKI certificates for authentication of gateways. We are concentrating on moving toward authentication via Secure DNS and opportunistic encryption; X.509 support is not (or at least not yet) on the priority list.

On the other hand, it is a priority for some users and user-contributed patches to add X.509 certificate support to FreeS/WAN have been available for some time. From mailing list reports, they seem to be quite widely used and to work well.

See the patches section of our web references document for details.

Does FreeS/WAN support user authentication (Radius, SecureID, ...)?

Not yet. So far, there is no standard way to authenticate users for IPsec, though there is a very active IETF working group looking at the problem, and several vendors have implemented various things already.

In the absence of a standard, user authentication has not been a priority for the FreeS/WAN team, and is unlikely to become one. This would be a good project for a volunteer, perhaps a staff member or contractor at some company that needs the feature. Certainly our team would co-operate with such an effort; we just don't have time to do it.

The patches section of our web links document has links to some user work on this.

Of course, there are various ways to avoid any requirement for user authentication in IPsec. Consider the situation where road warriors build IPsec tunnels to your office net and you are considering requiring user authentication during tunnel negotiation. Alternatives include:

If either of those is trustworthy, it is not clear that you need user authentication in IPsec.

Does FreeS/WAN support single DES encryption?

No, single DES is not used either at the IKE level for negotiating connections or at the IPsec level for actually building them.

Single DES is insecure. As we see it, it is more important to deliver real security than to comply with a standard which has been subverted into allowing use of inadequate methods. See this discussion.

If you want to interoperate with an IPsec implementation which offers only DES, see our interoperation document.

Can I ...

Can I reload connection info without restarting?

Yes, you can do this. Here are the details, in a mailing list message from Pluto programmer Hugh Redelmeier:
| How can I reload config's without restarting all of pluto and klips?  I am using
| FreeSWAN -> PGPNet in a medium sized production environment, and would like to be
| able to add new connections ( i am using include config/* ) without dropping current
| SA's.
| Can this be done?
| If not, are there plans to add this kind of feature?

        ipsec auto --add whatever
This will look in the usual place (/etc/ipsec.conf) for a conn named
whatever and add it.

If you added new secrets, you need to do
        ipsec auto --rereadsecrets
before Pluto needs to know those secrets.

| I have looked (perhaps not thoroughly enough tho) to see how to do this:

There may be more bits to look for, depending on what you are trying
to do.

Another useful command here is ipsec auto --replace <conn_name> which re-reads data for a named connection.

Can I use several masqueraded subnets?

Yes. This is done all the time. See the discussion in our setup document. The only restriction is that the subnets on the two ends must not overlap. See the next question.

Here is a mailing list message on the topic. The user incorrectly thinks you need a 2.4 kernel for this -- actually various people have been doing it on 2.0 and 2.2 for quite some time -- but he has it right for 2.4.

Subject: Double NAT and freeswan working :)
   Date: Sun, 11 Mar 2001
   From: Paul Wouters <>

Just to share my pleasure, and make an entry for people who are searching
the net on how to do this. Here's the very simple solution to have a double
NAT'ed network working with freeswan. (Not sure if this is old news, but I'm
not on the list (too much spam) and I didn't read this in any HOWTO/FAQ/doc
on the freeswan site yet (Sandy, put it in! :) --- a.b.c.d  ---- a.b.c.e {internet} ----+
                                                              | --- f.g.h.i  ---- f.g.h.j {internet} ----+

the goal is to have the first network do a VPN to the second one, yet also
have NAT in place for connections not destinated for the other side of the
NAT. Here the two Linux security gateways have one real IP number (cable
modem, dialup, whatever.

The problem with NAT is you don't want packets from 10.*.*.* to 10.*.*.*
to be NAT'ed. While with Linux 2.2, you can't, with Linux 2.4 you can.

(This has been tested and works for 2.4.2 with Freeswan snapshot2001mar8b)

relevant parts of /etc/ipsec.conf:

        # To authorize this connection, but not actually start it, at startup,
        # uncomment this.

and now the real trick. Setup the NAT correctly on both sites:

iptables -t nat -F
iptables -t nat -A POSTROUTING -o eth0 -d \! -j MASQUERADE

This tells the NAT code to only do NAT for packets with destination other then
10.* networks. note the backslash to mask the exclamation mark to protect it
against the shell.

Happy painting :)


Can I use subnets masqueraded to the same addresses?

No. The notion that IP addresses are unique is one of the fundamental principles of the IP protocol. Messing with it is exceedingly perilous.

Fairly often a situation comes up where a company has several branches, all using the same non-routable addresses, perhaps This works fine as long as those nets are kept distinct. The IP masquerading on their firewalls ensures that packets reaching the Internet carry the firewall address, not the private address.

This can break down when IPsec enters the picture. FreeS/WAN builds a tunnel that pokes through both masquerades and delivers packets from leftsubnet to rightsubnet and vice versa. For this to work, the two subnets must be distinct.

There are several solutions to this problem.

Can I assign a road warrior an address on my net?

Yes, but it is tricky. This has been discussed on the mailing list. The discussion was not entirely clear to me, so I cannot yet document the procedures. At this point, all I know is:

For example, you might have:

head office network
extruded to a road warrior
whole network, including both the above

Can I support many road warriors with one gateway?

Yes. This is easily done, using

either RSA authentication
standard in the FreeS/WAN distribution
or X.509 certificates
requires patches to FreeS/WAN

In either case, each Road Warrior must have a different key or certificate.

This cannot be made to work using pre-shared key authentication; see the next question for details.

If you expect to have more than a few dozen Road Warriors connecting simultaneously, you may need a fairly powerful gateway machine. See our document on FreeS/WAN performance.

Can I have many road warriors using shared secret authentication?

No. This does not work and there is no way to fix it.

This is a designed-in limitation of the IKE key negotiation protocol, not a problem with our implementation.

When using shared secrets, the protocol requires that the responding gateway be able to determine which secret to use at a time when all it knows about the initiator is an IP address. This works fine if you know the initiator's address in advance and can use it to look up the appropiriate secret. However, it fails for Road Warriors since the gateway cannot know their IP addresses in advance.

With RSA signatures (or certificates) the protocol is slightly different. The initiator provides an identifier early in the exchange and the responder can use that identifier to look up the correct key or certificate. See above.

Can I use Quality of Service routing with FreeS/WAN?

From project technical lead Henry Spencer:
> Do QoS add to FreeS/WAN?
>For example integrating DiffServ and FreeS/WAN?

With a current version of FreeS/WAN, you will have to add hidetos=no to
the config-setup section of your configuration file.  By default, the TOS
field of tunnel packets is zeroed; with hidetos=no, it is copied from the
packet inside.  (This is a modest security hole, which is why it is no
longer the default.)

DiffServ does not interact well with tunneling in general.  Ways of
improving this are being studied.

Copying the TOS information from the encapsulated packet to the outer header reveals the TOS information to an eavesdropper. It is not clear whether or how an attacker could use this information, but since we do not have to give it to him, our default is not to.

See ipsec.conf(5) for more on the hidetos= parameter.

Can I recognise dead tunnels and shut them down?

There is no general mechanism to do this is in the IPsec protocols.

From time to time, there is discussion on the IETF Working Group mailing list of adding a "keep-alive" mechanism (which some say should be called "make-dead"), but it is a fairly complex problem and no consensus has been reached on whether or how it should be done.

The protocol does have optional delete-SA messages which one side can send when it closes a connection in hopes this will cause the other side to do the same. FreeS/WAN does not currently support these. In any case, they would not solve the problem since:

However, connections do have limited lifetimes and you can control how many attempts your gateway makes to rekey before giving up. For example, you can set:

conn default
With these settings old connections will be cleaned up. Within 30 minutes of the other end dying, rekeying will be attempted. If it succeeds, the new connection replaces the old one. If it fails, no new connection is created. Either way, the old connection is taken down when its lifetime expires.

Here is a mailing list message on the topic from FreeS/WAN tech support person Claudia Schmeing:

You ask how to determine whether a tunnel is redundant:

> Can anybody explain the best way to determine this. Esp when a RW has
> disconnected? I thought 'ipsec auto --status' might be one way.

If a tunnel goes down from one end, Linux FreeS/WAN on the
other end has no way of knowing this until it attempts to rekey.
Once it tries to rekey and fails, it will 'know' that the tunnel is 

Because it doesn't have a way of knowing the state until this point, 
it will also not be able to tell you the state via ipsec auto --status.

> However, comparing output from a working tunnel with that of one that
> was closed 
> did not show clearly show tunnel status.

If your tunnel is down but not 'unrouted' (see man ipsec_auto), you
should not be able to ping the opposite side of the tunnel. You can
use this as an indicator of tunnel status.

On a related note, you may be interested to know that as of 1.7, 
redundant tunnels caused by RW disconnections are likely to be 
less of a pain. From doc/CHANGES:

    There is a new configuration parameter, uniqueids, to control a new Pluto
    option:  when a new connection is negotiated with the same ID as an old
    one, the old one is deleted immediately.  This should help eliminate
    dangling Road Warrior connections when the same Road Warrior reconnects. 
    It thus requires that IDs not be shared by hosts (a previously legal but
    probably useless capability).  NOTE WELL:  the sample ipsec.conf now has
    uniqueids=yes in its config-setup section.



Can I build IPsec tunnels over a demand-dialed link?

This is possible, but not easy. FreeS/WAN technical lead Henry Spencer wrote:
> 5. If the ISDN link goes down in between and is reestablished, the SAs
> are still up but the eroute are deleted and the IPsec interface shows
> garbage (with ifconfig)
> 6. Only restarting IPsec will bring the VPN back online.

This one is awkward to solve.  If the real interface that the IPsec
interface is mounted on goes down, it takes most of the IPsec machinery
down with it, and a restart is the only good way to recover. 

The only really clean fix, right now, is to split the machines in two: 

1. A minimal machine serves as the network router, and only it is aware
that the link goes up and down. 

2. The IPsec is done on a separate gateway machine, which thinks it has
a permanent network connection, via the router.

This is clumsy but it does work.  Trying to do both functions within a
single machine is tricky.  There is a software package (diald) which will
give the illusion of a permanent connection for demand-dialed modem
connections; I don't know whether it's usable for ISDN, or whether it can
be made to cooperate properly with FreeS/WAN. 

Doing a restart each time the interface comes up *does* work, although it
is a bit painful.  I did that with PPP when I was running on a modem link;
it wasn't hard to arrange the PPP scripts to bring IPsec up and down at
the right times.  (I'd meant to investigate diald but never found time.)

In principle you don't need to do a complete restart on reconnect, but you
do have to rebuild some things, and we have no nice clean way of doing
only the necessary parts.
In the same thread, one user commented:
Subject: Re: linux-ipsec: IPsec and Dial Up Connections
   Date: Wed, 22 Nov 2000
   From: Andy Bradford <>

On Wed, 22 Nov 2000 19:47:11 +0100, Philip Reetz wrote:

> Are there any ideas what might be the cause of the problem and any way
> to work around it.
> Any help is highly appreciated.

On my laptop, when using ppp there is a ip-up script in /etc/ppp that 
will be executed each time that the ppp interface is brought up.  
Likewise there is an ip-down script that is called when it is taken 
down.  You might consider custimzing those to stop and start FreeS/Wan 
with each connection.  I believe that ISDN uses the same files, though 
I could be wrong---there should be something similar though.

Can I build GRE tunnels over IPsec?

This is possible in theory, but we are short on practical details. If you do this, please let us know via the mailing lists.

There is a list message with links to relevant resources.

Life's little mysteries

FreeS/WAN is a fairly complex product. (Neither the networks it runs on nor the protocols it uses are simple, so it could hardly be otherwise.) It therefore sometimes exhibits behaviour which can be somewhat confusing, or has problems which are not easy to diagnose. This section tries to explain those problems.

Setup and configuration of FreeS/WAN are covered in other documentation sections:

However, we also list some of the commonest problems here.

I cannot ping ....

This question is dealt with in the configuration section under the heading multiple tunnels.

The standard subnet-to-subnet tunnel protects traffic only between the subnets. To test it, you must use pings that go from one subnet to the other.

For example, suppose you have:

      subnet a.b.c.0/24
      eth1 = a.b.c.1
      eth0 =

       ~ internet ~

      eth0 =
      eth1 = x.y.z.1
       subnet x.y.z.0/24

and the connection description:

conn abc-xyz

You can test this connection description only by sending a ping that will actually go through the tunnel. Assuming you have machines at addresses a.b.c.2 and x.y.z.2, pings you might consider trying are:

ping from x.y.z.2 to a.b.c.2 or vice versa
Succeeds if tunnel is working. This is the only valid test of the tunnel.
ping from gate2 to a.b.c.2 or vice versa
Does not use tunnel. gate2 is not on protected subnet.
ping from gate1 to x.y.z.2 or vice versa
Does not use tunnel. gate1 is not on protected subnet.
ping from gate1 to gate2 or vice versa
Does not use tunnel. Neither gate is on a protected subnet.

Only the first of these is a useful test of this tunnel. The others do not use the tunnel. Depending on other details of your setup and routing, they:

If required, you can of course build additional tunnels so that all the machines involved can talk to all the others. See multiple tunnels in the configuration document for details.

It takes forever to ...

Users fairly often report various problems involving long delays, sometimes on tunnel setup and sometimes on operations done through the tunnel, occasionally on simple things like ping or more often on more complex operations like doing NFS or Samba through the tunnel.

Almost always, these turn out to involve failure of a DNS lookup. The timeouts waiting for DNS are typically set long so that you won't time out when a query involves multiple lookups or long paths. Genuine failures therefore produce long delays before they are detected.

A mailing list message from project technical lead Henry Spencer:

> ... when i run /etc/rc.d/init.d/ipsec start, i get:
> ipsec_setup: Starting FreeS/WAN IPsec 1.5...
> and it just sits there, doesn't give back my bash prompt.

Almost certainly, the problem is that you're using DNS names in your
ipsec.conf, but DNS lookups are not working for some reason.  You will
get your prompt back... eventually.  But the DNS timeouts are long.
Doing something about this is on our list, but it is not easy.

In the meanwhile, we recommend that connection descriptions in ipsec.conf(5) use numeric IP addresses rather than names which will require a DNS lookup.

Names that do not require a lookup are fine. For example:

These are fine. The @ sign prevents any DNS lookup. However, do not attempt to give the gateway address as . That requires a lookup.

A post from one user after solving a problem with long delays:

Subject: Final Answer to Delay!!!
   Date: Mon, 19 Feb 2001
   From: "Felippe Solutions" <>

Sorry people, but seems like the Delay problem had nothing to do with

The problem was DNS as some people sad from the beginning, but not the way
they thought it was happening. Samba, ssh, telnet and other apps try to
reverse lookup addresses when you use IP numbers (Stupid that ahh).

I could ping very fast because I always ping with "-n" option, but I don't
know the option on the other apps to stop reverse addressing so I don't use
This post is fairly typical. These problems are often tricky and frustrating to diagnose, and most turn out to be DNS-related.

One suggestion for diagnosis: test with both names and addresses if possible. For example, try all of:

If these behave differently, the problem must be DNS-related since the three commands do exactly the same thing except for DNS lookups.

I send packets to the tunnel with route(8) but they vanish

IPsec connections are designed to carry only packets travelling between pre-defined connection endpoints. As project technical lead Henry Spencer put it:
IPsec tunnels are not just virtual wires; they are virtual wires with built-in access controls. Negotiation of an IPsec tunnel includes negotiation of access rights for it, which don't include packets to/from other IP addresses. (The protocols themselves are quite inflexible about this, so there are limits to what we can do about it.)
For fairly obvious security reasons, and to comply with the IPsec RFCs, KLIPS drops any packets it receives that are not allowed on the tunnels currently defined. So if you send it packets with route(8) , and suitable tunnels are not defined, the packets vanish. Whether this is reported in the logs depends on the setting of klipsdebug in your ipsec.conf(5) file.

To rescue vanishing packets, you must ensure that suitable tunnels for them exist, by editing the connection descriptions in ipsec.conf(5). For example, supposing you have a simple setup:

         leftsubnet -- leftgateway === internet === roadwarrior
If you want to give the roadwarrior access to some resource that is located behind the left gateway but is not in the currently defined left subnet, then the usual procedure is to define an additional tunnel for those packets by creating a new connection description.

In some cases, it may be easier to alter an existing connection description, enlarging the definition of leftsubnet. For example, instead of two connection descriptions with and as their leftsubnet parameters, you can use a single description with

If you have multiple endpoints on each side, you need to ensure that there is a route for each pair of endpoints. See this example.

When a tunnel goes down, packets vanish

This is a special case of the vanishing packet problem described in the previous question. Whenever KLIPS sees packets for which it does not have a tunnel, it drops them.

When a tunnel goes away, either because negotiations with the other gateway failed or because you gave an ipsec auto --down command, the route to its other end is left pointing into KLIPS, and KLIPS will drop packets it has no tunnel for.

This is a documented design decision, not a bug. FreeS/WAN must not automatically adjust things to send packets via another route. The other route might be insecure.

Of course, re-routing may be necessary in many cases. In those cases, you have to do it manually or via scripts. We provide the ipsec auto --unroute command for these cases.

From ipsec_auto(8):

Normally, pluto establishes a route to the destination specified for a connection as part of the --up operation. However, the route and only the route can be established with the --route operation. Until and unless an actual connection is established, this discards any packets sent there, which may be preferable to having them sent elsewhere based on a more general route (e.g., a default route).
Normally, pluto's route to a destination remains in place when a --down operation is used to take the connection down (or if connection setup, or later automatic rekeying, fails). This permits establishing a new connection (perhaps using a different specification; the route is altered as necessary) without having a ``window'' in which packets might go elsewhere based on a more general route. Such a route can be removed using the --unroute operation (and is implicitly removed by --delete).

See also this mailing list message.

The firewall ate my packets!

If firewalls filter out:

then IPsec cannot work. The first thing to check if packets seem to be vanishing is the firewall rules on the two gateway machines and any other machines along the path that you have access to.

For details, see our document on firewalls .

Some advice from technical lead Henry Spencer on diagnosing such problems:

> > Packets vanishing between the hardware interface and the ipsecN interface
> > is usually the result of firewalls not being configured to let them in...
> Thanks for the suggestion. If only it were that simple! My ipchains startup
> script does take care of that, but just in case I manually inserted rules 
> accepting everything from london on dublin. No difference.

The other thing to check is whether the "RX packets dropped" count on the
ipsecN interface (run "ifconfig ipsecN", for N=1 or whatever, to see the
counts) is rising.  If so, then there's some sort of configuration mismatch
between the two ends, and IPsec itself is rejecting them.  If none of the
ipsecN counts is rising, then the packets are never reaching the IPsec
machinery, and the problem is almost certainly in firewalls etc.

Dropped connections

Networks being what they are, IPsec connections can be broken for any number of reasons, ranging from hardware failures to various software problems such as the path MTU problems discussed elsewhere in the FAQ. Fortunately, various diagnostic tools exist that help you sort out many of the possible problems.

There is one situation, however, where FreeS/WAN (using default settings) may destroy a connection for no readily apparent reason. This occurs when things are misconfigured so that two tunnels from the same gateway expect the same subnet on the far end.

In this situation, the first tunnel comes up fine and works until the second is established. At that point, because of the way we track connections internally, the first tunnel ceases to exist as far as this gateway is concerned. Of course the far end does not know that, and a storm of error messages appears on both systems as it tries to use the tunnel.

If the far end gives up, goes back to square one and negotiates a new tunnel, then that wipes out the second tunnel and ...

The solution is simple. Do not build multiple conn descriptions with the same remote subnet.

This is actually intended to be a feature, rather than a bug. Consider the situation where a single remote system goes down, then comes back up and reconnects to the gateway. It is useful to have the gateway tear down the old tunnel and recover resources when the reconnection is made. It recognises that situation by checking the remote subnet for each tunnel it builds and discarding duplicates. This works fine as long as you don't configure multiple tunnels with the same remote subnet.

If this behaviour is inconvenient for you, you can disable it by setting uniqueids=no in ipsec.conf(5).

TCPdump on the gateway shows strange things

Attempting to look at IPsec packets by running monitoring tools on the IPsec gateway machine can produce silly results. That machine is mangling the packets for IPsec, and possibly for firewall or NAT purposes as well. If the internals of the machine's IP stack are not what the monitoring tool expects, then the tool can misinterpret them and produce nonsense output.

See our testing document for more detail.

Traceroute does not show anything between the gateways

As far as traceroute can see, the two gateways are one hop apart; the data packet goes directly from one to the other through the tunnel. Of course the outer packets that implement the tunnel pass through whatever lies between the gateways, but those packets are built and dismantled by the gateways. Traceroute does not see them and cannot report anything about their path.

Here is a mailing list message with more detail.

Date: Mon, 14 May 2001
From: "John S. Denker" <<
Subject: Re: traceroute: one virtual hop

At 02:20 PM 5/14/01 -0400, Claudia Schmeing wrote:
>> > A bonus question: traceroute in subnet to subnet enviroment looks like:
>> > 
>> > traceroute to andris.dmz (, 30 hops max, 38 byte packets
>> > 1  drama (  0.716 ms  0.942 ms  0.434 ms
>> > 2  * * *
>> > 3  andris.dmz (  73.576 ms  78.858 ms  79.434 ms
>> > 
>> > Why aren't there the other hosts which take part in the delivery during 
>    * * * ?
>If there is an ipsec tunnel between GateA and Gate B, this tunnel forms a 
>'virtual wire'.  When it is tunneled, the original packet becomes an inner 
>packet, and new ESP and/or AH headers are added to create an outer packet 
>around it. You can see an example of how this is done for AH at 
>doc/ipsec.html#AH . For ESP it is similar.
>Think about the packet's path from the inner packet's perspective.
>It leaves the subnet, goes into the tunnel, and re-emerges in the second
>subnet. This perspective is also the only one available to the
>'traceroute' command when the IPSec tunnel is up.

Claudia got this exactly right.  Let me just expand on a couple of points:

*) GateB is exactly one (virtual) hop away from GateA.  This is how it
would be if there were a physically private wire from A to B.  The
virtually private connection should work the same, and it does.

*) While the information is in transit from GateA to GateB, the hop count
of the outer header (the "envelope") is being decremented.  The hop count
of the inner header (the "contents" of the envelope) is not decremented and
should not be decremented.  The hop count of the outer header is not
derived from and should not be derived from the hop count of the inner header.

Indeed, even if the packets did time out in transit along the tunnel, there
would be no way for traceroute to find out what happened.  Just as
information cannot leak _out_ of the tunnel to the outside, information
cannot leak _into_ the tunnel from outside, and this includes ICMP messages
from routers along the path.

There are some cases where one might wish for information about what is
happening at the IP layer (below the tunnel layer) -- but the protocol
makes no provision for this.  This raises all sorts of conceptual issues.
AFAIK nobody has ever cared enough to really figure out what _should_
happen, let alone implement it and standardize it.

*) I consider the "* * *" to be a slight bug.  One might wish for it to be
replaced by "GateB GateB GateB".  It has to do with treating host-to-subnet
traffic different from subnet-to-subnet traffic (and other gory details).
I fervently hope KLIPS2 will make this problem go away.

*) If you want to ask questions about the link from GateA to GateB at the
IP level (below the tunnel level), you have to ssh to GateA and launch a
traceroute from there.

Testing in stages

It is often useful in debugging to test things one at a time:

FreeS/WAN releases are tested for all of these, so you can be reasonably certain they can do them all. Of course, that does not mean they will on the first try, especially if you have some unusual configuration.

The rest of this section gives information on diagnosing the problem when each of the above steps fails.

Manually keyed connections don't work

Suspect one of:

One manual connection works, but second one fails

This is fairly common problem when attempting to configure multiple manually keyed connections from a single gateway.

Each connection must be identified by a unique SPI value. For automatic connections, these values are assigned automatically. For manual connections, you must set them with spi= statements in ipsec.conf(5).

Each manual connection must have a unique SPI value in the range 0x100 to 0x999. Two or more with the same value will fail. For details, see our HTML doc section Using manual keying in production and the man page ipsec.conf(5).

Manual connections work, but automatic keying doesn't

The most common reason for this behaviour is a firewall dropping the UDP port 500 packets used in key negotiation.

Other possibilities:

IPsec works, but connections using compression fail

Suspect one of:

Small packets work, but large transfers fail

If tests with ping(1) and a small packet size succeed, but tests or transfers with larger packet sizes fail, suspect problems with packet fragmentation and perhaps path MTU discovery.

Our troubleshooting document covers these problems. Information on the underlying mechanism is in our background document.

Subnet-to-subnet works, but tests from the gateways don't

This is described under I cannot ping... above.

Compilation problems

gmp.h: No such file or directory

Pluto needs the GMP (GNU Multi- Precision) library for the large integer calculations it uses in public key cryptography. This error message indicates a failure to find the library. You must install it before Pluto will compile.

The GMP library is included in most Linux distributions. Typically, there are two RPMs, libgmp and libgmp-devel, You need to install both, either from your distribution CDs or from your vendor's web site.

On Debian, a mailing list message reports that the command to give is apt-get install gmp2.

For more information and the latest version, see the GMP home page.

... virtual memory exhausted

We have had several reports of this message appearing, all on SPARC Linux. Here is a mailing message on a solution:
> ipsec_sha1.c: In function `SHA1Transform':
> ipsec_sha1.c:95: virtual memory exhausted

I'm seeing exactly the same problem on an Ultra with 256MB ram and 500
MB swap.  Except I am compiling version 1.5 and its Red Hat 6.2.

I can get around this by using -O instead of -O2 for the optimization
level.  So it is probably a bug in the optimizer on the sparc complier. 
I'll try and chase this down on the sparc lists.

Interpreting error messages

route-client (or host) exited with status 7

Here is a discussion of this error from FreeS/WAN "listress" (mailing list tech support person) Claudia Schmeing. The "FAQ on the network unreachable error" which she refers to is the next question below.
> I reached the point where the two boxes (both on dial-up connections, but
> treated as static IPs by getting the IP and editing ipsec.conf after the
> connection is established) to the point where they exchange some info, but I
> get an error like "route-client command exited with status 7 \n internal
> error".
> Where can I find a description of this error?

In general, if the FAQ doesn't cover it, you can search the mailing list 
archives - I like to use
but you can see doc/mail.html for different archive formats.

Your error comes from the _updown script, which performs some
routing and firewall functions to help Linux FreeS/WAN. More info
is available at doc/firewall.html and man ipsec.conf. Its routing
is integral to the health of Linux FreeS/WAN; it also provides facility
to insert custom firewall rules to be executed when you create or destroy
a connection.

Yours is, of course, a routing error. You can be fairly sure the routing 
machinery is saying "network is unreachable". There's a FAQ on the 
"network is unreachable" error, but more information is available now; read on.

If your _updown script is recent (for example if it shipped with 
Linux FreeS/WAN 1.91), you will see another debugging line in your logs 
that looks something like this:

> output: /usr/local/lib/ipsec/_updown: `route add -net 
> netmask dev ipsec0 gw' failed

This is, of course, the system route command that exited with status 7, 
(ie. failed). Man route for details. Seeing the command typed out yields 
more information. If your _updown script is older, you may wish to update 
it to show the command explicitly.

Three parameters fed to the route command: net, netmask and gw [gateway] 
are derived from things you've put in ipsec.conf.

Net and netmask are derived from the peer's IP and mask. In more detail:

You may see a routing error when routing to a client (ie. subnet), or 
to a host (IPSec gateway or freestanding host; a box that does IPSec for
itself). In _updown, the "route-client" section  is responsible to set up 
the route for IPSec'd (usually, read 'tunneled') packets headed to a 
peer subnet. Similarly, route-host routes IPSec'd packets to a peer host
or IPSec gateway.

When routing to a 'client', net and netmask are ipsec.conf's left- or 
rightsubnet (whichever is not local). Similarly, when routing to a 
'host' the net is left or right. Host netmask is always /32, indicating a 
single machine.

Gw is nexthop's value. Again, the value in question is left- or rightnexthop,
whichever is local. Where left/right or left-/rightnexthop has the special 
value %defaultroute (described in man ipsec.conf), gw will automagically get
the value of the next hop on the default route.

Q: "What's a nexthop and why do I need one?"

A: 'nexthop' is a routing kluge; its value is the next hop away
   from the machine that's doing IPSec, and toward your IPSec peer. 
   You need it to get the processed packets out of the local system and 
   onto the wire. While we often route other packets through the machine 
   that's now doing IPSec, and are done with it, this does not suffice here. 
   After packets are processed with IPSec, this machine needs to know where 
   they go next. Of course using the 'IPSec gateway' as their routing gateway 
   would cause an infinite loop! [To visualize this, see the packet flow 
   diagram at doc/firewall.html.] To avoid this, we route packets through 
   the next hop down their projected path.

Now that you know the background, consider:
1. Did you test routing between the gateways in the absence of Linux
   FreeS/WAN, as recommended? You need to ensure the two machines that
   will be running Linux FreeS/WAN can route to one another before trying to 
   make a secure connection.
2. Is there anything obviously wrong with the sense of your route command?

Normally, this problem is caused by an incorrect local nexthop parameter.
Check out the use of %defaultroute, described in man ipsec.conf. This is
a simple way to set nexthop for most people. To figure nexthop out by hand,
traceroute in-the-clear to your IPSec peer. Nexthop is the traceroute's 
first hop after your IPSec gateway.

SIOCADDRT:Network is unreachable

This message is not from FreeS/WAN, but from the Linux IP stack itself. That stack is seeing packets it has no route for, either because your routing was broken before FreeS/WAN started or because FreeS/WAN's changes broke it.

Here is a message from Claudia suggesting ways to diagnose and fix such problems:

You write,
> I have correctly installed freeswan-1.8 on RH7.0 kernel 2.2.17, but when 
> I setup a VPN connection with the other machine(RH5.2 Kernel 2.0.36 
> freeswan-1.0, it works well.) it told me that 
> "SIOCADDRT:Network is unreachable"!  But the network connection is no 
> problem.

Often this error is the result of a misconfiguration. 

Be sure that you can route successfully in the absence of Linux
FreeS/WAN. (You say this is no problem, so proceed to the next step.)

Use a custom copy of the default updownscript. Do not change the route 
commands, but add a diagnostic message revealing the exact text of the 
route command. Is there a problem with the sense of the route command
that you can see? If so, then re-examine those ipsec.conf settings
that are being sent to the route command. 

You may wish to use the ipsec auto --route and --unroute commands to 
troubleshoot the problem. See man ipsec_auto for details.
Since the above message was written, we have modified the updown script to provide a better diagnostic for this problem. Check /var/log/messages.

See also the FAQ question route-client (or host) exited with status 7.

ipsec_setup: Fatal error, kernel appears to lack KLIPS

This message indicates an installation failure. The kernel you are running does not contain the KLIPS (kernel IPsec) code.

Commands you can quickly try are:

uname -a
to get details, including compilation date and time, of the currently running kernel
ls /
ls /boot
to ensure a new kernel is where it should be. If kernel compilation puts it in / but lilo wants it in /boot , then you should uncomment the INSTALL_PATH=/boot line in the kernel Makefile.
more /etc/lilo.conf
to see that lilo has correct information
to ensure that information in /etc/lilo.conf has been transferred to the boot sector
If those don't find the problem, you have to go back and check through the install procedure to see what was missed.

Here is one of Claudia's messages on the topic:

> I tried to install freeswan 1.8 on my mandrake 7.2 test box. ...

> It does show version and some output for whack.

Yes, because the Pluto (daemon) part of ipsec is installed correctly, but
as we see below the kernel portion is not.

> However, I get the following from /var/log/messages:
> Mar 11 22:11:55 pavillion ipsec_setup: Starting FreeS/WAN IPsec 1.8...
> Mar 11 22:12:02 pavillion ipsec_setup: modprobe: Can't locate module ipsec
> Mar 11 22:12:02 pavillion ipsec_setup: Fatal error, kernel appears to lack

This is your problem. You have not successfully installed a kernel with
IPSec machinery in it. 

Did you build Linux FreeS/WAN as a module? If so, you need to ensure that 
your new module has been installed in the directory where your kernel 
loader normally finds your modules. If not, you need to ensure
that the new IPSec-enabled kernel is being loaded correctly.

See also doc/install.html, and INSTALL in the distro.
Note that the "modprobe: Can't locate module ipsec" message appears even if you are not using modules. If there is no KLIPS in your kernel, FreeS/WAN tries to load it as a module. If that fails, you get this message.

ipsec_setup: ... failure to fetch key for ... from DNS

Quoting Henry:
Note that by default, FreeS/WAN is now set up to
     (a) authenticate with RSA keys, and
     (b) fetch the public key of the far end from DNS.
Explicit attention to  ipsec.conf will be needed if you want
to do something different.
and Claudia, responding to the same user:
You write,

>       My current setup in ipsec.conf is leftrsasigkey=%dns I have 
> commented this and authby=rsasig out. I am able to get ipsec running, 
> but what I find is that the documentation only specifies for %dns are 
> there any other values that can be placed in this variable other than 
> %dns and the key? I am also assuming that this is where I would place 
> my public key for the left and right side as well is this correct?

Valid values for authby= are rsasig and secret, which entail authentication
by RSA signature or by shared secret, respectively. Because you have 
commented authby=rsasig out, you are using the default value of authby=secret. 

When using RSA signatures, there are two ways to get the public key for the
IPSec peer: either copy it directly into *rsasigkey= in ipsec.conf, or
fetch it from dns. The magic value %dns for *rsasigkey parameters says to 
try to fetch the peer's key from dns.

For any parameters, you may find their significance and special values in
man ipsec.conf. If you are setting up keys or secrets, be sure also to
reference man ipsec.secrets.

ipsec_setup: ... interfaces ... and ... share address ...

This is a fatal error. FreeS/WAN cannot cope with two or more interfaces using the same IP address. You must re-configure to avoid this.

A mailing list message on the topic from Pluto developer Hugh Redelmeier:

| I'm trying to get freeswan working between two machine where one has a ppp
| interface.
| I've already suceeded with  two machines with ethernet ports but  the ppp
| interface is causing me problems.
|  basically when I run ipsec start  i get
| ipsec_setup: Starting FreeS/WAN IPsec 1.7...
| ipsec_setup: 003 IP interfaces ppp1 and ppp0 share address!
| ipsec_setup: 003 IP interfaces ppp1 and ppp2 share address!
| ipsec_setup: 003 IP interfaces ppp0 and ppp2 share address!
| ipsec_setup: 003 no public interfaces found
| followed by lots of cannot work out interface for connection messages
| now I can specify the interface in ipsec.conf to be ppp0 , but this does
| not affect the above behaviour. A quick look in server.c indicates that the
| interfaces value  is not used but some sort of raw detect happens.
| I guess I could prevent the formation of the extra ppp interfaces or
| allocate them different ip but I'd  rather not. if at all possible. Any
| suggestions please.

Pluto won't touch an interface that shares an IP address with another.
This will eventually change, but it probably won't happen soon.

For now, you will have to give the ppp1 and ppp2 different addresses.

ipsec_setup: Cannot adjust kernel flags

A mailing list message form technical lead Henry Spencer:
> When FreeS/WAN IPsec 1.7 is starting on my 2.0.38 Linux kernel the following
> error message is generated:
> ipsec_setup: Cannot adjust kernel flags, no /proc/sys/net/ipsec directory!
> What is supposed to create this directory and how can I fix this problem?

I think that directory is a 2.2ism, although I'm not certain (I don't have
a 2.0.xx system handy any more for testing).  Without it, some of the
ipsec.conf config-setup flags won't work, but otherwise things should
You also need to enable the /proc filesystem in your kernel configuration for these operations to work.

Connection names in Pluto error messages

From Pluto programmer Hugh Redelmeier:

| Jan 17 16:21:10 remus Pluto[13631]: "jumble" #1: responding to Main Mode from Road Warrior
| Jan 17 16:21:11 remus Pluto[13631]: "jumble" #1: no suitable connection for peer
|     The connection "jumble" has nothing to do with the incoming
| connection requests, which were meant for the connection "banshee".

You are right.  The message tells you which Connection Pluto is
currently using, which need not be the right one.  It need not be the
right one now for the negotiation to eventually succeed!  This is
described in ipsec_pluto(8) in the section "Road Warrior Support".

There are two times when Pluto will consider switching Connections for
a state object.  Both are in response to receiving ID payloads (one in
Phase 1 / Main Mode and one in Phase 2 / Quick Mode).  The second is
not unique to Road Warriors.  In fact, neither is the first any more
(two connections for the same pair of hosts could differ in Phase 1 ID
payload; probably nobody else has tried this).

Pluto: ... can't orient connection

Older versions of FreeS/WAN used this message. The same error now gives the "we have no ipsecN ..." error described just below.

... we have no ipsecN interface for either end of this connection

Each Pluto needs to know whether it is running on the machine which the connection description calls left or on right . It figures that out by:

Normally a match is found. Then Pluto knows where it is and can set up other things (for example, if it is left) using parameters such as leftsubnet and leftnexthop, and sending its outgoing packets to right.

If no match is found, it emits the above error message.

Pluto: ... no connection is known

This error message occurs when a remote system attempts to negotiate a connection and Pluto does not have a connection description that matches what the remote system has requested. The most common cause is a configuration error on one end or the other.

Parameters involved in this match are left, right , leftsubnet and rightsubnet.

The match must be exact. For example, if your left subnet is a.b.c.0/24 then neither a single machine in that net nor a smaller subnet such as a.b.c.64/26 will be considered a match.

The message can also occur when an appropriate description exists but Pluto has not loaded it. Use an auto=add statement in the connection description, or an ipsec auto --add <conn_name> command, to correct this.

An explanation from the Pluto developer:

| Jul 12 15:00:22 sohar58 Pluto[574]: "corp_road" #2: cannot respond to IPsec
| SA request because no connection is known for

This is the first message from the Pluto log showing a problem.  It
means that PGPnet is trying to negotiate a set of SAs with this
^^^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^
client on our side  our host         PGPnet host, no client

None of the conns you showed look like this.

        ipsec auto --status
to see a snapshot of what connections are in pluto, what
negotiations are going on, and what SAs are established.

The leftsubnet= (client) in your conn is  It must
exactly match what pluto is looking for, and it does not.

Pluto: ... no suitable connection ...

This is similar to the no connection known error, but occurs at a different point in Pluto processing.

Here is one of Claudia's messages explaining the problem:

You write,

> What could be the reason of the following error? 
> "no suitable connection for peer '@xforce'"

When a connection is initiated by the peer, Pluto must choose which entry in 
the conf file best matches the incoming connection. A preliminary choice is 
made on the basis of source and destination IPs, since that information is 
available at that time. 

A payload containing an ID arrives later in the negotiation. Based on this
id and the *id= parameters, Pluto refines its conn selection. ...

The message "no suitable connection" indicates that in this refining step,
Pluto does not find a connection that matches that ID.

Please see "Selecting a connection when responding" in man ipsec_pluto for
more details.

See also Connection names in Pluto error messages.

Pluto: ... no connection has been authorized

Here is one of Claudia's messages discussing this problem:
You write,

>  May 22 10:46:31 debian Pluto[25834]: packet from x.y.z.p:10014: 
>  initial Main Mode message from x.y.z.p:10014 
                            but no connection has been authorized

This error occurs early in the connection negotiation process,
at the first step of IKE negotiation (Main Mode), which is itself the 
first of two negotiation phases involved in creating an IPSec connection.

Here, Linux FreeS/WAN receives a packet from a potential peer, which 
requests that they begin discussing a connection.

The "no connection has been authorized" means that there is no connection 
description in Linux FreeS/WAN's internal database that can be used to 
link your ipsec interface with that peer.

"But of course I configured that connection!" 

It may be that the appropriate connection description exists in ipsec.conf 
but has not been added to the database with ipsec auto --add myconn or the 
auto=add method. Or, the connection description may be misconfigured.

The only parameters that are relevant in this decision are left= and right= .
Local and remote ports are also taken into account -- we see that the port 
is printed in the message above -- but there is no way to control these
in ipsec.conf.

Failure at "no connection has been authorized" is similar to the
"no connection is known for..." error in the FAQ, and the "no suitable
connection" error described in the snapshot's FAQ. In all three cases,
Linux FreeS/WAN is trying to match parameters received in the
negotiation with the connection description in the local config file.

As it receives more information, its matches take more parameters into 
account, and become more precise:  first the pair of potential peers,
then the peer IDs, then the endpoints (including any subnets).

The "no suitable connection for peer *" occurs toward the end of IKE 
(Main Mode) negotiation, when the IDs are matched.

"no connection is known for a/b===c...d" is seen at the beginning of IPSec 
(Quick Mode, phase 2) negotiation, when the connections are matched using
left, right, and any information about the subnets.

Pluto: ... OAKLEY_DES_CBC is not supported.

This message occurs when the other system attempts to negotiate a connection using single DES, which we do not support because it is insecure.

Our interoperation document has suggestions for how to deal with systems that attempt to use single DES.

Pluto: ... no acceptable transform

This message means that the other gateway has made a proposal for connection parameters, but nothing they proposed is acceptable to Pluto. Possible causes include: A more detailed explanation, from Pluto programmer Hugh Redelmeier:

When one IKE system (for example, Pluto) is negotiating with another
to create an SA, the Initiator proposes a bunch of choices and the
Responder replies with one that it has selected.

The structure of the choices is fairly complicated.  An SA payload
contains a list of lists of "Proposals".  The outer list is a set of
choices: the selection must be from one element of this list.

Each of these elements is a list of Proposals.  A selection must be
made from each of the elements of the inner list.  In other words,
*all* of them apply (that is how, for example, both AH and ESP can
apply at once).

Within each of these Proposals is a list of Transforms.  For each
Proposal selected, one Transform must be selected (in other words,
each Proposal provides a choice of Transforms).

Each Transform is made up of a list of Attributes describing, well,
attributes.  Such as lifetime of the SA.  Such as algorithm to be
used.  All the Attributes apply to a Transform.

You will have noticed a pattern here: layers alternate between being
disjunctions ("or") and conjunctions ("and").

For Phase 1 / Main Mode (negotiating an ISAKMP SA), this structure is
cut back.  There must be exactly one Proposal.  So this degenerates to
a list of Transforms, one of which must be chosen.

In your case, no proposal was considered acceptable to Pluto (the
Responder).  So negotiation ceased.  Pluto logs the reason it rejects
each Transform.  So look back in the log to see what is going wrong.

rsasigkey dumps core

A comment on this error from Henry:
On Fri, 29 Jun 2001, Rodrigo Gruppelli wrote:
> ...Well, it seem that there's
> another problem with it. When I try to generate a pair of RSA keys,
> rsasigkey cores dump...

*That* is a neon sign flashing "GMP LIBRARY IS BROKEN".  Rsasigkey calls
GMP a lot, and our own library a little bit, and that's very nearly all it
does.  Barring bugs in its code or our library -- which have happened, but
not very often -- a problem in rsasigkey is a problem in GMP.
See the next question for how to deal with GMP errors.

!Pluto failure!: ... exited with ... signal 4

Pluto has died. Signal 4 is SIGILL, illegal instruction.

The most likely cause is that your gmp (GNU multi-precision) library is compiled for a different processor than what you are running on. Pluto uses that library for its public key calculations.

Try getting the gmp sources and recompile for your processor type. Most Linux distributions will include this source, or you can download it from the GMP home page.

ECONNREFUSED error message

From John Denker, on the mailing list:

1)  The log message
  some IKE message we sent has been rejected with 
  ECONNREFUSED (kernel supplied no details)
is much more suitable than the previous version.  Thanks.

2) Minor suggestion for further improvement: it might be worth mentioning
that the command
  tcpdump -i eth1 icmp[0] != 8 and icmp[0] != 0
is useful for tracking down the details in question.  We shouldn't expect
all IPsec users to figure that out on their own.  The log message might
even provide a hint as to where to look in the docs.

Reply From Pluto developer Hugh Redelmeier

Good idea.

I've added a bit pluto(8)'s BUGS section along these lines.
I didn't have the heart to lengthen this message.

klips_debug: ... no eroute!

This message means KLIPS has received a packet for which no IPsec tunnel has been defined.

Here is a more detailed duscussion from the team's tech support person Claudia Schmeing, responding to a query on the mailing list:

> Why ipsec reports no eroute! ???? IP Masq... is disabled.

In general, more information is required so that people on the list may
give you informed input. See doc/
The document she refers to has since been replaced by a section of the troubleshooting document.
However, I can make some general comments on this type of error.

This error usually looks something like this (clipped from an archived

> ttl:64 proto:1 chk:45459 saddr: daddr:
> ... klips_debug:ipsec_findroute:>
> ... klips_debug:rj_match: * See if we match exactly as a host destination
> ... klips_debug:rj_match: ** try to match a leaf, t=0xc1a260b0
> ... klips_debug:rj_match: *** start searching up the tree, t=0xc1a260b0
> ... klips_debug:rj_match: **** t=0xc1a260c8
> ... klips_debug:rj_match: **** t=0xc1fe5960
> ... klips_debug:rj_match: ***** not found.
> ... klips_debug:ipsec_tunnel_start_xmit: Original head/tailroom: 2, 28
> ... klips_debug:ipsec_tunnel_start_xmit: no eroute!: ts=47.3030, dropping.

What does this mean?
- --------------------

"eroute" stands for "extended route", and is a special type of route 
internal to Linux FreeS/WAN. For more information about this type of route, 
see the section of man ipsec_auto on ipsec auto --route.

"no eroute!" here means, roughly, that Linux FreeS/WAN cannot find an 
appropriate tunnel that should have delivered this packet. Linux 
FreeS/WAN therefore drops the packet, with the message "no eroute! ...
dropping", on the assumption that this packet is not a legitimate 
transmission through a properly constructed tunnel.

How does this situation come about?
- -----------------------------------

Linux FreeS/WAN has a number of connection descriptions defined in 
ipsec.conf. These must be successfully brought "up" to form actual tunnels.
(see doc/setup.html's step 15, man ipsec.conf and man ipsec_auto 
for details).

Such connections are often specific to the endpoints' IPs. However, in 
some cases they may be more general, for example in the case of 
Road Warriors where left or right is the special value %any.

When Linux FreeS/WAN receives a packet, it verifies that the packet has
come through a legitimate channel, by checking that there is an
appropriate tunnel through which this packet might legitimately have
arrived. This is the process we see above.

First, it checks for an eroute that exactly matches the packet. In the 
example above, we see it checking for a route that begins at
and ends at This search favours the most specific match that
would apply to the route between these IPs. So, if there is a connection 
description exactly matching these IPs, the search will end there. If not, 
the code will search for a more general description matching the IPs.
If there is no match, either specific or general, the packet will be
dropped, as we see, above.

Unless you are working with Road Warriors, only the first, specific part 
of the matching process is likely to be relevant to you.

"But I defined the tunnel, and it came up, why do I have this error?"
- ---------------------------------------------------------------------

One of the most common causes of this error is failure to specify enough
connection descriptions to cover all needed tunnels between any two 
gateways and their respective subnets. As you have noticed, troubleshooting
this error may be complicated by the use of IP Masq. However, this error is
not limited to cases where IP Masq is used. 

See doc/configuration.html#multitunnel for a detailed example of the 
solution to this type of problem.

... trouble writing to /dev/ipsec ... SA already in use

From the mailing list:
> When I activate one manual tunnels it works, but when I try to activate
> another tunnel, it gives an error message...
> tunnel_2: Had trouble writing to /dev/ipsec SA:tun0x200@ --
> SA already in use.  Delete old one first.

Please read the Using manual keying in production discussion in
config.html, specifically the part about needing a different spi
(or spibase) setting for each connection. 
This problem is also discussed in this FAQ under the heading One manual connection works, but second one fails.

... ignoring ... payload

This message is harmless. The IKE protocol provides for a number of optional messages types: An implementation is never required to send these, but they are allowed to. The receiver is not required to do anything with them. FreeS/WAN ignores them, but notifies you via the logs.

For the "ignoring delete SA Payload" message, see also our discussion of cleaning up dead tunnels.

Why don't you restrict the mailing lists to reduce spam?

As a matter of policy, some of our mailing lists need to be open to non-subscribers. Project management feel strongly that maintaining this openness is more important than blocking spam.

This has been discussed several times at some length on the list. See the list archives. Bringing the topic up again is unlikely to be useful. Please don't. Or at the very least, please don't without reading the archives and being certain that whatever you are about to suggest has not yet been discussed.

Project technical lead Henry Spencer summarised one discussion:

For the third and last time: this list *will* *not* do address-based filtering. This is a policy decision, not an implementation problem. The decision is final, and is not open to discussion. This needs to be communicated better to people, and steps are being taken to do that.
Adding this FAQ section is one of the steps he refers to.

You have various options other than just putting up with the spam, filtering it yourself, or unsubscribing:

A number of tools are available to filter mail.

If you use your ISP's mail server rather than running your own, consider suggesting to the ISP that they tag suspected spam as this ISP does. They could just refuse mail from dubious sources, but that is tricky and runs some risk of losing valuable mail or senselessly annoying senders and their admins. However, they can safely tag and deliver dubious mail. The tags can greatly assist your filtering.

For information on tracking down spammers, see these HowTos, or the Sputum site. Sputum have a Linux anti-spam screensaver available for download.

Here is a more detailed message from Henry:

On Mon, 15 Jan 2001, Jay Vaughan wrote:
> I know I'm flogging a dead horse here, but I'm curious as to the reasons for
> an aversion for a subscriber-only mailing list?

Once again:  for legal reasons, it is important that discussions of these
things be held in a public place -- the list -- and we do not want to
force people to subscribe to the list just to ask one question, because
that may be more than merely inconvenient for them.  There are also real
difficulties with people who are temporarily forced to use alternate
addresses; that is precisely the time when they may be most in need of
help, yet a subscribers-only policy shuts them out.

These issues do not apply to most mailing lists, but for a list that is
(necessarily) the primary user support route for a crypto package, they
are very important.  This is *not* an ordinary mailing list; it has to
function under awkward constraints that make various simplistic solutions
inapplicable or undesirable. 

> We're *ALL* sick of hearing about list management problems, not just you
> old-timers, so why don't you DO SOMETHING EFFECTIVE ABOUT IT...

Because it's a lot harder than it looks, and many existing "solutions"
have problems when examined closely.

> A suggestion for you, based on 10 years of experience with management of my
> own mailing lists would be to use mailman, which includes pretty much every
> feature under the sun that you guys need and want, plus some.  The URL for
> mailman...

I assure you, we're aware of mailman.  Along with a whole bunch of others,
including some you almost certainly have never heard of (I hadn't!).

> As for the argument that the list shouldn't be configured to enforce
> subscription - I contend that it *SHOULD* AT LEAST require manual address
> verification in order for posts to be redirected.

You do realize, I hope, that interposing such a manual step might cause
your government to decide that this is not truly a public forum, and thus
you could go to jail if you don't get approval from them before mailing to
it?  If you think this sounds irrational, your government is noted for
making irrational decisions in this area; we can't assume that they will
suddenly start being sensible.  See above about awkward constraints.  You
may be willing to take the risk, but we can't, in good conscience, insist
that all users with problems do so. 

                                                          Henry Spencer
and a message on the topic from project leader John Gilmore:
Subject: Re: The linux-ipsec list's topic
   Date: Sat, 30 Dec 2000
   From: John Gilmore <>

I'll post this single message, once only, in this discussion, and then
not burden the list with any further off-topic messages.  I encourage
everyone on the list to restrain themself from posting ANY off-topic
messages to the linux-ipsec list.

The topic of the linux-ipsec mailing list is the FreeS/WAN software.

I frequently see "discussions about spam on a list" overwhelm the
volume of "actual spam" on a list. BOTH kinds of messages are
off-topic messages.  Twenty anti-spam messages take just as long to
detect and discard as twenty spam messages.

The Linux-ipsec list encourages on-topic messages from people who have
not joined the list itself.  We will not censor messages to the list
based on where they originate, or what return address they contain.
In other words, non-subscribers ARE allowed to post, and this will not
change.  My own valid contributions have been rejected out-of-hand by
too many other mailing lists for me to want to impose that censorship
on anybody else's contributions.  And every day I see the damage that
anti-spam zeal is causing in many other ways; that zeal is far more
damaging to the culture of the Internet than the nuisance of spam.

In general, it is the responsibility of recipients to filter,
prioritize, or otherwise manage the handling of email that comes to
them.  It is not the responsibility of the rest of the Internet
community to refrain from sending messages to recipients that they
might not want to see.  If your software infrastructure for managing
your incoming email is insufficient, then improve it.  If you think
the signal-to-noise ratio on linux-ipsec is too poor, then please
unsubscribe.  But don't further increase the noise by posting to the
linux-ipsec list about those topics.

        John Gilmore
        founder sponsor, FreeS/WAN project

Installing FreeS/WAN

Who needs to perform an installation?

Some Linux distributions, listed in the introduction, ship with FreeS/WAN included. If you are using one of them, you need not perform a FreeS/WAN installation. That should all be done for you already. All you have to do is:

Users of such distributions can skip ahead to our section on setting up FreeS/WAN.

Unfortunately, due to export laws restricting distribution of strong cryptography, not all distributions include FreeS/WAN. Moreover, the standard kernel does not include the kernel parts of FreeS/WAN. Many people will need to install FreeS/WAN, including patching and rebuilding their kernel.

Information on re-installing or un-installing is provided near the end of this document.

Before starting the install

Configure, compile, install, and test a Linux kernel, without FreeS/WAN.

If you have not done this before, you will need to read the Kernel HowTo. You might also look at this magazine article.

Choosing a kernel

2.2.x for many users

Many users can continue to run kernels from the 2.2 series of Linux production kernels.

We recommend using the latest release in that series. At time of writing (Nov 2001), that is 2.2.20.

If you need to use an older 2.2.x kernel for some reason, be warned that recent versions of FreeS/WAN will not compile out-of-the-box on a kernel earlier than 2.2.19. A workaround is described in the FreeS/WAN 1.91 section of our CHANGES file. See the mailing list archives, around June 2001, for more details if needed.

2.4.x is possible

The new 2.4 series of kernels began in January 2001 and are currently (late November) at 2.4.16.

Do not use 2.4.15; it has a bug that causes file system corruption. FreeS/WAN 1.93 was tested mainly on 2.4.14 and is known to work there.

2.4 has new firewalling code called netfilter. This may provide good reasons to move to 2.4, especially on for gateway machines.

2.0.x should still work

In the older 2.0.x kernel series, we no longer support versions earlier than 2.0.38. 2.0.38 has fixes for a number of small security-related glitches, worth having on a security gateway machine. FreeS/WAN has been tested on 2.0.39, and does work there.

Recent versions of FreeS/WAN are not heavily tested on 2.0 kernels. Most of both the development team and the user community are on 2.2, or even 2.4, by now.

We are likely to drop 2.0 support entirely if some problem crops up that would mean retaining it required significant work from our team.

Development kernels

Development kernels are a separate series, work-in-progress versions for use by kernel developers. By convention, production kernels have an even second digit in the version number (2.0, 2.2, 2.4) and development kernels have an odd digit there (2.1, 2.3, 2.5).

In late November 2001, the first 2.5 series kernel, 2.5.0, appeared. It has the same file system bug as 2.4.15. Do not use it.

Development kernels are not intended for production use . They change often and include new code which has not yet been thoroughly tested. This regularly breaks things, including FreeS/WAN. The FreeS/WAN team does not have the resources to chase the moving target; our priority is developing FreeS/WAN on stable kernels. If you encounter a problem on a development kernel, please solve it (you are a developer, aren't you?) and send us a patch. Of course, we will happily discuss problems and solutions on the mailing list, but we are unlikely to do much work on actually implementing a solution.

Fortunately we have a user who regularly fixes problems with FreeS/WAN on development kernels (merci, Marc), and we do fix some ourselves. FreeS/WAN often works just fine on a development kernel; it's just that there's no guarantee.

If you are going to test FreeS/WAN with a development kernel, we recommend you use our latest snapshot. This is the FreeS/WAN version most likely to have the patches required to work on a recent development kernel. The released version of FreeS/WAN is likely to be out of date for your purposes.

Things you must have installed

If you have a CD distribution of Linux, it should include everything you need.

Tools and libraries

Use your distribution's tools to load:

There are some common slips worth avoiding here:

Kernel source code

You need the source code for the kernel because you must patch and re-compile it to install FreeS/WAN. There are several places you can get this:
Kernel from CD
You can install the kernel from your distribution CD. It may be in two packages. However, if your CD is not recent, it may have an older kernel, in which case we suggest getting more recent kernel source from the net.
Vendor kernels

All the major distribution vendors provide kernel source. See for example:

Using a kernel from your distribution vendor may save you some annoyance later.

Different distributions put the kernel in different places (/vmlinuz, /boot/vmlinuz, /boot/vmlinuz-2.2.15 ...) and set lilo (the Linux loader) up differently. With a kernel from your distribution vendor, everything should work right. With other combinations, a newly compiled kernel may be installed in one place while lilo is looking in another. You can of course adjust the kernel Makefile and/or /etc/lilo.conf to solve this problem, but we suggest just avoiding it.

Also, distributions vendors may include patches or drivers which are not part of the standard kernel. If you install a standard kernel, you must either do without those features or download those patches and add them yourself.

Note: Some recent distributions (certainly Redhat 7.2, perhaps others) put kernel source code in a directory named linux-2.4 while FreeS/WAN expects to find it in linux, which is where all distributions used to put it and the kernels still do. If your distribution uses linux-2.4, then you must create a symbolic link to linux before proceeding with your FReeS/WAN install. See the man page for ln(1) for details of how to do this if required.

Kernels from
For kernels direct from Linus, without any distribution vendor's modifications, see the mirror list, or go directly to ftp.<country>, with the appropriate two-letter country code inserted.

Once you've found a kernel

Once you have found suitable kernel source, choose a mirror that is close to you and bookmark it.

Kernel source normally resides in /usr/src/linux, whether you load it from a distribution CD or download a tar file into /usr/src and untar it there. Unless you both have unusual requirements and know exactly what you're doing, we recommend you put it there.

Getting FreeS/WAN

You can download FreeS/WAN from our primary site or one of our mirrors.

Put the tarfile under /usr/src and untar it there. The command to use is:

This will give you a directory /usr/src/freeswan<version> .

Note that these methods don't work:

Kernel configuration

The gateway kernel must be configured before FreeS/WAN is added because some of our utilities rely on the results of configuration.

Note for Redhat 7.1 users: If you are using the Redhat-supplied kernel, then you must do a make mrproper command before starting the kernel configuration. This prevents some unpleasant interactions between Redhat's config and our patches.

On some distributions, you can get the configuration files for the vendor's standard kernel(s) off the CD, and use that. This allows you to skip this step; you need not configure the kernel if the vendor has and you have the vendor's config file installed. Here is a mailing list message describing the procedure for Redhat:

Subject: Re: [Users] Do I need to recompile kernel 2.2.17-14?
   Date: Wed, 6 Jun 2001 08:38:38 -0500
   From: "Corey J. Steele" <>

if you install the corresponding kernel-source-*.rpm, you can actually find
the config file used to build that kernel in /usr/src/linux/Configs, just
copy the one you want to use (based solely on architecture) to
/usr/src/linux/.config, and proceed!  It should work.
If you have ever configured the kernel yourself on this machine, you can also skip this step.

If the kernel has not been configured, do that now. This is done by giving one of the following commands in /usr/src/linux:

make config
command-line interface
make menuconfig
text menus (requires curses(3) libraries)
make xconfig
using the X window system (requires X, not recommended for gateways)

Any of these wiil do the job. If you have no established preference, we suggest trying menuconfig.

For more information on configuring your kernel, see our section on that topic.

Install and test a kernel before adding FreeS/WAN

You should compile, install and test the kernels as you have configured them, so that you have a known stable starting point. The series of commands involved is usually something like:

make menuconfig
choose kernel options, set up a kernel for your machine
make dep
find dependencies between files
make bzImage
build a loadable kernel image, compressed with bzip(1)
make install
install it
make modules
build modules which can be added to a running kernel
make modules_install
install them
ensure that the boot loader sees your changes

Doing this first means that if there is a problem after you add FreeS/WAN, tracking it down is much simpler.

If you need advice on this process, or general Linux background information, try our Linux web references. The most directly relevant document is the Kernel HowTo.

Building and installing the software

There are several ways to build and install the software. All require that you have kernel source, correctly configured for your machine, as a starting point. If you don't have that yet, see the previous section

Whatever method you choose, it will do all of the following:

You can do the whole install with two commands (recommended in most cases) or get into as much of the detail as you like.

Building RPMs

As of version 1.93, we provide a facilty to build FreeS/WAN RPMs.

Go to the FreeS/WAN directory and do whichever of the following commands you prefer:

make orpm
uses command-line kernel configuration
make menurpm
uses menu kernel configuration (requires ncurses library)
make xrpm
use X Window kernel configuration (requires X)

After the Makefile does the software and kernel build, it will make some RPMs and leave them in the rpms directory. The RPMs are:

the userland utilities
the ipsec.o kernel module, built only if your kernel configuration sets klips as a module
the Linux kernel and its modules
all of the above

Once you have the RPMs, you can install FreeS/WAN from them with rpm -i commands.

This makes it much easier to build FreeS/WAN on one system for installation on another.

This facility is based on work by Paul Lahaie at Steamballoon.

Building IPsec as a module

With the full procedure described in the next section, you can either build the kernel parts of FreeS/WAN into your kernel or build them as a kernel module, depending on how you set the kernel configuration options.

Since 1.91, we also provide an option to build only the FreeS/WAN module, without re-compiling the rest of your kernel.

Note, however, that this requires:

To do the module install, give two commands in the FreeS/WAN directory:

This is relatively new code and not yet tested on a wide range of systems. If it does not work for you, please report the problem. In the meanwhile, fall back to the older procedure described above.

Installing directly from source

You can also install FreeS/WAN directly from the source, without building RPMs as an intermediate step.

There are two steps here. First you do everything else, then you install the new FreeS/WAN-enabled kernel.

Everything but kernel installation

To do everything except install the new kernel, cd into the freeswan directory and become root. Give any one of the following commands:

make oldgo
Uses FreeS/WAN's default settings for some kernel configuration options. Leaves all other options unchanged from your last kernel configuration.
make ogo
Invokes config so you can configure the kernel from the command line.
make menugo
Invokes menuconfig so you can configure the kernel with text-mode menus.
make xgo
Invokes xconfig so you can configure the kernel in an X window.

You must save the new configuration even if you make no changes. This ensures that the FreeS/WAN changes are actually seen by the system.

There are few options in the FreeS/WAN part of kernel configuration. For most of them, we recommend that you make no changes.

Our scripts save the output of make commands they call in files with names like out.kbuild or out.kinstall. The last command of each script checks the appropriate out.* file for error messages.

For the above commands, the error files are out.kpatch and out.kbuild.

These scripts automatically build an RSA authentication key pair (a public key and the matching private key) for you, and put the result in /etc/ipsec.secrets. For information on using RSA authentication, see our configuration section. Here, we need only note that generating the key uses random(4) quite heavily and if random(4) runs out of randomness, it will block until it has enough input. You may need to provide input by moving the mouse around a lot, or going to another window and typing random characters, or using some command such as du -s /usr to generate disk activity.

Installing the new kernel

To install the kernel the easy way, just give this command in the FreeS/WAN directory:

make kinstall
Installs the new kernel and, if required, the modules to go with it. Errors, if any, are reported in out.kinstall

Using make kinstall from the FreeS/WAN directory is equivalent to giving the following sequence of commands in /usr/src/linux:

If you prefer that sequence, use it instead.

If you have some unusual setup such that the above sequence of commands won't work on your system, then our make kinstall will not work either. Use whatever method does work on your system. See our implementation notes file for additional information that may help in such situations.

Make sure Lilo knows about the new kernel

Check your lilo.conf(5) file to ensure it points to the right kernel, then run lilo(8) to read lilo.conf(5) and set up the bootloader.

Testing to see if install succeeded

To check that you have a sucessful install, you can reboot and check (by watching messages during boot or by looking at them later with dmesg(8)) that:

You can also try the commands:

Of course any status information at this point should be uninteresting since you have not yet configured connections.

Where to go from here

See the following section for information on configuring connections.

Alternately, you might want to look at background material on the protocols used before trying configuration.

Re-install or un-install

If you already have FreeS/WAN installed on this machine, and need to install a newer version or un-install FreeS/WAN, this section is for you.

If you need to install FreeS/WAN, go to the installation section.


The scripts are designed so that a re-install -- to upgrade to a later FreeS/WAN version or to a later kernel version -- can be done in exactly the same way as an original install.

The scripts know enough, for example, not to apply the same kernel patch twice and not to overwrite your ipsec.conf or ipsec.secrets files. However, they will overwrite the _updown script. If you have modified that, save your version under another name before doing the install.

Also, they may not always work exactly as designed. Check the BUGS file for any caveats in the current version.

to install a new version of FreeS/WAN, with your current kernel
Download and untar the new FreeS/WAN. Since kernel source has already been installed and configured, you can skip a few steps in the procedure below. Go to Building FreeS/WAN, and follow normal FreeS/WAN procedures from there.
to install a new kernel, on a machine which already has FreeS/WAN installed
Download and untar the new kernel source. Since this kernel is not yet configured, that is the next thing to do.Go to Kernel configuration, and follow normal FreeS/WAN procedures from there.
to upgrade both kernel and FreeS/WAN
You need both new kernel source and new FreeS/WAN source. Follow the full FreeS/WAN install procedure. See below.


Disabling FreeS/WAN

In many Linux distributions, you can easily disable FreeS/WAN with the command:

    chkconfig --del ipsec

This removes the symlinks in /etc/rc.d/rc?.d which cause ipsec(8) to be called at boot time or when switching run levels. If the kernel part of IPsec, KLIPS, has been compiled as a module, then this also prevents loading that module, so IPsec is completely disabled.

Other distributions may use another version of init(8), or may not provide the chkconfig(8) command. For these, you will have to manually edit the init scripts to achieve the same effect.

Removing FreeS/WAN files

To entirely remove the user-level FreeS/WAN components from your system, go to the FreeS/WAN install directory and give the command:

     make uninstall_freeswan

If that doesn't work for you -- for example, if FreeS/WAN was built on another system and copied here -- then you can do it manually. First disable FreeS/WAN as described above (to avoid problems with symlinks pointing to things you are about to remove), and then use these commands:

        rm -f /etc/ipsec.* /usr/local/sbin/ipsec /etc/rc.d/init.d/ipsec
        rm -rf /usr/local/lib/ipsec
        rm -f /usr/local/man/man?/ipsec[._]*
You may need to vary the commands slightly if you, or whoever packaged your distribution, changed the install directories when building FreeS/WAN.

Removing FreeS/WAN from the kernel

If you compiled KLIPS as a module, then just disabling FreeS/WAN as described above prevents loading the module.

If KLIPS is compiled into your kernel, then you can disable it by turning off IPsec in your kernel configuration (or by making it a module) and recompiling.

You can remove the FreeS/WAN patches from your kernel source by going to the FreeS/WAN install directory and giving the command:

     make unpatch

This does not remove all FreeS/WAN changes; some are not done with patch(1) and cannot be reversed in this way.

To remove all trace of IPsec in your kernel, you should revert to an unpatched version, or download fresh kernel source.


This section describes setting up and testing Linux FreeS/WAN.

There are separate documents on testbed configurations and performance measurement which some users may want to consult along with this one. If you just want to get a few connections up, this document should have everything you need.

Before attempting this, you should:

You also need to set up and test IP networking on all the machines you plan to install FreeS/WAN on or to use in testing, before trying to set up FreeS/WAN.

Our example networks

In our examples, we describe a setup with three networks -- two that want to talk to each other plus the Internet in the middle. The idea is to build an encrypted tunnel across the Internet so the two networks can talk securely.

We'll call the two gateways East and West. We'll have a client machine on each net: Sunrise in the East and Sunset in the West.

           local net       untrusted net       local net

Of course one does not always have a security gateway separate from the client machine. It is also quite common to use IPsec on a network that looks like this:

                                           telecommuter's PC or
                                           traveller's laptop
         corporate LAN     untrusted net

We treat this setup as degenerate cases of the network-to-network link. The East computer is a gateway for a one-client subnet, and it is also the client.

Our goal here is to tell you how to set up the two gateways, East and West. We assume your goal is to ensure that East and West encrypt all traffic between them.

More complicated network configurations are described later.

Set up and test networking

Before trying to get FreeS/WAN working, you should configure and test IP networking on both gateways and on at least one client machine behind each of them. IPsec cannot work without a working IP network beneath it.

Many reported "FreeS/WAN problems" turn out to actually be problems with routing or firewalling. If any actual IPsec problems turn up, you often cannot even recognise them (much less debug them) unless the underlying network is right.

If you need advice on this, your best sources are likely:

See also our bibliography.

Here is our network diagram again:

              local net       untrusted net       local net

The client machines, Sunrise and Sunset in our example, may have assigned routable IP addresses, or they may be using private non-routable addresses (as defined in RFC 1918 ) with the gateways doing IP masquerade. It doesn't matter which, as long as whatever it is works correctly. Note, however, that the two client subnets must have distinct addresses. You cannot have them both masqueraded to the same range of RFC 1918 addresses.

You must have a working IP network before you try to add IPsec:

It is not enough to just test that your gateways (East and West in the example) can communicate. You need to test routing to the clients (Sunrise and Sunset) as well.

If you want to run some service encapsulated in IP -- perhaps to use Novell protocols encapsulated in IPX or to make Windows file sharing or NT domains work across the IPsec tunnel -- then please build and test what you need for that service on plain IP before trying it over IPsec. It can be a real nightmare trying to debug such things when you don't know if the problem is in IPsec, firewall rules, routing, or the configuration of the service itself. Some advice on making such things work with IPsec is in our interoperation section.

Enabling packet forwarding

Some systems turn off packet forwarding by default. This is the safe default. You don't want systems forwarding packets in uncontrolled ways.

There are three places where you can enable or disable IP forwarding:

in kernel configuration, before compiling the kernel
for FreeS/WAN use, forwarding must always be enabled here. If forwarding is not compiled into your kernel, attempts to enable or use it will fail.
in the standard boot scripts.
If your kernel has forwarding, you can have it turned on automatically by the standard boot scripts. The exact method varies from distribution to distribution:
Older Redhat
in the file /etc/sysconfig/network, set FORWARD_IPV4=yes
Redhat 6.x and 7.0
in the file /etc/sysconfig/network, set net.ipv4.ip_forward=1
Debian r2.2 systems (and most likely Debian r2.2 derived systems):
in the file /etc/network/options, set ip_forward=yes
From the command line or your own scripts
use the command:
         echo "1" > /proc/sys/net/ipv4/ip_forward
You need root privileges to write to that file.

A gateway machine needs forwarding enabled or it will not route packets between the two networks it is attached to. The simplest way to ensure this is to enable forwarding using your distribution's standard boot scripts. See above.

A more conservative approach is to disable forwarding in your system configuration, and only enable it after appropriate firewall rules and IPsec tunnels are in place. This reduces the risk of something slipping past your defenses before they are fully set up. On most systems, this can conveniently be done by adding a line to /etc/rc.d/rc.local , which is usually the last script run at boot time.

Other software

Configure and test any other software you will want to use for testing once IPsec is up. For example, you might put an HTTP daemon on Sunset and a browser on Sunrise. Make sure these work without IPsec.

If these tests fail, figure out why and fix it. Do not proceed until it works.

RTFM (please Read The Fine Manuals)

As with most things on any Unix-like system, most parts of Linux FreeS/WAN are documented in online manual pages. We provide a list of FreeS/WAN man pages, with links to HTML versions of them.

The man pages describing configuration files are:

Man pages for commands used in this document include:

You can read these either in HTML using the links above or with the man(1) command.

Setting up RSA authentication keys

To build a connection, the two gateway machines must be able to authenticate each other. For FreeS/WAN, the default is public key authentication based on the RSA algorithm. IPsec does allow several other authentication methods; using some of them with FreeS/WAN is discussed in our advanced configuration section.

This section covers setting up RSA keys. The example connections to follow (VPN, road warrior and opportunistic) all use RSA.

How RSA works

RSA keys are created as matched pairs. Each pair includes:

For FreeS/WAN, both keys for your system are in the ipsec.secrets(5) file. Maintaining security of this file is essential since it holds your private key.

Public keys for the gateways you communicate with must be made available to your gateway. There are several ways to do this:

The first two methods are described in more detail below. See the X.509 patch documentation for details of the third, if required.

Remember that public key systems are designed so that it does not matter if an enemy knows the public keys. However, the private keys must be scrupulously protected.

Generating an RSA key pair

If you installed FreeS/WAN yourself, then the installation process has already generated an RSA key pair for you and placed it in the ipsec.secrets(5) file. If not, then you need to generate an RSA key pair (private and public).

If you have the common simple situation where:

then you can just give these commands as root:

        ipsec newhostkey > /etc/ipsec.secrets
        chmod 600 /etc/ipsec.secrets

For other options, for example if you want to use different identities with different partners, see the ipsec.secrets(5) and ipsec_newhostkey(8) man pages.

Key generation may take some time, even on a fast system. Also, it needs a lot of random numbers so you may need to switch consoles and do something like typing a lot of text or running du / > /dev/null. These give random(4) some inputs to work with.

The RSA keys we generate are suitable only for authentication, not for encryption. IPsec uses them only for authentication. See our IPsec section for details.

It is also possible to use keys in other formats, not generated by FreeS/WAN. This may be necessary for interoperation with other IPsec implementations. See our links to patches which add support for keys generated by PGP or embedded in X.509 certificates.

Exchanging authentication keys

Once your gateway's key is in ipsec.secrets(5), the next step is to send your public key to everyone you need to set up connections with and collect their public keys. The other players will be:
for a VPN
each gateway administrator needs public keys for all the other gateways his or her machine talks to
for a Road Warrior
the gateway needs public keys for all Warriors that connect to it, and each Warrior needs the gateway public key
for opportunistic encryption
no explicit key exchange is needed, but you must put your public key in DNS so others can find it when they need it

You need to extract the public part in a suitable suitable format. This done with the ipsec_showhostkey(8) command. For VPN or Road Warrior applications, use one of:

        ipsec showhostkey --left
        ipsec showhostkey --right

These two produce the key formatted for insertion in an ipsec.conf(5) file.

For opportunistic encryption, just use:

        ipsec showhostkey
This gives the key in a format suitable for use in DNS records.

Public keys need not be protected as fanatically as private keys. They are intended to be made public; the system is designed to work even if an enemy knows all the public keys used. You can safely make them publicly accessible -- for example, put a gateway key on a web page, make in available in DNS or via finger(1) -- or transmit it with an insecure method such as email. However, the recipient must be able to authenticate them, as described in the next section.

Authenticating public key exchange

Authentication of public keys is critical. It does not matter if an enemy knows your public keys, but if you can be tricked into trusting a public key supplied by an enemy, you are in deep trouble .

For example, consider the fellow who wants to communicate with his mistress, keeping messages secret from his wife.

The minute he begins to trust a bogus key, the cryptography does not just stop working for him. Instead, it becomes a powerful weapon against him.

You must authenticate any public keys received before using them. For remote sites, the simplest method is to exchange them using PGP-signed email (taking appropriate steps to authenticate the signing keys). Keys posted on the web or made available for finger(1) should also be PGP-signed. Keys in DNS should be protected by DNS Security. For nearby machines, a floppy disk or trusted network is fine.

Using RSA signatures for authentication

For each system you will communicate with, you need an RSA public key and an identifier associated with it. The identifiers go in the leftid= and rightid= lines of connection descriptions in ipsec.conf(5). They are the names the systems use to identify themselves during connection negotiation.

The syntax rules allow four types of identifier:

We recommend that only the @FQDN form be used in most applications. The other three forms have problems:

If your domain is, the gateway identifiers you use should be all be of the form "" with some convenient string replacing something.

In order to facilitate distributing keys through DNS, we recommend avoiding

For example, if you have a server, then you should not use "" to identify Alice's laptop for IPsec.

One convenient scheme is to

use DNS names for your gateways
their IPsec identifiers are things like or
add a "road" label in the identifiers for your remote users ("Road Warriors")
Alice's laptop uses the identifier

The configuration file

FreeS/WAN uses a configuration file, ipsec.conf(5).

This section describes setting up the parts of that file that apply to all connections:
config setup section
describes machine configuration
conn default section
default parameters which apply to all connections

and gives an introduction to the parts of the file that specify the actual connections. The following section covers setting up three common types of connection, all using automatic keying with RSA authentication of the gateways:

conventional VPN
two security gateways, each with a known fixed IP address and with a network of client machines behind it
Road Warrior
one player has a dynamically-assigned address
opportunistic encryption
the two machines have no prior knowledge of each other, but are set up to secure connections whenever possible

Setup is quite similar for each of these, but details differ.

Other types of connections are covered in later sections.

The easiest way to create a connection is by editing one of our examples. Here we will use the one in the installation ipsec.conf file. You could also start with one from our doc/examples file if one of those is closer to what you need to do.

General comments on ipsec.conf

The ipsec.conf(5) file is divided into sections, and the following rules apply:

For more detail, see the man page.

Which is which?

The confguration file uses left and right to refer to the two gateways involved in a connection, and has other parameters which come in left/right pairs. For example, leftsubnet is the subnet behind left.

Which gateway is left and which is right is arbitrary, entirely up to you.

We suggest that you name connections by their ends. For example, name the link between Fred and Susan's machines "fred-susan" or the link between your Reno and Vancouver offices "reno-van". You can then let "left" refer to the left half of the name, "fred" or "reno" in our examples, and "right" to the other half.

To simplify administration, we recommend that you use the same names in the ipsec.conf(5) files on both ends. The name reno , for example, should refer to the machine in Reno, no matter which city the file is in.

Then when you copy the file from one machine to the other, the only change you need to make on the second machine is changing the interfaces= line to match the interface that machine uses for IPsec.

Of course the software does not actually require this. The names are just arbitrary strings to it. If your administrator in Reno wants to refer to the machines as "Phobos" and "Demios" while the Vancouver admin calls them "George" and "Gracie", things should still work.

The setup section of ipsec.conf(5)

The first section of ipsec.conf(5) contains overall setup parameters for IPsec, which apply to all connections. In our example file, it is:

# basic configuration
config setup
        # THIS SETTING MUST BE CORRECT or almost nothing will work;
        # %defaultroute is okay for most simple cases.
        # Debug-logging controls:  "none" for (almost) none, "all" for lots.
        # Use auto= parameters in conn descriptions to control startup actions.
        # Close down old connection when new one using same ID shows up.

The variables set here are:

Tells the KLIPS IPsec code in the Linux kernel which network interface to use. The interfaces specified here are the only ones this gateway machine will use to communicate with other IPsec gateways. If this is not correct, nothing works.

In many cases, the appropriate interface is just your default connection to the world (the Internet, or your corporate network). In these cases, you can use the default setting:

To check what FreeS/WAN sees as the default route, you can use the command ipsec showdefaults. You may need to compare this with the output from route -n to get a more complete picture.

In other cases, you can name one or more specific interfaces to be used by FreeS/WAN. For example:

Both tell KLIPS to use eth0 as ipsec0. The second one also supports IPsec over PPP.

Note that

If you need to discover interface names, use the command:

If you have PCMCIA or other interfaces that are not available at boot time, special measures are required. See our section on that.
Debugging setting for the KLIPS kernel code
Debugging setting for the Pluto key and connection negotiation daemon.

klipsdebug and plutodebug can each be set to "none" or to "all" in most circumstances. There are other options; see the relevant man pages.

List of connections to be automatically loaded into memory when Pluto starts.
List of connections to be automatically negotiated when Pluto starts.

plutoload and plutostart can be quoted lists of connection names, but are often set to %search as in our example. Any connection with auto=add in its connection definition is then loaded, and any connection with auto=start is started.

In most cases, you want plutostart=%search here and auto=start in your connection descriptions. That way when a connection is broken, for example if one machine crashes or is taken down for some reason, it will be reliably rebuilt. If only one end is told to start the connection, then if the other end crashes, you may lose the connection for a long time. The end that could rebuild does not know it needs to.

The exception to the above is when you have many road warriors connecting to a single gateway. Having the gateway trying to rebuild tunnels to systems which are offline can waste considerable resources. In this case, the gateway should have auto=add for all connections, and let the remote systems start negotiations.

Controls whether two connections with the same subnet on the remote end are allowed. Normally this is set to yes so that when a remote system disconnects and reconnects, Pluto will automatically take the old connection down.

Connection defaults

There is a special name %default that lets you define things that apply to all connections.

You can also set general defaults here but override them later for specific connections. If both the %default section and the actual connection description set the same variable, then the connection description takes precedence.

Our example file has:

# defaults for subsequent connection descriptions
conn %default
        # How persistent to be in (re)keying negotiations (0 means very).
        # How to authenticate gateways
        # Load all connection descriptions by default
        # Some will override this with auto=start

Variables set here are:

How persistent to be in (re)keying negotiations (0 means very).

For testing, you might wish to set this to some small number, perhaps even to 1, to avoid wasting resources on incorrectly set up connections. In production, it is often set to zero (retry forever). Keeping the connection up is what machine resources are for, so if a connection is down you might as well waste resources retrying as waste them by sitting idle. Of course some caution should be exercised with this, since it can waste network resources as well.

authenticate gateways using RSA signatures. This is the preferred method and is what we will use in this section's examples. An alternate method is to use shared secrets.
automatically add connections descriptions to Pluto's in-memory database at startup. This is required before Pluto can recognise incoming requests for that connection, so we suggest making it the default here.

To actually start negotiations for a given connection, you need auto=start. You could make that the default here or leave auto=add as the default and override it where needed with auto=start in individual connection descriptions.

Once you are finished testing, you can edit these defaults, adding anything that is standard for all gateways in your organisation.

Editing a connection description

Edit our example connection to match what you want to do. Rename it appropriately for the connection you would like to build: "fred-susan", "reno-van" or whatever. The name is the second string in the line that begins with "conn", for example in:

        conn snt

The connection name is "snt" (subn et tunnel) and to define another connection you make a copy with a new name such as:

        conn reno-van

A sample connection description is:

# sample tunnel
# The network here looks like:
#   leftsubnet====left----leftnexthop......rightnexthop----right====rightsubnet
# If left and right are on the same Ethernet, omit leftnexthop and rightnexthop.
conn sample
        # left security gateway (public-network address)
        # next hop to reach right
        # subnet behind left (omit if there is no subnet)
        # right s.g., subnet behind it, and next hop to reach left

We omit here the variables we have shown as set in the default connection above. All of them could also be set here. If they are set in both places, settings here take precedence. Defaults are used only if the specific connection description has no value set.

The network described above looks like this:

         subnet              =leftsubnet
                |                          [whatever]
         inside interface
            left gateway machine
         interface                =left
         interface             =leftnexthop
         interface we don't know
         interface we don't know
         interface             =rightnexthop
         interface              =right
            right gateway machine
         inside interface
                 |                         [whatever]
         subnet             =rightsubnet
You need to edit the connection description, inserting appropriate IP addresses and subnet descriptions so that it describes your network.

The [whatever]s above indicate places where all that matters is routing.

It does not matter what is between the inside interface and the protected subnet, as long as the gateway knows how to get packets to the subnet.

You do not need to tell FreeS/WAN anything about the inside interfaces. In fact, there is no parameter you could use to do that. What you do have to do is make sure the gateway can route to its client subnet.

In most cases, you should use numeric IP addresses, not names, here. The file syntax allows names to be used, but this creates an additional risk. If someone can subvert the DNS service, then they can redirect packets whose addresses are looked up via that service.

Many of the variables in this file come in pairs such as "leftsubnet: and "rightsubnet", one for each end of the connection. The variables on the left side are:

The gateway's external interface, the one it uses to talk to the other gateway. This can be left=%defaultroute.
Where left should send packets whose destination is right, typically the first router in the appropriate direction.

This need not always be set.

However, in all other cases, you must provide nexthop information. KLIPS (Kernel IP Security) bypasses the normal routing machinery, so you must give KLIPS the information even though routing already knows it.

(Yes, we know that design is not ideal, and we plan to change it. See extensive discussions on the mailing list, mostly with "routing" or "KLIPS 2" in the subject lines.)

Addresses for the machines which left is protecting. For some applications, you may want to create two connections, one to protect traffic from the subnet behind left and another to protect traffic from the left gateway itself. This takes two connection descriptions. See below.
If the conn setup section has plutoload=%search , then all connections marked auto=add are loaded when Pluto starts.

If the conn setup section has plutostart=%search , then all connections marked auto=start are started when Pluto starts.

Initially, we suggest using auto=add on all connections. This lets you start them manually during testing. Once they are tested, you can change many of them to auto=start.

For each left* parameter, there is a corresponding right* parameter.

Note that a connection to a subnet behind left does not include left itself. The tunnel described above protects packets going from one subnet to the other. It does not apply to packets which either begin or end their journey on one of the gateways. If you need to protect those packets, you must build separate tunnel descriptions for them.

It is a common error to attempt testing a subnet-to-subnet connection by pinging from one of the gateways to the far end or vice versa. This does not work, even if the connection is functioning perfectly, because traffic to or from the gateway itself is not sent on that connection. If you want to protect traffic originating or terminating on the gateway, then you need a separate tunnel for that in addition to the subnet's tunnel. See the section on multiple tunnels.

Example setups

In this section we show examples of three common setups:

We use a, b, c ... to indicate components of IP addresses. Each letter is some number in the range 0 to 255, inclusive.

For additional examples, see our examples file.


For a site-to-site VPN, a simple network diagram looks like this:
              local net       untrusted net       local net

which we describe in our config files as:

     leftsubnet === West------------------East=== rightsubnet
                        ^left       right^ 

In most cases, we also have to provide next hop information. A more detailed diagram might look like this:
(using a, b, c, ... to refer to arbitrary numbers 0 to 255)

         subnet a.b.c.0/24                 =leftsubnet
                |          (head office has routable IP addresses)
         interface a.b.c.d
            left gateway machine
         interface e.f.g.h                 =left
                 |         (external address outside a.b.c.0 subnet)
         interface e.f.g.i                 =leftnexthop
         interface we don't know
         interface we don't know
         interface j.k.l.m                =rightnexthop
         interface j.k.l.n                =right
            right gateway machine
         interface 192.168.0.something
                 |        (branch office uses private IP addresses)
         subnet             =rightsubnet

The ipsec.conf(5) file for the above network would look like this (with RSA keys shortened for easy display):

# basic configuration
config setup

# defaults that apply to all connection descriptions
conn %default
        # How persistent to be in (re)keying negotiations (0 means very).
        # How to authenticate gatways

# VPN connection for head office and branch office
conn head-branch
        # identity we use in authentication exchanges
        # left security gateway (public-network address)
        # next hop to reach right
        # subnet behind left (omit if there is no subnet)
        # right s.g., subnet behind it, and next hop to reach left
        # right is masquerading
        # So you have three choices, none of them ideal
        # uncomment this to use our default script
        # which works only with ipfwadm(8) on 2.0 kernels
        # or ipchains(8) on 2.2 in ipfwadm(8) emulation mode
        # rightfirewall=yes
        # uncomment this and enter a name to write your own script
        # to use all features of ipchains(8) on 2.2
        # or to use iptables(8) on 2.4
        # rightupdown=whatever_you_want_to_name_the_script
        # if you uncomment neither and remove the rightsubnet= line
        # then the tunnel terminates on the outside of your gateway
        # and the masqueraded subnet is not visible to the remote
        # subnet; they all think they're talking to the gateway
        # try to start the connection

For more on the use of the firewalling parameters -- leftfirewall, rightfirewall, leftupdown and rightupdown -- see our IPsec and firewalls section.

The versions of this file at the two ends should be identical, except that each must have an interfaces= line appropriate for the local machine.

Routable and non-routable addresses

RFC 1918 reserves three groups of addresses for use on private networks:

Addresses in these ranges will never be assigned to anything on the Internet. Many routers automatically drop any packet with one of these addresses as either source or destination.

You can use FreeS/WAN to:

If you do this, the non-routable addresses still do not appear on the Internet. They are encapsulated inside IPsec packets which have the gateways' external addresses (from the left and right parameters of the connection description) in their headers.

Of course FreeS/WAN can also tunnel packets between subnets with normal routable IP address.

Road Warrior

For our purposes, a "road warrior" is any machine that does not have a fixed IP address. This includes:

The configuration for road warrior support looks slightly different from a VPN configuration. We cannot use the road warrior's IP address in the configuration file since we don't know it, and we don't want to have our server retrying connections to road warriors that are no longer online.

Some machines, such as home firewalls, may have a dynamic IP address and have a protected subnet behind them. For this example, however, we assume the Road Warrior is a standalone machine:

                                           telecommuter's PC or
                                           traveller's laptop
         corporate LAN     untrusted net

In more detail, the network looks like this:
(using a, b, c, ... to refer to arbitrary numbers 0 to 255)

         subnet a.b.c.0/24               =leftsubnet
                |          (head office has routable IP addresses)
         interface a.b.c.d
            left gateway machine
         interface e.f.g.h               =left
                 |         (external address outside a.b.c.0 subnet)
         interface e.f.g.i               =leftnexthop
     interface with dynamic IP address
          road warrior machine

Here the ipsec.conf(5) files on the two ends are slightly different. The one at the office might have exactly the same config setup and conn %default sections as in the VPN example.

# basic configuration
config setup

# defaults that apply to all connection descriptions
conn %default
        # How persistent to be in (re)keying negotiations (0 means very).
        # How to authenticate gatways

Then add a description for the road warrior connection:

# Connection for road warrior Fred 
conn head-fred
        # identity we use in authentication exchanges
        # left security gateway (public-network address)
        # next hop to reach right
        # subnet behind left (omit if there is no subnet)
        # accept any address for right
        # any address, provided authentication works
        # no subnet for a typical road warrior
        # it is possible, but usually not needed
        # so the rightsubnet= parameter is omitted
        # let the road warrior start the connection
        # override the default retry for road warriors
        # we don't want to retry if IP connectivity is gone

On the gateway end we use the values shown above:

The file on the road warrior end is nearly identical, except that it has:

Because we are using right=%defaultroute, we omit the rightnexthop parameter.

Additional road warriors can be added as required. Each should have his or her own connection description with unique settings for rightid and rightrsasigkey.

If necessary, a single road warrior can have multiple connections, all with the same rightid and rightrsasigkey, but with different values for leftsubnet to give access to different parts of the office network.

Jean-Francois Nadeau's Practical Configurations document also has an example of using RSA authentication for road warriors.

Opportunistic encryption

We use the term opportunistic encryption for encryption which does not rely on any pre-arranged connection, hence does not require that the administrators of the two gateways involved communicate with each other (for example, to exchange keys) before their systems can create a secure connection.

The idea is that each gateway check the destinations of outgoing packets, see if an encrypted connection is possible and, if so, take the opportuntity to encrypt. The opportunity will exist whenever the admins on both ends have set their systems up for opportunistic encryption.

This makes encryption the default behaviour, and could greatly increase the overall security of the Internet if it were widely enough adopted. See our documents:

history and politics
for the reasons we want to do this
IPsec protocols
for discussion of the general principle of encrypting as much as possible

The gateways must be able to authenticate each other for IPsec to be secure. For opportunistic encryption, we rely on the domain name system, DNS, to provide the RSA keys needed for this authentication. Note, that currently this is not entirely secure because the DNS mechanism it relies on is not fully secure. Eventually, as secure DNS becomes widely deployed, this will change.


The team have been working on this for some time, and testing internally. The code is now ready for wider testing. We encourage everyone to try it.

The main documentation items so far are:

I am playing catch up. HTML documentation so far is neither complete nor particularly clear. What I have so far is below.

We do not yet recommend this code for production use . You should still protect your critical data with explicitly configured IPsec tunnels, rather than relying on opportunistic for everything at this stage.

ipsec.conf entries for opportunism

The relevant lines in the config file might look like this:

conn subnet-to-anyone              # for our client subnet
        leftsubnet=   # any single client in our subnet
        left=%defaultroute         # our SG (defaults leftnexthop too)
        right=%opportunistic       # anyone we can authenticate via DNS
        rekey=no                   # let unused connections die

The public key, in our format, must be in a KEY record of the appropriate DNS entry for this to work. We provide some background information on DNS in another file.

Each opportunistic connection supports a single source/destination pair of IP addresses. There is no way to build an opportunistic connection for a larger subnet. Specifying a subnet in the connection description, as in the example above, just means that any host in that subnet may have opportunistic connections.

Putting IPsec information in DNS

To set up for opportunistic encryption, you add some KEY and TXT records to your DNS data. Specifically: ipsec_showhostkey(8) provides the key in DNS record format. You will need to put it in the appropriate place in the DNS records.

To be more precise, quoting the Opportunism Design document:

For reference, the minimum set of DNS records needed to make
this all work is either:

1.  TXT in Destination reverse  map,  identifying  Responder
    and providing public key.
2.  KEY in Initiator reverse map, providing public key.
3.  TXT  in  Source  reverse  map, verifying relationship to


1.  TXT in Destination reverse map, identifying Responder.
2.  KEY in Responder reverse map, providing public key.
3.  KEY in Initiator reverse map, providing public key.
4.  TXT in Source reverse  map,  verifying  relationship  to

Slight  complications  ensue  for dynamic addresses, lack of
control over reverse maps, etc.
DNS records for client systems
You must have control of the reverse maps for your client systems, or opportunistic IPsec cannot be made to work.

The client systems will be either Source or Destination, so they must have:

1.  TXT in Destination reverse  map,  identifying  Responder
    and providing public key.
2.  ...
3.  TXT  in  Source  reverse  map, verifying relationship to


1.  TXT in Destination reverse map, identifying Responder.
2.  ...
3.  ...
4.  TXT in Source reverse  map,  verifying  relationship  to
If you control the gateway's reverse map, example client records would look like this: IN PTR IN TXT "X-IPsec-Server(10)= AQNJjkKlIk9...nYyUkKK8"
which can also be written as just: IN PTR
                          IN TXT "X-IPsec-Server(10)= AQNJjkKlIk9...nYyUkKK8"
This provides the IP address of the security gateway and the public key which the gateway will use to authenticate itself. This is the preferred method.
DNS records for gateway systems
The gateways will be either Initiator or Responder so they need:
1.  ...
2.  KEY in Initiator reverse map, providing public key.
3.  ...


1.  ...
2.  KEY in Responder reverse map, providing public key.
3.  KEY in Initiator reverse map, providing public key.
4.  ...

If you control the gateway's reverse map, you just add a KEY record there. That is all the gateway reverse map needs, whether it is working as Initiator or Responder.

Here is an example, with many characters of the key itself left out: IN KEY 0x4200 4 1 AQNJjkKlIk9...nYyUkKK8
This allows lookups on the IP address of the gateway to retrieve the key.
If you don't control the gateway's reverse map
The approach must be different if you do not have control over the reverse map for your gateway. Perhaps your ISP controls that, and provides no way for you to put data into their maps. Without that, you cannot set your gateway up to respond to incoming opportunistic requests (short of changing ISPs, which you might consider).

However, suppose a friend over at will let you put things in their maps. That will allow you to set your gateway up to handle opportunistic connections for which it is the initiator.

You still need to be able to put data in the reverse map for your clients. However, that data is slightly different: IN PTR
                          IN TXT "X-IPsec-Server(10)"
Over at, your friend puts these lines in the DNS data files: IN A IN KEY 0x4200 4 1 AQNJjkKlIk9...nYyUkKK8
Your gateway must identify itself in IKE as, not as You set that up via leftid= or rightid= entries in ipsec.conf(5).

With this arrangement, the remote gateway receives an ID payload early in IKE with your (bogus) gateway name "". Then it looks up that name to get the IP address and key for the gateway.

Going beyond the examples

The examples above each described a single connection. This section discusses some issues in going beyond that, dealing with more complicated networks.

If your network is simple enough that one of the examples had all you need, then you can skip ahead to firewall setup .

Tunnels are cheap

Nearly all of the overhead in IPsec processing is in the encryption and authentication of packets. Our performance document discusses these overheads.

Beside those overheads, the cost of managing additional tunnels is trivial. Whether your gateway supports one tunnel or ten just does not matter. A hundred might be a problem; there is a section on this in the performance document.

So, in nearly all cases, if using multiple tunnels gives you a reasonable way to describe what you need to do, you should describe it that way in your configuration files.

For example, one user recently asked on a mailing list about this network configuration:


   netA and B are secured netC not.
   netA and gwA can not access netC
The user had constructed only one tunnel, netA to netB, and wanted to know how to use ip-route to get netC packets into it. This is entirely unnecessary. One of the replies was:
  The simplest way and indeed the right way to
  solve this problem is to set up two connections:


This would still be correct even if we added nets D, E, F, ... to the above diagram and needed twenty tunnels.

Of course another possibility would be to just use one tunnel, with a subnet mask that includes both netB and netC (or B, C, D, ...). See next section.

In general, you can construct as many tunnels as you need. Networks like netC in this example that do not connect directly to the gateway are fine, as long as the gateway can route to them.

The number of tunnels can become an issue if it reaches 50 or so. This is discussed in the performance document. Look there for information on supporting hundreds of Road Warriors from one gateway.

If you find yourself with too many tunnels for some reason like having eight subnets at one location and nine at another so you end up with 9*8=72tunnels, read the next section here.

Subnet sizes

The subnets used in leftsubnet and rightsubnet can be of any size that fits your needs, and they need not correspond to physical networks.

You adjust the size by changing the subnet mask , the number after the slash in the subnet description. For example

As an example of using these in connection descriptions, suppose your company's head office has four physical networks using the address ranges:
You can use exactly those subnets in your connection descriptions, or use larger subnets to grant broad access if required:
remote hosts can access only development
remote hosts can access development or production
remote hosts can access marketing or administration
remote hosts can access any of the four departments
or use smaller subnets to restrict access:
remote hosts can access any machine in administration
remote hosts can access only certain machines in administration.
remote hosts can access only one particular machine in administration
To be exact, means all addresses whose top 28 bits match There are 16 of these because there are 16 possibilities for the remainingg 4 bits. Their addresses are to

Each connection description can use a different subnet if required.

It is possible to use all the examples above on the same FreeS/WAN gateway, each in a different connection description, perhaps for different classes of user or for different remote offices.

It is also possible to have multiple tunnels using different leftsubnet descriptions with the same right. For example, when the marketing manager is on the road he or she might have access to:

all machines in marketing
some machines in production
one particular machine in administration

This takes three tunnels, but tunnels are cheap. If the laptop is set up to build all three tunnels automatically, then he or she can access all these machines concurrently, perhaps from different windows.

Other network layouts

In our VPN example, we used a simple network picture:

           local net       untrusted net       local net

And for the Road Warrior, another:

                                           telecommuter's PC or
                                           traveller's laptop
         corporate LAN     untrusted net

Other configurations are also possible.

The Internet as a big subnet

A telecommuter might have:

     Sunset==========West------------------East ================= firewall --- the Internet
         home network      untrusted net        corporate network
This can be described as a special case of the general subnet-to-subnet connection. The subnet on the right is, the whole Internet.

West (the home gateway) can have its firewall rules set up so that only IPsec packets to East are allowed out. It will then behave as if its only connection to the world was a wire to East.

When machines on the home network need to reach the Internet, they do so via the tunnel, East and the corporate firewall. From the viewpoint of the Internet (perhaps of some EvilDoer trying to break in!), those home office machines are behind the firewall and protected by it.


Another possible configuration comes up when you do not trust the local network, either because you have very high security standards or because your are using easily-intercepted wireless signals.

Some wireless networks have built-in encryption called WEP, but its security is dubious. It is a fairly common practice to use IPsec instead.

In this case, part of your network may look like this:

          West-----------------------------East == the rest of your network
     workstation   untrusted wireless net
Of course, there would likely be several wireless workstations, each with its own IPsec tunnel to the East gateway.

In this example, the rightsubnet= parameter might be set in any of several ways:

allowing workstations to access the entire Internet (see above)
allowing access to your entire local network
restricting the workstation to connecting to a particular server

Of course you can mix and match these as required. For example, a university might allow faculty full Internet access while letting student laptops connect only to a group of lab machines.

Simplifying ipsec.conf files

We provide several features in the syntax of the ipsec.conf(5) file that are intended to simplify the work of managing complex multi-connection setups:

These can be combined in whatever way suits your application. One example is this ipsec.conf file for a gateway supporting multiple road warriors, all using RSA authentication:

conn %default
        # set some defaults appropriate for the gateway
        # these should be changed or overridden on the road warriors 
        keyingtries=1                   # road warrior can retry, we shouldn't
        auto=add                        # default is to load, but not start
        # some parameters are common to all remote systems
        authby=rsasig                   # all connections use RSA authentication
        right=%any                      # accept from any address

# pick up all remote system descriptions
# uses shell wildcards
include /etc/ipsec/remote.*.conn

# left side of all connections is the same
# define it after the descriptions which use it
conn leftstuff
        # left security gateway (public-network address)
        # next hop to reach right
        # subnet behind left (omit if there is no subnet)

On the left gateway, we can omit leftrsasig. That gateway uses the private key stored in ipsec.secrets(5) and has no need for its own public key. Similarly, the road warriors need not have their own public keys in ipsec.conf(5), only the gateway's public key.

The remote connection descriptions in /etc/ipsec/remote.*.conn need then have only a few lines each:

conn myname
        # pick up common info for all connections
        # identify the remote machine

Be careful with the order of sections in ipsec.conf(5) and any included files. The parser requires that a definition comes after the also= line which uses it. In our example, the include inserts the files with the also=leftstuff lines before the definition of conn leftstuff so things are parsed in the correct order.

The above method, using conn leftstuff and also= , is only one alternative. In simple cases, you can just put all the information about the left gateway in the conn default section instead and use no also= lines.

Is there a firewall in play?

If firewall packet filtering is being done on either of the FreeS/WAN gateway machines, or on any machine on the path between them, then you will probably need to adjust the filters before FreeS/WAN can work. The filters must allow:

For more detail, see our IPsec and firewalls document.

Testing the installation

This section covers testing connections once you have FreeS/WAN installed and your ipsec.conf(5) file set up. A separate testing document has more information if required.

We assume all your connection descriptions use auto=add so that ipsec_pluto(8) loads the descriptions into its internal database at startup but does not attempt to start the connections until you tell it to.

Matching numbers

It is important that the numbers in your connection descriptions match the network configuration. FreeS/WAN is almost certain to fail if they do not.

Suppose you are at the Reno office and your ipsec.conf file now has, among others, these lines:

config setup

conn reno-van

When you tell FreeS/WAN to start the reno-van connection, it doesn't automagically know that it is in Reno, or that it is left in the configuration. It discovers that by comparing the IP address for ipsec0 (and, if it is set, for ipsec1) to the addresses for left and right. ipsec0 inherits its address from the underlying device, eth0 in our example.

So in our example, if eth0 has IP address then ipsec0 inherits that address, the correct match is found, and this FreeS/WAN discovers that it is left. (If no match is found, Pluto reports "unable to orient connection".) It then sets itself up with any other left* parameters in use -- some of leftnexthop, leftsubnet, and leftid.

Once it has these parameters, FreeS/WAN sets things so that

All should be well.

Of course, there must also be interfaces and routes set up so that this machine can exchange IP packets both with the right gateway and with clients on leftsubnet. This is done with standard Linux utilities such as ifconfig(8) and route(8). Also, things must be correct on right in Vancouver. It takes two to tunnel.

A data mismatch anywhere in this configuration will cause FreeS/WAN to fail and to log various error messages. Depending on just how confused FreeS/WAN is and about what, the error messages may be somewhat confusing. See our troubleshooting section and the FAQ section om error messages to get help interpreting them if required.

We recommend double-checking for consistency here before starting actual tests..

Sanity checking

Reboot both gateways to get FreeS/WAN started. No connections are actually made yet, but the stage is set.

Examine /var/log/messages for any signs of trouble.

On both gateways, the following entries should now exist in the /proc/net/ directory:

and the IPsec interfaces should be attached on top of the specified physical interfaces. Confirm that with:

        cat /proc/net/ipsec_tncfg

You should see at least device ipsec0, and each ipsec device should point to a physical device, eg. 'ipsec0 -> eth0 mtu=16260 -> 1500'.

Routing connections through these ipsec pseudo-devices causes the data to be encrypted before being delivered to the underlying network interface. This can be done manually with our eroute(8) utility, but in most cases you do not need to use that utility directly. Just bring the connections up and down and the scripts call it as required.

Don't be surprised when you cannot find /dev/ipsec0 or /dev/ipsec1. They do not exist. Other network pseudo-devices such as eth0 and eth1 do not have entries in /dev either. In general, network devices do not need such entries.

Starting a connection

On one gateway, start IPsec with:

        ipsec auto --up name

replacing name with the connection name you used in ipsec.conf(5).

For this to work, the connection description must already be loaded into Pluto's database, either via auto=add in the connection description or with an explicit ipsec auto --add name command.

Note that to shut down a connection, you must do:

        ipsec auto --down name

on both gateway machines, even though you only start it from one.

If the ipsec auto --up command doesn't generate any errors, do

        ipsec look

and see if the output looks something like this: Wed Nov 25 22:51:45 EST 1998
------------------------- -> => tun0x200@ esp0x202@
tun0x200@ IPv4_Encapsulation: dir=out ->
esp0x203@ 3DES-MD5-96_Encryption: dir=in  iv=0xc2cbca5ba42ffbb6  seq=0  bit=0x00000000  win=0  flags=0x0<>
esp0x202@ 3DES-MD5-96_Encryption: dir=out  iv=0xc2cbca5ba42ffbb6  seq=0  bit=0x00000000  win=0  flags=0x0<>
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface   U      1500 0          0 eth1   UG     1404 0          0 ipsec0

If it does, you're probably in business.

This example shows:

        a tunnel              tun0x200 going to
        outgoing connection   esp0x202
        incoming connection   esp0x203

Both connections use ESP with 3DES encryption and MD5 authentication.

The routing is:    via eth1 and the Internet    via ipsec0 which encrypts and then sends to

This routes all traffic to the protected network through an IPsec tunnel to the gateway

Ping tests

If that works, test whether Sunrise can ping Sunset and vice versa. Our example setup again is:

              local net       untrusted net       local net

There is no point in testing to or from the gateways themselves; the goal is to secure traffic between the subnets, not between the security gateways themselves.

In general, pings or other tests using the public interfaces of East and/or West are entirely useless. The IPsec tunnel is for packets between the two protected subnets and the outside interfaces are not on those subnets. Depending on your routing configuration, test packets sent via those interfaces will be:

In either case, they tell you nothing about the tunnel .

Sometimes it will be inconvenient to use the client machines (Sunrise and Sunset in our example) for testing. In these cases, use a command such as:

     traceroute -i eth0 -f 20

where each of the interfaces specified (eth0 and in the example) are on one of the protected subnets, eth0 being the local gateway's interface on that side and the remote gateway's subnet interface. This forces the packets through the IPsec tunnel you want to test.

For information on setting things up so that gateways can do IPsec to each other or to remote subnets, see below .

If you have other software set up, test with it as well. Telnet from Sunrise to Sunset, browse a web server on the remote net and so on.

Testing with tcpdump

To verify that all is working, run tcpdump(8) on a machine which can listen to the traffic between the gateways.

This is most easily done from a third machine, rather than from one of the gateways. On the gateways you may see packets at intermediate stages of processing and the result may be confusing.

If the results make no sense at all, or you see "bad physical medium" error messages, you probably have an outdated version of tcpdump(8) that does not handle IPsec at all. See this discussion.

If packets look like total garbage, nothing recognizable, all is well.

See our document on testing for more detail if required.

Shutting down connections

To shut down a connection, you must do:

        ipsec auto --down name

on both gateway machines, even though you only start it from one.

Again, you can verify with the same commands. Repeat the ping test. Repeat the tcpdump test.

If everything succeeds, congratulations. You now have a working Linux FreeS/WAN installation.

What next?

At this point you should have a working FreeS/WAN setup. If not, you could go back and doublecheck various things above or try:

If all is well so far, you could go to:

Of course you might just go off for a beverage or meal at this point as well.

FreeS/WAN manual pages

The various components of Linux FreeS/WAN are of course documented in standard Unix manual pages, accessible via the man(1) command.

Links here take you to an HTML version of the man pages.


IPsec configuration and connections
preshared secrets for IKE/IPsec authentication

These files are also discussed in the configuration section.


Many users will never give most of the FreeS/WAN commands directly. Configure the files listed above correctly and everything should be automatic.

The exceptions are commands for mainpulating the RSA keys used in Pluto authentication:

generate keys
generate keys in a convenient format
extract RSA from ipsec.secrets(5) (or optionally, another file) and format them for insertion in ipsec.conf(5) or in DNS records

Note that:

The following commands are fairly likely to be used, if only for testing and status checks:

invoke IPsec utilities
control IPsec subsystem
control automatically-keyed IPsec connections
take manually-keyed IPsec connections up and down
generate random bits in ASCII form
show minimal debugging information
spew out collected IPsec debugging information

The lower-level utilities listed below are normally invoked via scripts listed above, but they can also be used directly when required.

manipulate IPsec extended routing tables
set Klips (kernel IPsec support) debug features and level
IPsec IKE keying daemon
manage IPsec Security Associations
group/ungroup IPsec Security Associations
associate IPsec virtual interface with real interface
control interface for IPsec keying daemon

Library routines

convert Internet addresses to and from ASCII
convert subnet/mask ASCII form to and from addresses
convert ASCII to Internet address, subnet, or range
convert Internet address range to ASCII
convert binary data from and to ASCII formats
convert IPsec Security Association IDs to and from ASCII
convert unsigned-long numbers to and from ASCII
is this Internet subnet mask a valid one?
convert Internet subnet mask to bit count
convert bit count to Internet subnet mask
read additional ``command-line'' options from file
given Internet address and subnet mask, return subnet number
given Internet address and subnet mask, return host part
given Internet address and subnet mask, return broadcast address

FreeS/WAN and firewalls

FreeS/WAN, or other IPsec implementations, frequently run on gateway machines, the same machines running firewall or packet filtering code. This document discusses the relation between the two.

The firewall code in 2.4 and later kernels is called Netfilter. The user-space utility to manage a firwewall is iptables(8). See the netfilter/iptables web site for details.

Filtering rules for IPsec packets

The basic constraint is that an IPsec gateway must have packet filters that allow IPsec packets, at least when talking to other IPsec gateways:

Your gateway and the other IPsec gateways it communicates with must be able to exchange these packets for IPsec to work. Firewall rules must allow UDP 500 and at least one of AH or ESP on the interface that communicates with the other gateway.

For nearly all FreeS/WAN applications, you must allow UDP port 500 and the ESP protocol.

There are two ways to set this up:

easier but less flexible
Just set up your firewall scripts at boot time to allow IPsec packets to and from your gateway. Let FreeS/WAN reject any bogus packets.
more work, giving you more precise control
Have the ipsec_pluto(8) daemon call scripts to adjust firewall rules dynamically as required. This is done by naming the scripts in the ipsec.conf(5) variables prepluto=, postpluto= , leftupdown= and rightupdown=.

Both methods are described in more detail below.

Firewall configuration at boot

It is possible to set up both firewalling and IPsec with appropriate scripts at boot and then not use leftupdown= and rightupdown=, or use them only for simple up and down operations.

Basically, the technique is

Since Pluto authenticates its partners during the negotiation, and KLIPS drops packets for which no tunnel has been negotiated, this may be all you need.

A simple set of rules

In simple cases, you need only a few rules, as in this example:

# allow IPsec
# IKE negotiations
iptables -A INPUT  -p udp --sport 500 --dport 500 -j ACCEPT
iptables -A OUTPUT -p udp --sport 500 --dport 500 -j ACCEPT
# ESP encrypton and authentication
iptables -A INPUT  -p 50 -j ACCEPT
iptables -A OUTPUT -p 50 -j ACCEPT
# uncomment for AH authentication header
# iptables -A INPUT  -p 51 -j ACCEPT
# iptables -A OUTPUT -p 51 -j ACCEPT

Other rules

You can add aditional rules, or modify existing ones, to work with IPsec and with your network and policies. We give a some examples in this section.

However, while it is certainly possible to create an elaborate set of rules yourself (please let us know via the mailing list if you do), it may be both easier and more secure to use a set which has already been published and tested.

The published rule sets we know of are described in the next section.

Adding additional rules

If necessary, you can add additional rules to:
reject IPsec packets that are not to or from known gateways
This possibility is discussed in more detail later
allow systems behind your gateway to build IPsec tunnels that pass through the gateway
This possibility is discussed in more detail later
filter incoming packets emerging from KLIPS.
Firewall rules can recognise packets emerging from IPsec. They are marked as arriving on an interface such as ipsec0, rather than eth0, ppp0 or whatever.
It is therefore reasonably straightforward to filter these packets in whatever way suits your situation.

Modifying existing rules

In some cases rules that work fine before you add IPsec may require modification to work with IPsec.

This is especially likely for rules that deal with interfaces on the Internet side of your system. IPsec adds a new interface; often the rules must change to take account of that.

For example, consider the rules given in this section of the Netfilter documentation:

Most people just have a single PPP connection to the Internet, and don't
want anyone coming back into their network, or the firewall:

    ## Insert connection-tracking modules (not needed if built into kernel).
    # insmod ip_conntrack
    # insmod ip_conntrack_ftp

    ## Create chain which blocks new connections, except if coming from inside.
    # iptables -N block
    # iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
    # iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
    # iptables -A block -j DROP

    ## Jump to that chain from INPUT and FORWARD chains.
    # iptables -A INPUT -j block
    # iptables -A FORWARD -j block

On an IPsec gateway, those rules may need to be modified. The above allows new connections from anywhere except ppp0. That means new connections from ipsec0 are allowed.

Do you want to allow anyone who can establish an IPsec connection to your gateway to initiate TCP connections to any service on your network? Almost certainly not if you are using opportunistic encryption. Quite possibly not even if you have only explicitly configured connections.

To disallow incoming connections from ipsec0, change the middle section above to:

    ## Create chain which blocks new connections, except if coming from inside.
    # iptables -N block
    # iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
    # iptables -A block -m state --state NEW -i ppp+ -j DROP
    # iptables -A block -m state --state NEW -i ipsec+ -j DROP
    # iptables -A block -m state --state NEW -i -j ACCEPT
    # iptables -A block -j DROP
The original rules accepted NEW connections from anywhere except ppp0. This version drops NEW connections from any PPP interface (ppp+) and from any ipsec interface (ipsec+), then accepts the survivors.

Of course, these are only examples. You will need to adapt them to your own situation.

Published rule sets

Several sets of firewall rules that work with FreeS/WAN are available.

Scripts based on Ranch's work

One user, Rob Hutton, posted his boot time scripts to the mailing list, and we included them in previous versions of this documentation. They are still available from our web site. However, they were for an earlier FreeS/WAN version so we no longer recommend them. Also, they had some bugs. See this message.

Those scripts were based on David Ranch's scripts for his "Trinity OS" for setting up a secure Linux. Check his home page for the latest version and for information on his book on securing Linux. If you are going to base your firewalling on Ranch's scripts, we recommend using his latest version, and sending him any IPsec modifications you make for incorporation into later versions.

The Seattle firewall

We have had several mailing lists reports of good results using FreeS/WAN with Seawall (the Seattle Firewall). See that project's home page on Sourceforge.

The RCF scripts

Another set of firewall scripts with IPsec support are the RCF or rc.firewall scripts. See their home page.

Asgard scripts

Asgard's Realm has set of firewall scripts with FreeS/WAN support, for 2.4 kernels and iptables.

User scripts from the mailing list

One user gave considerable detail on his scripts, including supporting IPX through the tunnel. His message was too long to conveniently be quoted here, so I've put it in a separate file.

Calling firewall scripts, named in ipsec.conf(5)

The ipsec.conf(5) configuration file has three pairs of parameters used to specify an interface between FreeS/WAN and firewalling code.

Note that using these is not required if you have a static firewall setup. In that case, you just set your firewall up at boot time (in a way that permits the IPsec connections you want) and do not change it thereafter. Omit all the FreeS/WAN firewall parameters and FreeS/WAN will not attempt to adjust firewall rules at all. See above for some information on appropriate scripts.

However, if you want your firewall rules to change when IPsec connections change, then you need to use these parameters.

Scripts called at IPsec start and stop

One pair of parmeters are set in the config setup section of the ipsec.conf(5) file and affect all connections:

specify scripts to be called before our pluto(8) IKE daemon is started and after it is stopped.
These parameters allow you to change firewall parameters whenever IPsec is started or stopped.

They can also be used in other ways. For example, you might have prepluto add a module to your kernel for the secure network interface or make a dialup connection, and then have postpluto remove the module or take the connection down.

Scripts called at connection up and down

The other parameters are set in connection descriptions. They can be set in individual connection descriptions, and could even call different scripts for each connection for maximum flexibility. In most applications, however, it makes sense to use only one script and to call it from conn %default section so that it applies to all connections.

You can:

set leftfirewall=yes or rightfirewall=yes to use our supplied default script
assign a name in a leftupdown= or rightupdown= line to use your own script

Note that only one of these should be used. You cannot sensibly use both. Since our default script is obsolete (designed for firewalls using ipfwadm(8) on 2.0 kernels), most users who need this service will need to write a custom script.

The default script

We supply a default script named _updown.
indicates that the gateway is doing firewalling and that pluto(8) should poke holes in the firewall as required.
Set these to yes and Pluto will call our default script _updown with appropriate arguments whenever it: The supplied default _updown script is appropriate for simple cases using the ipfwadm(8) firewalling package.

User-written scripts

You can also write your own script and have Pluto call it. Just put the script's name in one of these ipsec.conf(5) lines:
specifies a script to call instead of our default script _updown.
Your script should take the same arguments and use the same environment variables as _updown. See the "updown command" section of the ipsec_pluto(8) man page for details.

Note that you should not modify our _updown script in place . If you did that, then upgraded FreeS/WAN, the upgrade would install a new default script, overwriting your changes.

Scripts for ipchains or iptables

Our _updown is for firewalls using ipfwadm(8) , the firewall code for the 2.0 series of Linux kernels. If you are using the more recent packages ipchains(8) (for 2.2 kernels) or iptables(8) (2.4 kernels), then you must do one of:

You can write a script to do whatever you need with firewalling. Specify its name in a [left|right]updown= parameter in ipsec.conf(5) and Pluto will automatically call it for you.

The arguments Pluto passes such a script are the same ones it passes to our default _updown script, so the best way to build yours is to copy ours and modify the copy.

Note, however, that you should not modify our _updown script in place. If you did that, then upgraded FreeS/WAN, the upgrade would install a new default script, overwriting your changes.

A complication: IPsec vs. NAT

Network Address Translation, also known as IP masquerading, is a method of allocating IP addresses dynamically, typically in circumstances where the total number of machines which need to access the Internet exceeds the supply of IP addresses.

Any attempt to perform NAT operations on IPsec packets between the IPsec gateways creates a basic conflict:

For AH, which authenticates parts of the packet header including source and destination IP addresses, this is fatal. If NAT changes those fields, AH authentication fails.

For IKE and ESP it is not necessarily fatal, but is certainly an unwelcome complication.

NAT on or behind the IPsec gateway works

This problem can be avoided by having the masquerading take place on or behind the IPsec gateway.

This can be done physically with two machines, one physically behind the other. A picture, using SG to indicate IPsec S ecurity Gateways, is:

      clients --- NAT ----- SG ---------- SG
                  two machines

In this configuration, the actual client addresses need not be given in the leftsubnet= parameter of the FreeS/WAN connection description. The security gateway just delivers packets to the NAT box; it needs only that machine's address. What that machine does with them does not affect FreeS/WAN.

A more common setup has one machine performing both functions:

      clients ----- NAT/SG ---------------SG
                  one machine
Here you have a choice of techniques depending on whether you want to make your client subnet visible to clients on the other end:

NAT between gateways is problematic

We recommend not trying to build IPsec connections which pass through a NAT machine. This setup poses problems:

      clients --- SG --- NAT ---------- SG
If you must try it, some references are:

Other references on NAT and IPsec

Other documents which may be relevant include:

Other complications

Of course simply allowing UDP 500 and ESP packets is not the whole story. Various other issues arise in making IPsec and packet filters co-exist and even co-operate. Some of them are summarised below.

IPsec through the gateway

Basic IPsec packet filtering rules deal only with packets addressed to or sent from your IPsec gateway.

It is a separate policy decision whether to permit such packets to pass through the gateway so that client machines can build end-to-end IPsec tunnels of their own. This may not be practical if you are using NAT (IP masquerade) on your gateway, and may conflict with some corporate security policies.

Where possible, allowing this is almost certainly a good idea. Using IPsec on an end-to-end basis is more secure than gateway-to-gateway.

Doing it is quite simple. You just need firewall rules that allow UDP port 500 and protocols 50 and 51 to pass through your gateway. If you wish, you can of course restrict this to certain hosts.

Preventing non-IPsec traffic

You can of course also filter everything but UDP port 500 and ESP or AH to restrict traffic to IPsec only, either for anyone communicating with your host or just for specific partners.

One application of this is for the telecommuter who might have:

     Sunset==========West------------------East ================= firewall --- the Internet
         home network      untrusted net        corporate network
The subnet on the right is, the whole Internet. The West gateway is set up so that it allows only IPsec packets to East in or out.

This configuration is used in AT&T Research's network. For details, see the papers links in our introduction.

Filtering packets from unknown gateways

It is possible to use firewall rules to restrict UDP 500, ESP and AH packets so that these packets are accepted only from known gateways. This is not strictly necessary since FreeS/WAN will discard packets from unknown gateways. You might, however, want to do it for any of a number of reasons. For example:

It is not possible to use only static firewall rules for this filtering if you do not know the other gateways' IP addresses in advance, for example if you have "road warriors" who may connect from a different address each time or if want to do opportunistic encryption to arbitrary gateways. In these cases, you can accept UDP 500 IKE packets from anywhere, then use the updown script feature of pluto(8) to dynamically adjust firewalling for each negotiated tunnel.

Firewall packet filtering does not much reduce the risk of a denial of service attack on FreeS/WAN. The firewall can drop packets from unknown gateways, but KLIPS does that quite efficiently anyway, so you gain little. The firewall cannot drop otherwise legitmate packets that fail KLIPS authentication, so it cannot protect against an attack designed to exhaust resources by making FreeS/WAN perform many expensive authentication operations.

In summary, firewall filtering of IPsec packets from unknown gateways is possible but not strictly necessary.

Other packet filters

When the IPsec gateway is also acting as your firewall, other packet filtering rules will be in play. In general, those are outside the scope of this document. See our Linux firewall links for information. There are a few types of packet, however, which can affect the operation of FreeS/WAN or of diagnostic tools commonly used with it. These are discussed below.

ICMP filtering

ICMP is the Internet Control Message Protocol. It is used for messages between IP implementations themselves, whereas IP used is used between the clients of those implementations. ICMP is, unsurprisingly, used for control messages. For example, it is used to notify a sender that a desination is not reachable, or to tell a router to reroute certain packets elsewhere.

ICMP handling is tricky for firewalls.

ICMP does not use ports. Messages are distinguished by a "message type" field and, for some types, by an additional "code" field. The definitive list of types and codes is on the IANA site.

One expert uses this definition for ICMP message types to be dropped at the firewall.

# ICMP types which lack socially redeeming value.
#  5     Redirect
#  9     Router Advertisement
# 10     Router Selection
# 15     Information Request
# 16     Information Reply
# 17     Address Mask Request
# 18     Address Mask Reply

badicmp='5 9 10 15 16 17 18'

A more conservative approach would be to make a list of allowed types and drop everything else.

Whichever way you do it, your ICMP filtering rules on a FreeS/WAN gateway should allow at least the following ICMP packet types:

echo (type 8)
echo reply (type 0)
These are used by ping(1). We recommend allowing both types through the tunnel and to or from your gateway's external interface, since ping(1) is an essential testing tool.

It is fairly common for firewalls to drop ICMP echo packets addressed to machines behind the firewall. If that is your policy, please create an exception for such packets arriving via an IPsec tunnel, at least during intial testing of those tunnels.

destination unreachable (type 3)
This is used, with code 4 (Fragmentation Needed and Don't Fragment was Set) in the code field, to control path MTU discovery. Since IPsec processing adds headers, enlarges packets and may cause fragmentation, an IPsec gateway should be able to send and receive these ICMP messages on both inside and outside interfaces.

UDP packets for traceroute

The traceroute(1) utility uses UDP port numbers from 33434 to approximately 33633. Generally, these should be allowed through for troubleshooting.

Some firewalls drop these packets to prevent outsiders exploring the protected network with traceroute(1). If that is your policy, consider creating an exception for such packets arriving via an IPsec tunnel, at least during intial testing of those tunnels.

UDP for L2TP

Windows 2000 does, and products designed for compatibility with it may, build L2TP tunnels over IPsec connections.

For this to work, you must allow UDP protocol 1701 packets coming out of your tunnels to continue to their destination. You can, and probably should, block such packets to or from your external interfaces, but allow them from ipsec0.

See also our Windows 2000 interoperation discussion .

How it all works: IPsec packet details

IPsec uses three main types of packet:

IKE uses the UDP protocol and port 500 .
Unless you are using only (less secure, not recommended) manual keying, you need IKE to negotiate connection parameters, acceptable algorithms, key sizes and key setup. IKE handles everything required to set up, rekey, repair or tear down IPsec connections.
ESP is protocol number 50
This is required for encrypted connections.
AH is protocol number 51
This can be used where only authentication, not encryption, is required.

All of those packets should have appropriate IPsec gateway addresses in both the to and from IP header fields. Firewall rules can check this if you wish, though it is not strictly necessary. This is discussed in more detail later.

IPsec processing of incoming packets authenticates them then removes the ESP or AH header and decrypts if necessary. Successful processing exposes an inner packet which is then delivered back to the firewall machinery, marked as having arrived on an ipsec[0-3] interface. Firewall rules can use that interface label to distinguish these packets from unencrypted packets which are labelled with the physical interface they arrived on (or perhaps with a non-IPsec virtual interface such as ppp0).

One of our users sent a mailing list message with a diagram of the packet flow.

ESP and AH do not have ports

Some protocols, such as TCP and UDP, have the notion of ports. Others protocols, including ESP and AH, do not. Quite a few IPsec newcomers have become confused on this point. There are no ports in the ESP or AH protocols, and no ports used for them. For these protocols, the idea of ports is completely irrelevant.

Header layout

The protocol numbers for ESP or AH are used in the 'next header' field of the IP header. On most non-IPsec packets, that field would have one of:

Each header in the sequence tells what the next header will be. IPsec adds headers for ESP or AH near the beginning of the sequence. The original headers are kept and the 'next header' fields adjusted so that all headers can be correctly interpreted.

For example, using [ ] to indicate data protected by ESP and unintelligible to an eavesdropper between the gateways:

Part of the ESP header itself is encrypted, which is why the [ indicating protected data appears in the middle of some lines above. The next header field of the ESP header is protected. This makes traffic analysis more difficult. The next header field would tell an eavesdropper whether your packet was UDP to the gateway, TCP to the gateway, or encapsulated IP. It is better not to give this information away. A clever attacker may deduce some of it from the pattern of packet sizes and timings, but we need not make it easy.

IPsec allows various combinations of these to match local policies, including combinations that use both AH and ESP headers or that nest multiple copies of these headers.

For example, suppose my employer has an IPsec VPN running between two offices so all packets travelling between the gateways for those offices are encrypted. If gateway policies allow it (The admins could block UDP 500 and protocols 50 and 51 to disallow it), I can build an IPsec tunnel from my desktop to a machine in some remote office. Those packets will have one ESP header throughout their life, for my end-to-end tunnel. For part of the route, however, they will also have another ESP layer for the corporate VPN's encapsulation. The whole header scheme for a packet on the Internet might be:

The first ESP (outermost) header is for the corporate VPN. The inner ESP header is for the secure machine-to-machine link.

DHR on the updown script

Here are some mailing list comments from pluto(8) developer Hugh Redelmeier on an earlier draft of this document:

There are many important things left out

- firewalling is important but must reflect (implement) policy.  Since
  policy isn't the same for all our customers, and we're not experts,
  we should concentrate on FW and MASQ interactions with FreeS/WAN.

- we need a diagram to show packet flow WITHIN ONE MACHINE, assuming
  IKE, IPsec, FW, and MASQ are all done on that machine.  The flow is
  obvious if the components are run on different machines (trace the

  IKE input:
        + packet appears on public IF, as UDP port 500
        + input firewalling rules are applied (may discard)
        + Pluto sees the packet.

  IKE output:
        + Pluto generates the packet & writes to public IF, UDP port 500
        + output firewalling rules are applied (may discard)
        + packet sent out public IF

  IPsec input, with encapsulated packet, outer destination of this host:
        + packet appears on public IF, protocol 50 or 51.  If this
          packet is the result of decapsulation, it will appear
          instead on the paired ipsec IF.
        + input firewalling rules are applied (but packet is opaque)
        + KLIPS decapsulates it, writes result to paired ipsec IF
        + input firewalling rules are applied to resulting packet
          as input on ipsec IF
        + if the destination of the packet is this machine, the
          packet is passed on to the appropriate protocol handler.
          If the original packet was encapsulated more than once
          and the new outer destination is this machine, that
          handler will be KLIPS.
        + otherwise:
          * routing is done for the resulting packet.  This may well
            direct it into KLIPS for encoding or encrypting.  What
            happens then is described elsewhere.
          * forwarding firewalling rules are applied
          * output firewalling rules are applied
          * the packet is sent where routing specified

 IPsec input, with encapsulated packet, outer destination of another host:
        + packet appears on some IF, protocol 50 or 51
        + input firewalling rules are applied (but packet is opaque)
        + routing selects where to send the packet
        + forwarding firewalling rules are applied (but packet is opaque)
        + packet forwarded, still encapsulated

  IPsec output, from this host or from a client:
        + if from a client, input firewalling rules are applied as the
          packet arrives on the private IF
        + routing directs the packet to an ipsec IF (this is how the
          system decides KLIPS processing is required)
        + if from a client, forwarding firewalling rules are applied
        + KLIPS eroute mechanism matches the source and destination
          to registered eroutes, yielding a SPI group.  This dictates
          processing, and where the resulting packet is to be sent
          (the destinations SG and the nexthop).
        + output firewalling is not applied to the resulting
          encapsulated packet

- Until quite recently, KLIPS would double encapsulate packets that
  didn't strictly need to be.  Firewalling should be prepared for
  those packets showing up as ESP and AH protocol input packets on
  an ipsec IF.

- MASQ processing seems to be done as if it were part of the
  forwarding firewall processing (this should be verified).

- If a firewall is being used, it is likely the case that it needs to
  be adjusted whenever IPsec SAs are added or removed.  Pluto invokes
  a script to do this (and to adjust routing) at suitable times.  The
  default script is only suitable for ipfwadm-managed firewalls.  Under
  LINUX 2.2.x kernels, ipchains can be managed by ipfwadm (emulation),
  but ipchains more powerful if manipulated using the ipchains command.
  In this case, a custom updown script must be used.

  We think that the flexibility of ipchains precludes us supplying an
  updown script that would be widely appropriate.

Linux FreeS/WAN Troubleshooting Guide


Before we launch into how to establish, test and troubleshoot a connection step by step, here's a brief overview that may allow you to skim ahead to the most useful part of this document for you.

There are several general places where you might have a problem:

  1. During install.

  2. Before connection negotiation.

  3. During the negotiation process.

  4. Using an established connection.

This document also contains notes which expand on points made in these sections, and tips for problem reporting. If the other end of your connection is not FreeS/WAN, see also our interoperation document.

1. During Install

Instructions and tips are in our install document. If you encounter a problem, it may be:

2. Before Connection Negotiation

2.1 Plan your tunnels

Know your IPSec needs. For most people who begin experimenting with IPSec in the field, your configuration will more or less resemble one of the samples in our configuration document . Other folks may wish to create a testbed network in a lab environment for intensive IPSec testing or proof-of-concept. They may be interested in this description of a sample testbed network.

2.2 Configure Step by Step

Draw a network schematic. This will assist others in helping you should you mail the list for help.

For example:

         corporate LAN     untrusted net

Add IPs to the diagram.

         corporate LAN     untrusted net                           N.N.N.235


         gateway for

How many tunnels do you need to connect your sites sufficiently for your purposes? Check our configuration document for why you might need multiple tunnels. How would you name each tunnel?

Above, it is possible to make two tunnels: sunset-east and west-east. If East does not need to access any resources on West, and West is not masquerading (see below) we may only want to create sunset-east. Sunset-east is a tunnel between IPSec-enabled machines West and East, created to protect traffic between the net on which Sunset resides, and East. However, it is safest to create all potential tunnels in your configuration. This lowers the risk that you will forget to configure a needed tunnel, and send important data cleartext. As an added safeguard, by default, (at the time of writing) Linux FreeS/WAN prevents you from routing cleartext packets between IPSec gateways which are also linked by a tunnel. Reliance on this behaviour is not a substitute for secure network design.

Take into account any masquerading and Network Address Translation rules on your gateway. If you are masquerading packets from Sunset as they leave West, Linux FreeS/WAN will treat these as though they originated at West, not Sunset. For more detail, see this packet flow diagram.

For each tunnel, think of a packet path that will allow you to test that tunnel. Refer to this discussion.

2.3 Ensure a Clear Path for IKE

Ensure that IKE packets can travel freely between your IPSec gateways, as described here. If they cannot, the connection negotiation, described below, will be stuck in its first state.

3. During Negotiation

3.1 Create a connection

Bring up one of your connections, using the ipsec auto commands at the command line:

    ipsec auto --add west-east


    ipsec auto --up west-east

At this stage of testing, do not bring up the connections by the alternate method, using ipsec.conf's auto= directive. You want to view the output as it happens.

If the resulting status report shows that you have established an ISAKMP and an IPSec Security Association (aka "SA", loosely translated as "tunnel" or "connection"), your tunnel is up. Repeat for each tunnel you are testing.

If negotiations for any one tunnel fail, troubleshoot as indicated in the next section. If you have successfully established all desired tunnels, proceed to test your connection(s) below. If you find that each tunnel in a multitunnel config may be created individually, but all may not be created at once, you may have encountered an old (1.6) bug. Update your Linux FreeS/WAN.

3.1.1 Determine Connection State

When you bring a tunnel up from the command line, you see a report on the negotiations involved in creating the connection, as these happen. There are also:

Often, the most relevant state information appears last in a log or status report.

Negotiations will proceed through various states. You will know these are done and a connection is established when you see both messages:

    000 #21: "myconn" STATE_MAIN_I4 (ISAKMP SA established)...
    000 #2: "myconn" STATE_QUICK_I2 (sent QI2, IPsec SA established)...

The key phrases are "ISAKMP SA established" and "IPSec SA established", which should appear with the relevant connection name. Often, this happens at STATE_MAIN_I4 and STATE_QUICK_I2, respectively.

A note on ipsec auto --status: this will tell you what states have been achieved, rather than the current state. Since determining the current state is rather more difficult to do, current state information is not available from Linux FreeS/WAN. If you are actively bringing a connection up, the status report's last states for that connection likely reflect its current state. Beware, though, of the case where a connection was correctly brought up but is now downed: Linux FreeS/WAN will not notice this until it attempts to rekey. Meanwhile, the last known state indicates that the connection has been established.

Linux FreeS/WAN proceeds though IKE (Phase 1, Main Mode, STATE_MAIN_*) negotiations first, then begins IPSec (Phase 2, Quick Mode, STATE_QUICK_*) negotiations. If you do not see success, note the place where negotiations stopped. This information is useful, since there are common errors specific to certain points in the process.

3.2 Find a Negotiation Error

Look for verbose error text in the logs. While ipsec --auto dialog will tell you at which state Linux FreeS/WAN failed, it lacks detail. You can get more detail by modifying it with the --verbose flag on each invocation. For example:

    ipsec auto --verbose --up west-east

Complete information can be gleaned from the log files.

The amount of description in the logs depends on ipsec.conf debug settings, klipsdebug= and plutodebug=. See the ipsec.conf(5) man page for details. You will normally want to set these to either "none" or "all". Note that you must have enabled the klipsdebug compile-time option for the klipsdebug configuration switch to work.

For negotiation problems, plutodebug is most relevant. klipsdebug applies mainly to attempts to use an already-established connection. See also this description of the division of duties within Linux FreeS/WAN.

After raising your debug levels, restart Linux FreeS/WAN to ensure that ipsec.conf is reread, then recreate the error to generate verbose logs.

This is a good time to produce a barf file, a collection of information useful for debugging Linux FreeS/WAN on your system. Use the command

    ipsec barf > barf.west

See also the ipsec_barf(8) man page.

Look at the logs within the resulting file, and find the failure point. Are there a handful of lines which succinctly describe how things are going wrong or contrary to your expectation? Sometimes the failure point is not immediately obvious: Linux FreeS/WAN's errors are usually not marked "Error". Have a look in the FAQ for what some common failures look like. Tip: problems snowball. Focus your efforts on the first problem, which is likely to be the cause of later errors.

Repeat the process to find meaningful error text on the peer IPSec box. If the other end is not Linux FreeS/WAN, get it to produce detailed log output while you replicate the error, then capture that output to a file.

It is useful if both ends store information about the same event from two perspectives. Sometimes you will require information which only one side has. In this case, the peer can merely indicate the presence of an error, and its approximate point in the negotiations. If one side keeps retrying, it may be because there is a show stopper on the other side. Have a look at the other side and figure out what it doesn't like.

3.2.1 Interpret a Negotiation Error

To interpret Linux FreeS/WAN log text, use the following resources:

If you have failed to solve your problem with the help of these resources, send a detailed problem report to the users list, following these guidelines.

4. Using a Connection

4.1 Ping test

Test a connection by sending packets through it. The simplest way to do this is with ping. Remember, in the planning stage , choosing a path which would test the tunnel? Now, ping along that path.

If your ping returns, test any other connections you've brought up. If they all check out, great. You may wish to test with large packets for MTU problems.

If your ping fails to return, generate an ipsec barf debugging report on each IPSec gateway. On a non-Linux FreeS/WAN implementation, gather equivalent information. Use this, and the tips in the next sections, to troubleshoot. Are you sure that both endpoints are capable of hearing and responding to ping?

4.1.1 Check your ping path

IPSec may be dropping your ping packets since it does not "think" they belong in the tunnels you have constructed. This is an error about assumptions, and it takes two forms.

In the first, your ping is not returning because its path does not fall within your tunnel. This ping does not test the tunnel you intend to test. Referring to this discussion about appropriate tests, determine an alternate ping path which would test the tunnel.

In the second form of this error, you have not correctly configured the functionality you want. In the example above, you may have configured one of the possible tunnels between West and East (say west-east) but not the tunnel required to secure the important traffic you're now testing (say a sunset-east tunnel to secure traffic between office net Sunset and laptop East). See also this FAQ titled "I can't ping". NAT and masquerading may have an effect on which tunnels you need to configure; that's discussed in " Before there's trouble".

Both forms show identical symptoms. After all, the difference is one of intent. In both forms, Linux FreeS/WAN receives a packet destined for a peer IPSec gateway. Finding no active tunnel in which this packet belongs, it drops the packet on the floor. If your debug levels are appropriate, it logs this with a "klipsdebug... no eroute" message, which is discussed in this FAQ.

Note: When testing a tunnel that protects a multi-node subnet, you may wish to try several subnet nodes as ping targets, in case one node is routing incorrectly.

4.2 Check Routing and Firewalling

If you've confirmed your configuration assumptions, the problem is almost certainly with routing or firewalling. Isolate the problem using interface statistics, firewall statistics, or a packet sniffer.


4.2.1 View Interface and Firewall Statistics

Interface reports and firewall statistics can help you track down lost packets at a glance.

Check any firewall statistics you may be keeping on your IPSec gateways, for dropped packets.

Both cat /proc/net/dev and ifconfig display interface statistics, and both are included in an ipsec barf. Use either to check if any interface has dropped packets. If you find that one has, test whether this is related to your ping. While you ping continuously, print that interface's statistics several times. Does its drop count increase in proportion to the ping? If so, check why the packets are dropped there.

Check the firewall rules that apply to that interface. If the interface is an IPSec interface, more information may be available in the log. Grep for the word "drop" in a log which was created with klipsdebug=all as the error happened.

See also this detailed discussion by KLIPS programmer Richard Guy Briggs on interpreting ifconfig.

4.3 Sniff packets

If you have checked configuration assumptions, routing, and firewall rules, and your interface statistics yield no clue, it remains for you to investigate the mystery of the lost packet by the most thorough method: with a packet sniffer. Sniff packets at each interface along the projected ping path until you find where your packets disappear. In this way, you can isolate the problem area, and narrow your troubleshooting focus.

Install an up-to-date sniffer (tcpdump, ethereal, ksnuffle) on your IPSec gateway machines. A sniffer on the ping endpoints is also useful. The sniffer should be somewhat modern (tcpdump 3.3+, ethereal-0.8.18) so that you may view packets on the ipsec virtual interface as well as the underlying physical one.

Working from your schematic, anticipate your ping's path. Which machines will your ping be visible on, in which order? Now, which interfaces will it be visible on, in which order? Within a machine running Linux FreeS/WAN, this packet flow diagram will help you anticipate the packet's path. Note that from the perspective of the tunneled packet, the entire tunnel is one hop. That's explained in this FAQ.

Ping, and as you do, sniff the packets. Examine each interface along the projected path, checking for your ping's arrival. If it doesn't arrive at the next stop, you have narrowed down where to look for it.

Note that an encapsulated IPSec packet will look different, when sniffed, from the plaintext packet which generated it. However, you can observe plaintext packets entering an IPSec interface and the resulting cyphertext packets as they emerge from the corresponding physical interface.

Once you isolate where the packet is lost, take a closer look at firewall rules, routing and configuration assumptions as they affect that specific area. If the packet is lost on an IPSec gateway, comb through klipsdebug output for anomalies.

If the packet goes through both gateways successfully and reaches the ping target, but does not return, suspect routing. Check that the ping target routes packets back to the IPSec gateway.

4.4 Find a Connection Use Error

The guidelines are the same as for the Pluto logs, above.

For connection use problems, set klipsdebug=all. Note that you must have enabled the klipsdebug compile-time option to do this. Restart Linux FreeS/WAN so that it rereads the configuration file, then recreate the error condition. When searching through klipsdebug data, look especially for the keywords "drop" (as in dropped packets) and "error".

Often the problem with connection use is not software error, but rather that the software is behaving contrary to expectation.

4.4.1 Interpret a Connection Use Error

To interpret the Linux FreeS/WAN log text you've found, use the same resources as indicated for troubleshooting connection negotiation: the FAQ , our background document , and the list archives. Looking in the KLIPS code is recommended only for the brave.

If you are still stuck, send a detailed problem report to the users' list.

4.4 Test with Large Packets

If each of your connections passed the ping test, you may wish to test by pinging with large packets (2000 bytes or larger). If it does not return, suspect MTU issues, and see this discussion.

4.5 Stress Tests

In most users' view, a simple ping test, and perhaps a large-packet ping test suffice to indicate a working IPSec connection.

Some people might like to do additional stress tests prior to production use. They may be interested in this testing protocol we use at interoperation conferences, aka "bakeoffs". We also have a testing directory that ships with the release.

5. Problem Reporting

5.1 How to ask for help

Ask for troubleshooting help on the users' mailing list, While sometimes an initial query with a quick description of your intent and error will twig someone's memory of a similar problem, it's often necessary to send a second mail with a complete problem report.

The essay How to Report Bugs Effectively contains good guidelines.

When reporting problems to the mailing list(s), please include:

5.2 Where to ask

To report a problem, send mail about it to the users' list. If you are certain that you have found a bug, report it to the bugs list. If you encounter a problem while doing your own coding on the Linux FreeS/WAN codebase and think it is of interest to the design team, notify the design list. When in doubt, default to the users' list. More information about the mailing lists is found here.

For a number of reasons -- including export-control regulations affecting almost any private discussion of encryption software -- we prefer that problem reports and discussions go to the lists, not directly to the team. Beware that the list goes worldwide; US citizens, read this important information about your export laws. If you're using this software, you really should be on the lists. To get onto them, visit

If you do send private mail to our coders or want a private reply from them, please make sure that the return address on your mail (From or Reply-To header) is a valid one. They have more important things to do than to unravel addresses that have been mangled in an attempt to confuse spammers.

6. Additional Notes on Troubleshooting

The following sections supplement the Guide: information available on your system; testing between security gateways; ifconfig reports for KLIPS debugging; using GDB on Pluto.

6.1 Information available on your system

6.1.1 Logs used

Linux FreeS/WAN logs to:

Check both places to get full information. If you find nothing, check your syslogd.conf(5) to see where your /etc/syslog.conf or equivalent is directing authpriv messages.

6.1.2 man pages provided

Manual page for IPSEC configuration file.
Primary man page for ipsec utilities.

Other man pages are on this list and in

6.1.3 Status information

ipsec auto --status
Command to get status report from running system. Displays Pluto's state. Includes the list of connections which are currently "added" to Pluto's internal database; lists state objects reflecting ISAKMP and IPsec SAs being negotiated or installed.
ipsec look
Brief status info.
ipsec barf
Copious debugging info.

6.2 Testing between security gateways

Sometimes you need to test a subnet-subnet tunnel. This is a tunnel between two security gateways, which protects traffic on behalf of the subnets behind these gateways. On this network:

                     IPSec gateway         IPSec gateway
           local net       untrusted net       local net

you might name this tunnel sunset-sunrise. You can test this tunnel by having a machine behind one gateway ping a machine behind the other gateway, but this is not always convenient or even possible.

Simply pinging one gateway from the other is not useful. Such a ping does not normally go through the tunnel. The tunnel handles traffic between the two protected subnets, not between the gateways . Depending on the routing in place, a ping might

Neither event tells you anything about the tunnel. You can explicitly create an eroute to force such packets through the tunnel, or you can create additional tunnels as described in our configuration document, but those may be unnecessary complications in your situation.

The trick is to explicitly test between both gateways' private-side IP addresses. Since the private-side interfaces are on the protected subnets, the resulting packets do go via the tunnel. Use either ping -I or traceroute -i, both of which allow you to specify a source interface. (Note: unsupported on older Linuxes). The same principles apply for a road warrior (or other) case where only one end of your tunnel is a subnet.

6.3 ifconfig reports for KLIPS debugging

When diagnosing problems using ifconfig statistics, you may wonder what type of activity increments a particular counter for an ipsecN device. Here's an index, posted by KLIPS developer Richard Guy Briggs:

Here is a catalogue of the types of errors that can occur for which
statistics are kept when transmitting and receiving packets via klips.
I notice that they are not necessarily logged in the right counter.
. . .

Sources of ifconfig statistics for ipsec devices

- packet handed to ipsec_rcv that is not an ipsec packet.
- ipsec packet with payload length not modulo 4.
- ipsec packet with bad authenticator length.
- incoming packet with no SA.
- replayed packet.
- incoming authentication failed.
- got esp packet with length not modulo 8.

- cannot process ip_options.
- packet ttl expired.
- packet with no eroute.
- eroute with no SA.
- cannot allocate sk_buff.
- cannot allocate kernel memory.
- sk_buff internal error.

The standard counters are:

struct enet_statistics
        int        rx_packets;                /* total packets received */
        int        tx_packets;                /* total packets transmitted */
        int        rx_errors;                /* bad packets received */
        int        tx_errors;                /* packet transmit problems */
        int        rx_dropped;                /* no space in linux buffers */
        int        tx_dropped;                /* no space available in linux */
        int        multicast;                /* multicast packets received */
        int        collisions;

        /* detailed rx_errors: */
        int        rx_length_errors;
        int        rx_over_errors;                /* receiver ring buff overflow */
        int        rx_crc_errors;                /* recved pkt with crc error */
        int        rx_frame_errors;        /* recv'd frame alignment error */
        int        rx_fifo_errors;                /* recv'r fifo overrun */
        int        rx_missed_errors;        /* receiver missed packet */

        /* detailed tx_errors */
        int        tx_aborted_errors;
        int        tx_carrier_errors;
        int        tx_fifo_errors;
        int        tx_heartbeat_errors;
        int        tx_window_errors;

of which I think only the first 6 are useful.

6.4 Using GDB on Pluto

You may need to use the GNU debugger, gdb(1), on Pluto. This should be necessary only in unusual cases, for example if you encounter a problem which the Pluto developer cannot readily reproduce or if you are modifying Pluto.

Here are the Pluto developer's suggestions for doing this:

Can you get a core dump and use gdb to find out what Pluto was doing
when it died?

To get a core dump, you will have to set dumpdir to point to a
suitable directory (see ipsec.conf(5)).

To get gdb to tell you interesting stuff:
        $ script
        $ cd dump-directory-you-chose
        $ gdb /usr/local/lib/ipsec/pluto core
        (gdb) where
        (gdb) quit
        $ exit

The resulting output will have been captured by the script command in
a file called "typescript".  Send it to the list.

Do not delete the core file.  I may need to ask you to print out some
more relevant stuff.

Note that the dumpdir parameter takes effect only when the IPsec subsystem is restarted -- reboot or ipsec setup restart.

Linux FreeS/WAN Compatibility Guide

Most of this document is quoted directly from the Linux FreeS/WAN mailing list. Thanks very much to the community of testers, patchers and commenters there, especially the ones quoted below but also various contributors we haven't quoted.

Implemented parts of the IPsec Specification

In general, do not expect Linux FreeS/WAN to do everything yet. This is a work-in-progress and some parts of the IPsec specification are not yet implemented.

In Linux FreeS/WAN

Things we do, as of version 1.9:

All combinations of implemented transforms are supported. Note that some form of packet-level authentication is required whenever encryption is used. Without it, the encryption will not be secure.

Deliberately omitted

We do not implement everything in the RFCs because some of those things are insecure. See our discussions of avoiding bogus security.

Things we deliberately omit which are required in the RFCs are:

Since these are the only encryption algorithms and DH group the RFCs require, it is possible in theory to have a standards-conforming implementation which will not interpoperate with FreeS/WAN. Such an implementation would be inherently insecure, so we do not consider this a problem. Anyway, most implementations sensibly include more secure options as well, so dropping null encryption, single DES and Group 1 does not greatly hinder interoperation in practice.

We also do not implement some optional features allowed by the RFCs:

In theory, this should cause no interoperation problems since all implementations are required to support the more secure main mode, whether or not they also allow aggressive mode.

In practice, it does sometimes produce problems with implementations such as Windows 2000 where aggressive mode is the default. Typically, these are easily solved with a configuration change that overrides that default.

Not (yet) in Linux FreeS/WAN

Things we don't yet do, as of version 1.91:

  • encryption transforms
  • Currently Triple DES is the only encryption method Pluto will negotiate.

    No additional encryption transforms are yet implemented, though the RFCs allow them and some other IPsec implementations support various of them. We are not eager to add more, since they complicate both our work and that of the gateway administrator without any obvious security improvement. We would certainly not want to incorporate any cryptographic method that had inadequate key length or had not been subjected to intensive review over some time.

    Rjindael, which just won the AES competition to choose a successor to the DES standard, is an excellent candidate for inclusion in FreeS/WAN. This might be a good project for a volunteer.

  • authentication transforms
  • No optional additional authentication transforms are currently implemented and we do not forsee a need to add any soon.

  • Policy checking on decrypted packets
  • To fully comply with the RFCs, it is not enough just to accept only packets which survive any firewall rules in place to limit what IPsec packets get in, and then pass KLIPS authentication. That is what FreeS/WAN currently does.

    We should also apply additional tests, for example ensuring that all packets emerging from a particular tunnel have sourcen and destination addresses that fall within the subnets defined for that tunnel, and that packets with those addresses that did not emerge from the appropriate tunnel are disallowed.

    This will be done as part of the KLIPS rewrite currently in progress. See these links and the design mailing list for discussion.

    Our PF-Key implementation

    We use PF-key Version Two for communication between the KLIPS kernel code and the Pluto Daemon. PF-Key v2 is defined by RFC 2367.

    The "PF" stands for Protocol Family. PF-Inet defines a kernel/userspace interface for the TCP/IP Internet protocols (TCP/IP), and other members of the PF series handle Netware, Appletalk, etc. PF-Key is just a PF for key-related matters.

    Our PF-Key implementation is not yet (mid-July 2000) complete. In particular, it is mostly one-way, used for Pluto to talk to KLIPS but not yet doing much upward communication from kernel to user space. This will change, but is not at the top of our priority list.

    PF-Key portability

    PF-Key came out of Berkeley Unix work and is used in the various BSD IPsec implementations, and in Solaris. This means there is some hope of porting our Pluto(8) to one of the BSD distributions, or of running their photurisd(8) on Linux if you prefer Photuris key management over IKE.

    It is, however, more complex than that. The PK-Key RFC deliberately deals only with keying, not policy management. The three PF-Key implementations we have looked at -- ours, OpenBSD and KAME -- all have extensions to deal with security policy, and the extensions are different. There have been discussions aimed at sorting out the differences, perhaps for a version three PF-Key spec. All players are in favour of this, but everyone involved is busy and it is not clear whether or when these discussions might bear fruit.

    Kernels other than the latest 2.2.x and 2.4.y

    We develop and test on Redhat Linux using the most recent kernel in the 2.2 and 2.4 series. In general, we recommend you use the latest kernel in one of those series. Complications and caveats are discussed below.

    Other 2.0.x Intel Kernels

    Consider upgrading to the 2.2 kernel series. If you want to stay with the 2.0 series, then we strongly recommend 2.0.39. Some useful security patches were added in 2.0.38.

    Various versions of the code have run at various times on most 2.0.xx kernels, but the current version is only lightly tested on 2.0.39, and not at all on older kernels.

    Some of our patches for older kernels are shipped in 2.0.37 and later, so they are no longer provided in FreeS/WAN. This means recent versions of FreeS/WAN will probably not compile om anything earlier than 2.0.37.

    2.2 and 2.4 Kernels

    FreeS/WAN 1.0
    ran only on 2.0 kernels
    FreeS/WAN 1.1 to 1.8
    ran on 2.0 or 2.2 kernels
    ran on some development kernels, 2.3 or 2.4-test
    FreeS/WAN 1.9
    runs on 2.0, 2.2 or 2.4 kernels

    In general, we suggest the latest 2.2 kernel or 2.4 for production use. At time of writing (early June 2001, just before our 1.91 release) these are 2.2.19 and 2.4.5.

    Of course no release can be guaranteed to run on kernels more recent than it is, so quite often there will be no stable FreeS/WAN for the absolute latest kernel. See the FAQ for discussion.

    Intel Linux distributions other than Redhat

    We develop and test on Redhat 6.1 for 2.2 kernels, and on Redhat 7.1 for 2.4, so minor changes may be required for other distributions.

    Redhat 7.0

    There are some problems with FreeS/WAN on Redhat 7.0, but they are soluble.

    Redhat 7 ships with two compilers.

    Kernel Makefiles have gcc as a default, and must be adjusted to use kgcc before a kernel will compile on 7.0. This mailing list message gives details:

    Subject: Re: AW: Installing IPsec on Redhat 7.0
       Date: Thu, 1 Feb 2001 14:32:52 -0200 (BRST)
      From: Mads Rasmussen <>
    > From
    cd to /usr/src/linux and open the Makefile in your favorite editor. You
    will need to look for a line similar to this:
    This line specifies which C compiler to use to build the kernel. It should
    be changed to:
    for Red Hat Linux 7. The kgcc compiler is egcs 2.91.66. From here you can
    proceed with the typical compiling steps.
    Check the mailing list archive for more recent news.

    SuSE Linux

    SuSE 6.3 and later versions, at least in Europe, ship with FreeS/WAN included.

    Here are some notes for an earlier SuSE version.

    SuSE Linux 5.3

    Date: Mon, 30 Nov 1998
    From: Peter Onion <>
    ... I got Saturdays snapshot working between my two SUSE5.3 machines at home.
    The mods to the install process are quite simple.  From memory and looking at
    the files on the SUSE53 machine here at work....
    And extra link in each of the /etc/init.d/rc?.d directories called K35ipsec
    which SUSE use to shut a service down.
    A few mods in /etc/init.d/ipsec  to cope with the different places that SUSE
    put config info, and remove the inculsion of /etc/rc.d/init.d/functions and .
    /etc/sysconfig/network as they don't exists and 1st one isn't needed anyway.
    insert ". /etc/rc.config" to pick up the SUSE config info and use 
      if test -n "$NETCONFIG" -a "$NETCONFIG" != "YAST_ASK" ; then
    to replace 
      [ ${NETWORKING} = "no" ] amp; exit 0
    Create /etc/sysconfig  as SUSE doesn't have one.
    I think that was all (but I prob forgot something)....

    You may also need to fiddle initialisation scripts to ensure that /var/run/ is removed when rebooting. If this file is present, Pluto does not come up correctly.


    Subject: Re: linux-IPsec: Slackware distribution
      Date:  Thu, 15 Apr 1999 12:07:01 -0700
      From:  Evan Brewer <>
    > Very shortly, I will be needing to install IPsec on at least gateways that
    > are running Slackware. . . .
    The only trick to getting it up is that on the slackware dist there is no
    init.d directory in /etc/rc.d .. so create one.  Then, what I do is take the
    IPsec startup script which normally gets put into the init.d directory, and
    put it in /etc/rc.d and name ir rc.ipsec .. then I symlink it to the file
    in init.d.  The only file in the dist you need to really edit is the
    utils/Makefile, setup4:
    Everything else should be just fine.
    A year or so later:
    Subject: Re: HTML Docs- Need some cleanup?
       Date: Mon, 8 Jan 2001
       From: Jody McIntyre <>
    I have successfully installed FreeS/WAN on several Slackware 7.1 machines.
    FreeS/WAN installed its rc.ipsec file in /etc/rc.d.  I had to manually call
    this script from rc.inet2.  This seems to be an easier method than Evan


    A recent (Nov 2001) mailing list points to a web page on setting up several types of tunnel, including IPsec, on Debian.

    Some older information:

    Subject: FreeS/WAN 1.0 on Debian 2.1
       Date: Tue, 20 Apr 1999
      From:  Tim Miller <>
            Compiled and installed without error on a Debian 2.1 system
    with kernel-source-2.0.36 after pointing RCDIR in utils/Makefile to
            /var/lock/subsys/ doesn't exist on Debian boxen, needs to be
    created; not a fatal error.
            Finally, IPsec scripts appear to be dependant on GNU awk
    (gawk); the default Debian awk (mawk-1.3.3-2) had fatal difficulties.
    With gawk installed and /etc/alternatives/awk linked to /usr/bin/gawk
    operation appears flawless.

    The scripts in question have been modified since this was posted. Awk versions should no longer be a problem.


    Subject: Re: HTML Docs- Need some cleanup?
       Date: Mon, 08 Jan 2001
       From: Andy Bradford <>
    On Sun, 07 Jan 2001 22:59:05 EST, Sandy Harris wrote:
    >     Intel Linux distributions other than Redhat 5.x and 6.x 
    >         Redhat 7.0 
    >         SuSE Linux 
    >             SuSE Linux 5.3 
    >         Slackware 
    >         Debian 
    Can you please include Caldera in this list?  I have tested it since 
    FreeS/Wan 1.1 and it works great with our systems---provided one 
    follows the FreeS/Wan documentation. :-)
    Thank you,

    CPUs other than Intel

    FreeS/WAN has been run sucessfully on a number of different CPU architectures. If you have tried it on one not listed here, please post to the mailing list.

    Corel Netwinder (StrongARM CPU)

    Subject: linux-ipsec: Netwinder diffs
    Date: Wed, 06 Jan 1999
    I had a mistake in my IPsec-auto, so I got things working this morning.
    Following are the diffs for my changes.  Probably not the best and cleanest way 
    of doing it, but it works. . . . 

    These diffs are in the 0.92 distribution and any snapshot after Feb 20 1999, so these should work out-of-the-box on Netwinder.

    Yellow Dog Linux on Power PC

    Subject:  Compiling FreeS/WAN 1.1 on YellowDog Linux (PPC)
       Date:  11 Dec 1999
       From:  Darron Froese <>
    I'm summarizing here for the record - because it's taken me many hours to do
    this (multiple times) and because I want to see IPsec on more linuxes than
    just x86.
    Also, I can't remember if I actually did summarize it before... ;-) I'm
    working too many late hours.
    That said - here goes.
    1. Get your linux kernel and unpack into /usr/src/linux/ - I used 2.2.13.
    2. Get FreeS/WAN and unpack into /usr/src/freeswan-1.1
    3. Get the gmp src rpm from here:
    4. Su to root and do this: rpm --rebuild gmp-2.0.2-9a.src.rpm
    You will see a lot of text fly by and when you start to see the rpm
    recompiling like this:
    Executing: %build
    + umask 022
    + cd /usr/src/redhat/BUILD
    + cd gmp-2.0.2
    + libtoolize --copy --force
    Remember to add `AM_PROG_LIBTOOL' to `'.
    You should add the contents of `/usr/share/aclocal/libtool.m4' to
    + CFLAGS=-O2 -fsigned-char
    + ./configure --prefix=/usr
    Hit Control-C to stop the rebuild. NOTE: We're doing this because for some
    reason the gmp source provided with FreeS/WAN 1.1 won't build properly on
    cd /usr/src/redhat/BUILD/
    cp -ar gmp-2.0.2 /usr/src/freeswan-1.1/
    cd /usr/src/freeswan-1.1/
    rm -rf gmp
    mv gmp-2.0.2 gmp
    5. Open the freeswan Makefile and change the line that says:
    KERNEL=$(b)zimage (or something like that) to
    6. cd ../linux/
    7. make menuconfig
    Select an option or two and then exit - saving your changes.
    8. cd ../freeswan-1.1/ ; make menugo
    That will start the whole process going - once that's finished compiling,
    you have to install your new kernel and reboot.
    That should build FreeS/WAN on ydl (I tried it on 1.1).
    And a later message on the same topic:
    Subject: Re: FreeS/WAN, PGPnet and E-mail
       Date: Sat, 22 Jan 2000
       From: Darron Froese <>
    on 1/22/00 6:47 PM, Philip Trauring at wrote:
    > I have a PowerMac G3 ...
    The PowerMac G3 can run YDL 1.1 just fine. It should also be able to run
    FreeS/WAN 1.2patch1 with a couple minor modifications:
    1. In the Makefile it specifies a bzimage for the kernel compile - you have
    to change that to vmlinux for the PPC.
    2. The gmp source that comes with FreeS/WAN (for whatever reason) fails to
    compile. I have gotten around this by getting the gmp src rpm from here:
    If you rip the source out of there - and place it where the gmp source
    resides it will compile just fine.

    FreeS/WAN no longer includes GMP source.


    One user reports success on the Mach-based micro kernel Linux.

    Subject: Smiles on sparc and ppc
       Date: Fri, 10 Mar 2000
       From: Jake Hill <>
    You may or may not be interested to know that I have successfully built
    FreeS/WAN on a number of non intel alpha architectures; namely on ppc
    and sparc and also on osfmach3/ppc (MkLinux). I can report that it just
    works, mostly, with few changes.

    Alpha 64-bit processors

    Subject: IT WORKS (again) between intel & alpha :-)))))
       Date: Fri, 29 Jan 1999
       From: Peter Onion <>
    Well I'm happy to report that I've got an IPsec connection between by intel & alpha machines again :-))
    If you look back on this list to 7th of December I wrote...
    -On 07-Dec-98 Peter Onion wrote:
    -> I've about had enuf of wandering around inside the kernel trying to find out
    -> just what is corrupting outgoing packets...
    -Its 7:30 in the evening .....
    -I FIXED IT  :-))))))))))))))))))))))))))))))))
    -It was my own fault :-((((((((((((((((((
    -If you ask me very nicly I'll tell you where I was a little too over keen to
    -change unsigned long int __u32 :-)  OPSE ...
    -So tomorrow it will full steam ahead to produce a set of diffs/patches against
    -Peter Onion.

    In general (there have been some glitches), FreeS/WAN has been running on Alphas since then.

    Sun SPARC processors

    Several users have reported success with FreeS/WAN on SPARC Linux. Here is one mailing list message:

    Subject: Smiles on sparc and ppc
       Date: Fri, 10 Mar 2000
       From: Jake Hill <>
    You may or may not be interested to know that I have successfully built
    FreeS/WAN on a number of non intel alpha architectures; namely on ppc
    and sparc and also on osfmach3/ppc (MkLinux). I can report that it just
    works, mostly, with few changes.
    I have a question, before I make up some patches. I need to hack
    gmp/mpn/powerpc32/*.s to build them. Is this ok? The changes are
    trivial, but could I also use a different version of gmp? Is it vanilla
    I guess my only real headache is from ipchains, which appears to stop
    running when IPsec has been started for a while. This is with 2.2.14 on

    This message, from a different mailing list, may be relevant for anyone working with FreeS/WAN on Suns:

    Subject: UltraSPARC DES assembler
       Date: Thu, 13 Apr 2000
       From: (Svend Olaf Mikkelsen)
    An UltraSPARC assembler version of the LibDES/SSLeay/OpenSSL des_enc.c
    file is available at
    This brings DES on UltraSPARC from slower than Pentium at the same
    clock speed to significantly faster.

    MIPS processors

    We know FreeS/WAN runs on at least some MIPS processors because Lasat manufacture an IPsec box based on an embedded MIPS running Linux with FreeS/WAN. We have no details.

    Transmeta Crusoe

    The Merilus Firecard, a Linux firewall on a PCI card, is based on a Crusoe processor and supports FreeS/WAN.

    Motorola Coldfire

    Subject: Re: Crypto hardware support
       Date: Mon, 03 Jul 2000
       From: Dan DeVault <>
    .... I have been running
    uClinux with FreeS/WAN 1.4 on a system built by Moreton Bay  ( )  and it was using a Coldfire processor
    and was able to do the Triple DES encryption at just about
    1 mbit / sec rate.......  they put a Hi/Fn 7901 hardware encryption
    chip on their board and now their system does over 25 mbit of 3DES
    encryption........ pretty significant increase if you ask me.

    Multiprocessor machines

    FreeS/WAN is designed to work on SMP (symmetric multi-processing) Linux machines and is regularly tested on dual processor x86 machines.

    We do not know of any testing on multi-processor machines with other CPU architectures or with more than two CPUs. Anyone who does test this, please report results to the mailing list .

    The current design does not make particularly efficient use of multiprocessor machines; some of the kernel work is single-threaded. This issue is being addressed in the KLIPS II redesign.

    Support for crypto hardware

    Supporting hardware cryptography accelerators has not been a high priority for the development team because it raises a number of fairly complex issues:

    That said, we have a report of FreeS/WAN working with one crypto accelerator and some work is going on to modify KLIPS to create a clean generic interface to such products. See this web page for some of the design discussion.

    More recently, a patch to support some hardware accelerators has been posted:

    Subject: [Design] [PATCH] H/W acceleration patch
       Date: Tue, 18 Sep 2001
       From: "Martin Gadbois" <>
    Here's a web site with H/W acceleration patch for FreeS/WAN 1.91, including
    S/W and Hifn 7901 crypto support.
    Martin Gadbois

    Hardware accelerators could take performance well beyond what FreeS/WAN can do in software (discussed here ). Here is some discussion off the IETF IPsec list, October 2001:

     ... Currently shipping chips deliver, 600 mbps throughput on a single
     stream of 3DES IPsec traffic.  There are also chips that use multiple
     cores to do 2.4 gbps.  We (Cavium) and others have announced even faster
     chips. ... Mid 2002 versions will handle at line rate (OC48 and OC192)
     IPsec and SSL/TLS traffic not only 3DES CBC but also AES and arc4.

    The patches to date support chips that have been in production for some time, not the state-of-the-art latest-and-greatest devices described in that post. However, they may still outperform software and they almost certainly reduce CPU overhead.

    IP version 6 (IPng)

    The Internet currently runs on version four of the IP protocols. IPv4 is what is in the standard Linux IP stack, and what FreeS/WAN was built for. In IPv4, IPsec is an optional feature.

    The next version of the IP protocol suite is version six, usually abbreviated either as "IPv6" or as "IPng" for "IP: the next generation". For IPv6, IPsec is a required feature. Any machine doing IPv6 is required to support IPsec, much as any machine doing (any version of) IP is required to support ICMP.

    There is a Linux implementation of IPv6 in Linux kernels 2.2 and above. For details, see the FAQ. It does not yet support IPsec. The USAGI project are also working on IPv6 for Linux.

    FreeS/WAN was originally built for the current standard, IPv4, but we are interested in seeing it work with IPv6. Some progress has been made, but at time of writing (February 2001), the job is not complete. For more recent information, check the mailing list .

    The first release of FreeS/WAN (1.8) patched for IPv6 support is now available.

    IPv6 background

    IPv6 has been specified by an IETF working group. The group's page lists over 30 RFCs to date, and many Internet Drafts as well. The overview is RFC 2460. Major features include:

    A number of projects are working on IPv6 implementation. A prominent Open Source effort is KAME , a collaboration among several large Japanese companies to implement IPv6 for Berkeley Unix. Other major players are also working on IPv6. For example, see pages at:

    The 6bone (IPv6 backbone) testbed network has been up for some time. There is an active IPv6 user group.

    One of the design goals for IPv6 was that it must be possible to convert from v4 to v6 via a gradual transition process. Imagine the mess if there were a "flag day" after which the entire Internet used v6, and all software designed for v4 stopped working. Almost every computer on the planet would need major software changes! There would be huge costs to replace older equipment. Implementers would be worked to death before "the day", systems administrators and technical support would be completely swamped after it. The bugs in every implementation would all bite simultaneously. Large chunks of the net would almost certainly be down for substantial time periods. ...

    Fortunately, the design avoids any "flag day". It is therefore a little tricky to tell how quickly IPv6 will take over. The transition has certainly begun. For examples, see announcements from NTT and Nokia. However, it is not yet clear how quickly the process will gain momentum, or when it will be completed. Likely large parts of the Internet will remain with IPv4 for years to come.

    Interoperation with other IPsec implementations

    The IPsec protocols are designed to allow interoperation between different implementations. Other sections of this documentation have more detail on:

    FreeS/WAN does interoperate successfully with many other implementations. The ones we know about are listed below.

    Of course "the devil is in the details" and the IPsec protocols have a lot of details. At least one critique has argued that the protocols should be simplified. Various of those details can and do cause difficulties for interoperation. Should you encounter such problems, please let us know via the mailing list. We will likely be able to help you, and your report may be useful both to other users and to the implementation teams.

    Note: This file is updated often, whenever I notice an interesting interop report on the mailing list. If you are reading the version that ships with a FreeS/WAN release or is posted on the web, and what you need isn't here, consider downloading the latest snapshot to get the latest version of the doc. Perhaps I've added what you need since the last release.

    There is additional information on interoperability testing in our web links section.

    Interoperability problems

    The IPsec RFCs are complex and include a number of optional features. There is considerable opportunity for even two correct, standard-conforming, implementations to disagree on details in a way that blocks interoperation. Errors in either implementation -- either misinterpretations of the standards or just bugs -- can also foul things up.

    The commonest cause of problems, however, seems to be configuration errors. Any IPsec implementation is somewhat complex. It has to be; neither the networks it runs on nor the protocols it implements are simple. When you have two of them to deal with, the problem you face is not trivial.

    That said, FreeS/WAN interoperates successfully with many other implementations. There is a list below, with configuration details provided by various users who have already solved these problems.

    Compatibility between FreeS/WAN and other implementations

    Known areas where problems may appear are:

    The general rule is that to interoperate with FreeS/WAN, the other implementation should be configured for:

    This is possible for most implementations.

    For a more detailed discussion of which parts of the IPsec specification FreeS/WAN implements, and reasons for that, see our compatibility document.

    If it only works in one direction

    A few users have encountered situations in which interoperation is fine when one end initiates, but fails if the other end starts the negotiation.

    In such cases, you can set rekey=no in the FreeS/WAN connection description. This prevents FreeS/WAN from initiating re-keying of that connection, but it will still respond if the partner initiates.

    Even if that trick solves your problem, please report the difficulty. It is definitely supposed to work no matter who initiates.

    "Clients" and "servers"

    The IPsec protocols are specified as peer-to-peer. You might call the machines protected by a gateway "clients" of that gateway, but it is not a client/server protocol. The gateways on the two ends of an IPsec connection are peers, both doing the same thing. In a client/server protocol, the two ends have quite different roles. For example, consider the web. The client runs a browser and mostly does display. The server runs completely different software and does no display at all. IPsec is nothing like that.

    Despite this, various vendors produce products they call "clients" and others they call "servers". Typically, the "clients" do not support a subnet behind them. They are designed only to let a single remote machine connect. To get full IPsec with subnet support, you pay more for the "server version".

    For example, the free version of PGP is only a "client"; for subnet support you need to purchase the product. Also, Windows 2000 Professional has only "client" IPsec; for subnet support you need to purchase the server version, or put Linux and FreeS/WAN on your gateways.

    This difference does not cause interoperation problems as such. FreeS/WAN will happily interoperate with either a "client" or a "server" product, and will happily play either role itself, depending on how it is configured. In their marketing terms, FreeS/WAN acts as a "server" if you define a subnet behind the gateway, and as a "client" if you do not.

    It does, however, often complicate things when users discover that the product they have will not do what they need because it is "only a client".

    Systems that want to use single DES

    Linux FreeS/WAN does not support single DES transforms. Neither Pluto's IKE connections nor KLIPS' IPsec connections can use DES. Since DES is insecure we do not, and will not at any future time, provide it.

    DES is, unfortunately, a mandatory part of the IPsec standard. Despite that, we will not implement DES. We believe it is more important to provide security than to comply with a standard which has been subverted into allowing weak algorithms. See our history and politics section for discussion.

    Some implementations may offer DES as the default. In such cases we urge you to change them to Triple DES. If this is not possible, for example because export laws prevent your vendor from offerring you adequate crytography, we urge you to complain vigorously to one or more of:

    Consider using FreeS/WAN instead. PCs are cheap and we deliver 3DES now.

    FreeS/WAN does have DES code in it as a sort of historical accident, since we need it to implement our default (currently, our only) block cipher, Triple DES. However, since DES is insecure, we do not provide any interface to that code.

    As a matter of project policy, we will not help anyone subvert FreeS/WAN to provide insecure DES encryption .

    Patches to extend interoperability

    Sometimes interoperation requires user-contributed patches or add-ons on the FreeS/WAN end. In general, no patches to the actual IPsec code are required. The problem is to make FreeS/WAN recognise RSA keys stored in formats other than ours. Each such format needs either a patch to make FreeS/WAN understand that format or a utility to translate it to the FreeS/WAN format.

    For example, unmodified FreeS/WAN cannot use RSA keys generated by PGP or keys stored in X.509 certificates, but patches or utilities are available for both those formats. See this list of patches and add-ons .

    Interop HowTo documents

    The FreeS/WAN team does not have the resources to test with anything like the full range of other IPsec implementations out there. Fortunately, some of our users are doing a fine job of filling the gap by providing HowTo information:

    See also our lists of:

    IPsec 2001 files

    From a mailing list report:

         Subject: IPsec 2001 interop demo data available
            Date: Tue, 13 Nov 2001
            From: Ghislaine Labouret <>
    Organization: HSC (Herve Schauer Consultants)
    During the IPsec 2001 conference held in Paris last month, an
    interoperability demonstration including FreeS/WAN was set up.
    FreeS/WAN 1.91 + X.509 patch 0.9.3 was tested with the following
    devices: 6WINDGate, Cisco IOS, Cisco PIX, Cisco VPN 3000, Netasq F100,
    Netcelo VPN gateway, NetScreen NS100, Nortel Contivity, OpenBSD 3.0.
    The results and configuration files are now available online:

    Interoperation with specific products

    Most of the information in this section is gleaned from the mailing list. For additional information, search one of the list archives.

    A large thank you is in order to all the list contributors. This document would not exist without you.

    Anyone who has tested with an implementation not listed here, please report results to the mailing list. I generally include the sender's email address when I quote list messages here; "credit where credit is due". If you would prefer that I not do that with yours, please mention that.

    Older versions of FreeS/WAN

    Any two versions of FreeS/WAN should interoperate, and many combinations have been tested doing so successfully. In particular, every release is tested against its predecessor before it goes out.

    However, if you do encounter a problem involving an older version, we are likely to suggest you upgrade. We do not have the resources to support multiple versions.

    Using your old config files

    In general, new versions will use existing configuration files, at least until the next major version number change. For example, 1.8 can use files created for 1.7, 1.6, even back to 1.0, but not from 0.92. This behaviour will continue until we release 2.0.

    As of 1.8, however, conf file checking has become stricter, so that an error that may have slipped past the checks in an earlier version may be caught in a later one. From 1.8's doc/CHANGES:

          The internal configuration-file reader is progressively getting 
          fussier about what it will accept, which may cause problems for 
          illegal ipsec.conf files whose sins previously passed unnoticed.  
          IN PARTICULAR, the "auto" parameter's values are now checked for 
          legality everywhere.


    Two user-written HowTos we know of cover interoperation between FreeS/WAN and Open BSD IPsec: The OpenBSD FAQ includes information on their IPsec implementation.

    This report is from one of the OpenBSD IPsec developers, a regular participant on our mailing list:

    Subject: spi.c bug
       Date: Tue, 23 Feb 1999
       From: Niklas Hallqvist <>
    PS.  I don't know if you have an interop list anywhere, but you should
    know FreeS/WAN interops with OpenBSD both at the IPSec level and at
    the IKE level.

    There is one known problem with FreeS/WAN-OpenBSD IKE interoperation. Here is a mailing message from our Pluto implementer, discussing that with the user who discovered it:

    Subject: Re: [Bugs] Interoperability with OpenBSD
       Date: Sun, 30 Sep 2001
    | Yes, the problem was the pre-shared key.  It seems that it cannot be
    | more than 64 characters long.  I was using a longer key.
    | Is this documented behaviour for either FreeS/WAN or isakmpd?  (Anyway
    | a reasonable error message would not hurt.)
    The limit is not in FreeS/WAN, so we don't document it :-)
    I guess we could mention this on our interop pages.  Claudia?
    The error message from isakmpd was not very helpful (I realize that
    I'm in a glass house when I throw this stone).
    - isamkpd documentation should state the limit
    - isakmpd should diagnose that the PSK was too long
    - isakmpd should suggest that this type of problem (undigestable
      message) might be caused by mis-matched PSK
    I hope that you would have gotten a better message from Pluto.  So if
    you had initiated from the isakmpd side, the resulting diagnostic from
    Pluto might have lead you to the problem more quickly.
    When Pluto cannot parse the first encrypted IKE message, it prints a
    diagnosis of the parse failure (just like isakmpd did), but it prefixes it
            probable authentication (preshared secret) failure:
    I just noticed that it will print this even if authentication is via
    RSA Sig -- I will fix that.  I'll reword the prefix too:
            probable authentication failure (mismatch of preshared secrets?):


    FreeBSD uses the KAME IPsec and IPv6 code.

    Here is a mailing list message on FreeBSD interoperation:

    Subject: Re: Interop with [Free|Open|Net]BSD
       Date: Fri, 29 Dec 2000
       From: Ghislaine Labouret <>
    On Thu, 28 Dec 2000 13:53:01 -0500, Sandy Harris wrote:
    > FreeBSD:
    > For FreeBSD, I find list discussion of 3DES key formats, presumably for manual
    > keying. We have 192-bit, 3 64-bit keys including parity bits, while FreeBSD 4.0
    > used 168-bit, 3 56-bit keys without the parity bits. Has FreeBSD changed this?
    I still don't understand what made Spike Gronim say that KAME wants a
    168 bits key; I have always been using 192 bits keys with KAME and had
    no interoperability problem between KAME and FreeS/WAN using manual
    > For auto keying, I find reports of sucessful use of pre-shared secrets, but
    > nothing on RSA authentication.
    I had KAME (20001023 snapshot) and FreeS/WAN 1.6 successfully
    interoperate using both PSK and RSA-sig authentication. The config
    files, certificates and test keys used are available online:
    Not much details though, as this is just a report and not a how-to. Will
    improve it if I can find spare time.
    > Does FreeBSD support that? 
    KAME can use RSA-sig and can either exchange certificates online or get
    them from a file. I tested the latter. No test with the X.509 patch for
    FreeS/WAN yet, though that's in my short term plans too.
    > Are the key formats compatible, or has anyone written translation code?
    KAME wants the keys inside certificates, in PEM format. To extract the
    keys for FreeS/WAN I used the fswcert utility, but it can be done "by
    hand" using openssl.


    NetBSD has an IPsec implementation based on KAME. It is described in this FAQ.

    Cisco Routers

    Information from Cisco

    Useful pages on Cisco sites include:

    To work with FreeS/WAN, a Cisco router must have 3DES software. A page on Cisco's site gives this list:

    | Triple DES Encryption for IPSec
    | ...
    | This feature is supported only on the following platforms:
    |     1720
    |     2600 Series
    |     3600 Series
    |     4000 Series
    |     4500 Series
    |     AS5300 Series
    |     7200 Series
    |     7500 Series

    From our mailing list

    Our first Cisco interop success reports were from Ian Calderbank in January and February 1999. They included configuration information for his Cisco 3640. These messages can be found in the mailing list archives or in older versions of this document, still available on the web. We no longer include them here.

    One user has posted configuration files for FreeS/WAN 1.91, Cisco PIX and Cisco 2611 router on this web page.

    This mailing list message with subject "FreeS/WAN and Cisco 3030 VPN Concentrator" has an attached MS-Word document on the setup.

    A sample FreeS/WAN configuration, used in testing with Cisco at an interop conference, is in another list message. Unfortunately, it does not give the matching Cisco configuration.

    Shared secrets work
    Several users report successful interoperation using shared secrets. Here is one such message:
    Subject: [Users] cisco - freeswan summary
       Date: Fri, 31 Aug 2001 
    I finally got a vpn linked up between a 3000 series cisco router and a
    redhat linux box using shared secrets.  The linux box is running 2.2.19 with
    freeswan 1.9. The shared secret has no spaces in it, as I read somewhere
    that it might break the connection.
    Several people have asked me the configuration that I have used to
    make this happen, so I thought I should publish it here.  
    Here is the network...
                    linux router
                  freeswan linux
                       |    yyy.yyy.yyy.1
                       |    yyy.yyy.yyy.21
                   cisco router
    My ipsec.conf looks like this...
    config setup
            # THIS SETTING MUST BE CORRECT or almost nothing will work;
            # %defaultroute is okay for most simple cases.
            # Debug-logging controls:  "none" for (almost) none, "all" for lots.
            # Close down old connection when new one using same ID shows up.
    # defaults for subsequent connection descriptions
    conn %default
            # How persistent to be in (re)keying negotiations (0 means very).
    conn cisco1
    My cisco configuration looks like this...
    crypto map VPN 30 ipsec-isakmp   
     set peer
     set transform-set 3des-md5 
     match address 130
    crypto ipsec transform-set 3des-md5 esp-3des esp-md5-hmac 
    crypto isakmp key ******** address 
    crypto isakmp policy 3
     encr 3des
     hash md5
     authentication pre-share
     group 2
    access-list 130 permit ip
    Sorry it took so long to send this out, but there is apparently an ipchains
    firewall on the host behind the cisco, and it took some time to get
    these rules straight.
    I hope this helps someone....
    Another similar post:
    Subject: [Users] freeswan <--$gt; Cisco: success!
       Date: Thu, 20 Sep 2001
       From: "Wolfgang Tremmel" <>
    I have seen several requests on the list for example
    configurations for connection of Freeswan to Cisco, since
    about 1 hour ago I had success, here my example configuration.
    Background: My router is a 80486, connecting to the internet using
                    PPPoE via DSL. Kernel is 2.4.9, Freeswan is snap2001sep13b
                    And yes, I get a dynamic IP address
                    Router is a Cisco 4700, running IOS 12.2(2)T1 with 3DES
    config setup
    conn %default
    conn cisco
            right=x.x.x.x                   # fastEthernet0 of ciscorouter
            rightnexthop=            # next hop of ppp0
            rightsubnet=           # defaultroute via ipsec
    %any x.x.x.x: PSK "thisisatestkey"
    - ---- thats all on the linux router
    - ---- now for the Cisco:
    crypto isakmp policy 100
     encr 3des
     authentication pre-share
     group 2
     lifetime 3600
    crypto isakmp key diesisteintest address y.y.0.0  !
    network and mask of any possible address your ppp0 can have
    crypto ipsec transform-set linuxbox esp-3des esp-sha-hmac
    crypto dynamic-map linuxbox 100
     set transform-set linuxbox
     match address linuxbox
    crypto map linuxbox local-address FastEthernet0
    crypto map linuxbox isakmp authorization list linuxbox          ! not sure if
    that is really needed
    crypto map linuxbox 100 ipsec-isakmp dynamic linuxbox discover
    interface FastEthernet0
     ip address x.x.x.x
     crypto map linuxbox
    ip access-list extended wtremmel
     permit ip host x.x.x.x mask
    RSA keys are tricky
    A message from another user about using RSA keys with Cisco:
    Subject: Re: [Users] RSA public key and Cisco (3640)
    Date: Sat, 2 Jun 2001
    We use Cisco IOS 12.1.5(T) and freeswan 1.8
    Here an example on how I copied the key from cisco:
    Key Data:
      117C311E 16192D86 8886C71D 11111115 11138B11 31881241 11C7E23B D6DB22 
    Will become
    We used at least 1024 bits long keys.
    But it doesn´t matter. The problem is that cisco doesn´t agree with the RSA
    schema from freeswan, I think. In Cisco, rsasig is to use with a CA, and
    rsaencript did not work as well. 
    My case is worse than it. My first intention was to use freeswan in a road
    warrior config. I really need to use CA, as Cisco needs a fix address to
    use rsa public key. The public key to cisco is always associated to an IP
    address ou FQDN. I quit. Will try the X509 patch and the Open CA software.
    >>(off list)
    >Yes, I was just going to mention that the Cisco's key should be in
    >ipsec.conf (just received your correction).
    >I think that I have the Cisco side configured correctly ( I can't be sure
    >because I can't test against the Freeswan).
    >Starting from having the IPsec tunnel working with pre-share, I did the
    >following on the Cisco side:
    >#config t
    >(config)# crypto key pubkey-chain rsa
    >(config-pubkey-chain)# addressed-key 
    >(config-pubkey-key)# key-string
    ># config t
    >(config)# crypto isakmp policy 1
    >(config-isakmp)# no authentication pre-share
    >(config-isakmp)# authentication rsa-sig
    >(config-isakmp)# exit
    >How long is your RSA key that was generated on the Cisco? I tried copying
    >the key out of the 3640 and pasting it into ipsec.conf, removing the spaces
    >and adding a '0x' in the front. I get the 'key too small' error still. What
    >version of freeswan are you using?
    >I'm using Freeswan 1.9 w/ IOS 12.1(6).

    Cisco VPN Concentrator

    Another mailing list thread discussed using FreeS/WAN with the Cisco VPN Concentrator. Here is one user describing his problem:
    Subject: [Users] FreeSWAN and Cisco VPN CONCENTRATOR
       Date: Thu, 25 Oct 2001
       From: "M. Sticki" <>
    i have to establish a vpn tunnel between two companies.
    one of the company is using the cisco vpn concentrator
    and the other company is using redhat 7.1 and freeswan.
    it is no a problem to estblish the tunnel between two freeswan
    gateways or between a cisco vpn-client and the concentrator.
    but the companies don't want to change their equipment.
    and with this constellation i can't establish the tunnel.
    the responce from cisco is: "THAT IS NOT SUPPORTED"
    so this mailing list is my last chance, because i don't know how to go on
    and another user's answer:
    Subject: Re: [Users] FreeSWAN and Cisco VPN CONCENTRATOR
       Date: Thu, 25 Oct 2001 14:21:17 +0200
       From: Ghislaine Labouret <>
    > i have to establish a vpn tunnel between two companies.
    > one of the company is using the cisco vpn concentrator
    > and the other company is using redhat 7.1 and freeswan.
    > the responce from cisco is: "THAT IS NOT SUPPORTED"
    At the IPsec 2001 conference which is behing held right now, we have
    set up an interop demo platform which includes those two devices.
    They are successfully interoperating using certificates.
    Later in the same thread:
    Subject: Re: [Users] FreeSWAN and Cisco VPN CONCENTRATOR
       Date: Thu, 25 Oct 2001
       From: Ghislaine Labouret <>
    Organization: HSC (Herve Schauer Consultants)
    Juri Jensen wrote:
    > I've been trying to get those two to interoperate with certificates, but
    > I have only succeeded with PSK. Can you shed some light on how you did
    > it....?
    I will put the config files and different tests results from the demo on next week.
    With VPN3000, we had a problem with the DN comparison because of
    encoding issues. The solution was to specify the VPN 3000 DN in binary
    format in ipsec.conf. I leave it to Andreas Steffen to explain the exact
    issue, as he is the one who solved it.

    Nortel (Bay Networks) Contivity switch

    There is one known issue in FreeS/WAN-to-Contivity interoperation. Recent versions of FreeS/WAN no longer support DH group 1 for key exchange. Older versions of Contivity software support nothing else. Group 2 was added in more recent releases. So: We recommend using the latest Contivity release.

    Some messages from the mailing list:

    Subject: Contivity Extranet Switch
       Date: Fri, 11 Jun 1999
       From: Matthias David Siebler <>
    Organization: Nortel Networks
    More interoperability results:
    I successfully established a tunnel with a Nortel (formerly Bay (formerly New Oak)) Contivity
    Extranet Switch running the latest release versions.
    The CES is running V2.50 of the software and the Linux server is running V1.0.0 of the Free/SWAN
    code on a RedHat 5.2 unit with the kernel upgraded to 2.0.36-3
    I am using IKE with 3DES-HMAC-MD5
    Note however, that tunnels cannot yet be configured as client tunnels since Free/SWAN does not yet
    support aggressive mode.  Hopefully, that will arrive soon, which would allow remote users to
    connect to a CES using the Free/SWAN code as clients.

    and apparently Nortel want their product to work with FreeS/WAN:

    Subject: Is FreeSwan 3.1 a legitamate ipsec implementation when compared to its commercial competitors?
       Date: Tue, 02 May 2000
       From: Bill Stewart <>
    Nortel's Contivity IPsec server has a formal policy of interoperability
    with FreeS/WAN.   I was quite pleased to hear it when they last talked to us,
    and it makes sense in their business environment, since they let you use
    their WinXX client software free, so this gives them support for Linux

    A more recent mailing list report is:

    Subject: Nortel Contivity and Free-S/WAN
       Date: Wed, 7 Mar 2001
       From:  "JJ Streicher-Bremer" <>
    OK, here is a very brief nuts and bolts breakdown on how to get this
    combo working.  I want to thank everyone at Free-S/WAN and everyone on
    the list for your help in getting this to work.
    Connecting FreeS/WAN to the Nortel Networks Contivity Extranet Switch:
    What you need:
    FreeS/WAN v1.5 and Contivity ver 2.5 - 3.5 (might work with earlier
    versions, but I have not tested it with this config)
    FreeS/WAN v1.8 and COntivity ver 3.5 (the 3.5 version supports Diffe
    Hilman group 2 key exchange)
    What to do:
    1 - Configure the Contivity:
       Set up a branch office tunnel group with the following settings:
            Nailed Up: Disabled
            Access Hours: Anytime
            Call Admission Priority: Highest Priority
            Forwarding Priority: Low Priority
            Idle Timeout: 00:00:00
            Forced Logoff: 00:00:00
            RSVP: Disabled
            RSVP: Token Bucket Depth: 3000 Bytes
            RSVP: Token Bucket Rate: 28 Kbps
            Branch Office Bandwidth Policy: 
            - Committed Rate: 56 Kbps
            - Excess Rate: 128 Kbps
            - Excess Action: Mark
            - ESP - Triple DES with SHA1 Integrity: Enabled
            - ESP - Triple DES with MD5 Integrity: Enabled
            - ESP - 56-bit DES with SHA1 Integrity: Disabled
            - ESP - 56-bit DES with MD5 Integrity: Disabled
            *IKE Encryption and Diffie-Hellman Group: Triple DES with Group
    2 (1024-bit prime)
            Vendor ID: Disabled
            Perfect Forward Secrecy: Enabled
            Compression: Disabled
            Rekey Timeout: 08:00:00
            Rekey Data Count:  (None) 
            *ISAKMP Retransmission Interval: 16
            *ISAKMP Retransmission Max Attempts: 4
            Set up a branch office tunnel inside this new group with the
    following settings:
            Endpoint Addresses
            Local - Public address of your COntivity
            Remote - Your Free-S/WAN interface Address
                    Tunnel Type - IPSEC
            IPSEC Authentication - Text Pre-Shared Key
                    One note here, I have had some trouble trying to use HEX
    or Non alphanumeric chars in this key.
            Under IP:
            Static Routing
            Local - networks you want to be able to access through the
            Remote - networks that will be allowed through the tunnel
            NAT - None
       Get routing setup on your office network:
            You will need to get a routing entry that will point all traffic
    bound for your home network (the one that will be acciessible through
    the tunnel) to the internal interface of the contivity system.
       Configure Free-S/WAN:
            Install, compile, and test Free-S/WAN
            Edit ipsec.conf for your new tunnel:
    ipsec.conf --
    config setup
    conn net1
    conn net2
    ipsec.secrets -- "Your big secret"
    The above config is for this imaginary network:
             +------+ |      | Internet  
    ---------|      |-------------------++===========
             +------+            Home Router         
             Free-S/WAN host
    Internet ++ These
    =========++--------------####--------- are here somewhere
       Office Router       Contivity
       This has worked for me.  I am still having trouble with the tunnels
    dying after about 30-40 minutes of non-use.  Don't know what that is
    about, but I'll keep you posted.

    Raptor Firewall

    Raptor 5 on NT (old info)

       Subject: Interoperability with Raptor 5 (Success!)
       Date: Wed, 06 Jan 1999 16:19:27 -0500
       From: Chuck Bushong <>
    I don't know if this is useful information for anyone, but I have
    successfully established a VPN between RedHat 5.1 (kernel 2.0.34) running
    FreeS/WAN 0.91 and NT4 running Raptor 5.  However, Pluto does not appear
    compatible with the Raptor IKE implementation. . . .
    Subject: RE: Interoperability with Raptor 5 (Success!)
    Date: Thu, 28 Jan 1999 17:22:55 -0500
    From: Chuck Bushong <> 
    ... this VPN (at least the klips end) has been up under minimal
    utilization for three weeks plus without interruption.  The
    machine seems very stable.  Pat yourself on the back, gentlemen.
    Your beta release is more stable than certain companies' shipping
    Keep up the good work.

    Raptor 6 on Solaris

    Subject: Re: successful interop. with Raptor 6.02 
       From: "Charles G. Griebel" <> 
       Date: Tue, 25 Jul 2000
    On Thu, Jul 20, 2000 at 12:04:40PM -0700, Kevin Traas wrote:
    > Great!  I'm just about to start looking into this as well, so any
    > docs/info you can provide would be *greatly* appreciated.  Immortalize
    > yourself!  Get something written and added to the compatibility.html
    > file.  Many will thank you.
    Can't be that hard.  I'm just a freeswan newbie who hasn't even done a FS
    tunnel yet :)
    Anyway, I hope you find this helpful.
    Automatically keyed 3DES VPN between Raptor 6.02 on Solaris 2.6 (left) and
       FreeS/WAN 1.5 on 2.2.16 Intel (right)
    FreeS/WAN (right) information:
    config setup
            interfaces="ipsec0=ppp0"    # change to suite
    conn sample
    # note I haven't verified that underscores will actually work PSK "some_long_secret_with_plenty_of_chars"
    Raptor 6.02 (left) information:
    Key Profiles:
        Name: left-external-kp-dynamic
        Type: Dynamic
        Profile Describing: local entity
        Identification Type: Address
        ISAKMP Hash Method: MD5
        ISAKMP Authentication: Shared_Key
        Shared Secret: some_long_secret_with_plenty_of_chars
        Time Expiration: 1080
        Name: right-external-kp-dynamic
        Type: Dynamic
        Profile Describing: remote entity
        Identification Type: Address
    Secure Subnets:
        Name: left-ss-dynamic
        Key Profile: left-ss-dynamic
        Name: right-ss-dynamic
        Key Profile: right-ss-dynamic
    Secure Tunnel:
        Name: left-to-right-tunnel
        Entity A: right-ss-dynamic
        Entity B: left-ss-dynamic
        Encapsulation: ISAKMP
        Filter: [none]
        Pass traffic through proxies: [unchecked]
        Use Authentication Header: [unchecked]
        Use Encryption Header: [checked]
        Data Integrity Algorithm: MD5
        Data Privacy Algorithm: 3DES
        [Advanced settings]
        Data volume timeout: 2100000
        Lifetime timeout: 480
        Inactivity timeout: 0
        Transport mode: [unchecked]
        Perfect forward secrecy: [unchecked]
        Proxy: [checked]
    I made the addresses fictitious RFC1918 addresses.
    I haven't tried PFS.
    I had problems getting an SA with manual keying -- I think it may be with the

    Raptor manual keying

    A mailing list suggestion from FreeS/WAN technical lead Henry Spencer:
    > In the Raptor settings, there are 2 sets of data (1 for each end). Each set
    > contains an SPI, 3 DES Keys and 1 MD5 hash. I only know how to include one
    > set, how do I include the other set? Is the other set needed?
    They may be using different keys for each direction, which is a bit
    unusual for manual keying, but not impossible.  The simplest thing is
    probably to just give it two identical sets of data -- that should work.
    FreeS/WAN has provisions for asymmetric keys etc. in manual keying, but
    that stuff is lightly documented and lightly tested.

    Gauntlet firewall GVPN

    Subject:  Successful interop: FreeS/WAN 1.7 
     Gauntlet Firewall GVPN 5.5
       Date: Tue, 21 Nov 2000
    Sending the following to the list, at Hugh's request.
    -----Original Message-----
    From: Reiner, Richard 
    Sent: Tuesday, November 21, 2000 11:34 AM
    To: ''
    > Good.  But we don't think that you should be using our IPCOMP just
    > yet.  It is flaky :-(
    I've seen no anomalies, although "allow ipcomp" is on at the Gauntlet 
    end.  Looking at my ipsec.conf I actually find no refereence to ipcomp. 
     I presume it is disabled by default.  In addition, reviewing my logs 
    both on the Gauntlet end and the Linux end, I see nothing I can 
    interpret as an indication that ipcomp was enabled during negotiation.  
    So I have to correct my previous posting - I believe the link is *not* 
    using ipcomp.
    > This is interesting and we'd love a more complete writeup.  It should
    > get incorporated into our interop documentation.
    Here are the relevant bits from ipsec.conf:
    config setup
    conn freeswan17-gauntlet55
    All settings on the Gauntlet side are the same (not shown here, as GUI 
    screenshots are hard to show in ASCII... and the textual format that is 
    generated by the Gauntlet GUI is ugly in the extreme).
    Note that ikelifetime is 1440m by default on the Gauntlet end, but 
    freeswan does not support this value (max appears to be 480m), thus the 
    Gauntlet end is also set to 480m to match freeswan's value.
    Also worth noting: I am using the excellent Seawall scripts to manage 
    ipchains configuration on the freeswan end.  It automatically generates 
    a correct set of firewall rules for the link (along with doing many 
    other convenient things).
    For more information on Seawall (the Seattle Firewall), see that project's home page on Sourceforge.

    Checkpoint Firewall-1

    A PDF HowTo for connecting FreeS/WAN and this product can be downloaded from the vendor's site or browsed at a VPN mailing list site.

    A resource page full of Firewall-1 information.

    The mailing list reports success with this combination, but also some problems. Search the archives for the full story.

    Here is one message, about what seems to be the biggest problem:

    Subject: Re: Pb establishing connection from FW1/3DES/SP2 with freeswan 1.5 - ACTE 2
       Date: Tue, 6 Feb 2001
       From: Claudia Schmeing <>
    > Thanx to Michael and Claudia, but this doesn't work from VPN1 to linux (as
    > linux to VPN1 is OK).
    > I think that VPN1 doesn't send "" but "" and,
    > as Claudia said, IPSEC SA need to match Exactly. 
    I don't know about the rules on the VPN-1. You'll have to rely on people 
    with applicable experience there...
    > Is it possible that freeswan doesn't do the inclusion process (ie if he
    > receive, i doesn't match that this is include in
    > ?
    Yes, that's correct. It needs to match exactly, and inclusion is not
    part of this process.
    > Btw why VPN/1 send and not (the value that
    > Freeswan is waiting for)? A bug?
    I think Michael may be able to help you with this.
    > Have i a way to force Freeswan to do the "inclusion" (ie accept 
    > as a part of, even if the 2 IPSEC Sa 
    > doesn't match exactly) ?
    No, but...
    Another strategy is to accept the fact that the Checkpoint 
    proposes separate connections for each machine. If you define 
    and add each of these connections on the Linux FreeS/WAN side, then 
    Linux FreeS/WAN ought to accept the Checkpoint's proposals.
    The only possible difficulty with this strategy is that I don't know 
    how Linux FreeS/WAN handles the concept of overlapping tunnels. I
    believe, though, that these tunnels can coexist, and if for any 
    packet there are two options, a more general and a less general, the
    packet will be handled by the more specific tunnel. You would need
    to do a little testing to ensure you understand the behaviour and
    that this does actually solve your problem.
    I think it would be simplest to try to get the Checkpoint to propose the 
    more general tunnel. Since I don't recall having seen this problem before, 
    I suspect the simpler solution is doable.

    Redcreek Ravlin

    We have reports of successful interoperation at an interop conference, but there is also a mailing list thread discussing difficulties some users have encountered.

    SSH Sentinel

    The vendor's web site has configuration examples for use with FreeS/WAN.

    One user reports:

    Subject: [Users] Very Useful document, can a link to it be put on the FreeS/WAN web site?
       Date: Fri, 19 Oct 2001
       From: Simon Matthews <>
    This is a very useful document on getting SSH Sentinel to work with 
    FreeS/WAN using x509 certificates.
    Perhaps a link to it could be put on the web site.
    There is also another document on FreeS/WAN  SSH Sentinel interoperability:

    The vendor seems serious about interop with us. Here is a message one of their staff posted on our list:

    From: Jussi Torhonen <>
    Organization: SSH Communications Security Corp -
    Subject: [Users] SSH Sentinel VPN client public beta #3 now available
    Date: Thu, 31 May 2001
    Hello, FreeS/WAN community !
    SSH Communications Security Corp has released a new public beta #3
    version of SSH Sentinel VPN client for Windows. We've got a lot of
    reports also from FreeS/WAN community and with that feedback we've
    improved interoperability and stability. 
    For example PFS (Perfect Forward Secrecy in IKE rekey) can now be used
    between SSH Sentinel and FreeSWAN, and if using that user contributed
    X.509 patch and exporting the certificate from SSH Sentinel, now those
    -----[BEGIN|END] CERTIFICATE----- headers/footers are properly included
    in the exported PEM formatted certificate, so it can be imported to
    FreeSWAN with fswcert utility and OpenSSL tools. 
    Thank you a lot for your feedback, colleagues !
    You can get that new public beta #3 and PDF formatted User Manual from or via website
    For more information about the product, please check our website
    We eagerly want to make SSH Sentinel as the best VPN client on the
    market. If you want to contact our support, please send e-mail to or fill up our feedback form at
    Best regards,
    Jussi Torhonen, SSH Sentinel Team
    Kuopio, Finland
    There is one known problem withh SSH-FreeS/WAN interoperation, described in this message:
    Subject: Re: [Users] Any plans for AES / Rijndael support ?
       Date: Tue, 11 Dec 2001
       From: Jussi Torhonen 
    Organization: SSH Communications Security Corp -
    Markus Weber wrote:
    > ... the installation
    > of Sentinel don't let you set 3DES as the default!
    > And when your want to add a connection the first
    > diagnostic-test goes wrong ! :-( ...
    In current SSH Sentinel release you can select 'Legacy proposal' option, 
      when setting up a VPN Connection profile. That causes it to use 3DES 
    as a default cipher and DES as a alternative one. The option was added 
    there just to improve interoperability with legacy systems supporting 
    3DES or even DES only.
    If no selecting Legacy Proposal option, SSH Sentinel sends quite a huge 
    proposal list to the responder to find automatically one common cipher 
    supported to be used for the connection. That proposal list is known to 
    be problematic for some VPN gateway implementations like FreeSWAN. 
    Typically the long proposal list itself may the a problem or fragmented 
    packets of the long proposal list may be a probem.
    Now we've been living in a world of DES and 3DES, but hopefully in a 
    near future the use of AES/Rijndael will increase. ...

    FreeS/WAN does not yet support AES.

    F-Secure VPN for Windows

       Subject: Identification through other than IP number
       Date: Tue, 13 Apr 1999
       From: Thomas Bellman <>
    ... Currently we are trying to interop FreeS/WAN
    with F-Secure VPN+ Client 4.0 (for MS Windows), and as long as
    the Windows machine has a fix IP address, and are initiating the
    IKE negotiations, things are working well.  However, when the IP
    address is changing, it doesn't work. ...
    (I'll try to write something up about the problems we are having
    when Pluto is initiatior in another message.)


    Watchguard make a Linux-based firewall product. Ipchains author Rusty Russell thanks them for support and recommends them in one of his HowTos . On the other hand, some comments on our mailing list about the Watchguard product have been quite unfavourable. See, for example, this archive message.

    Watchguard do not use FreeS/WAN in their product. They have their own IPsec implementation.

    We have had mailing list reports of successful interoperation between FreeS/WAN and the Watchguard firewall, using manually keyed connections. The user could not get automatically keyed connections to work; the message below explains this.

    Here is some mail from a Watchguard employee about interoperation:

    Subject: FreeS/WAN and WatchGuard Firebox interop
       Date: Mon, 18 Dec 2000
       From: Max Enders <>
    I was recently given the task of testing IPSec interoperability
    with our product, the Firebox. I just wanted to let you know that
    I had success with a manual keyed tunnel. Here's what I used for
    my test:
    RedHat Linux 6.2
    Linux 2.2.18 i686 unknown
    Linux FreeS/WAN 1.8
    "Trusted" interface:
    "External" interface:
    Firebox II FastVPN
    WatchGuard Live Security System v4.5
    Trusted interface:
    External interface:
    Because FreeS/WAN does not implement single DES, a dynamic keyed
    tunnel will not work. Our product strictly uses DES for main mode.
    We hope to address this in a future release. Here are instructions
    for configuring the Firebox:
    Open the Policy Manager and create a new IPSec gateway. Set the Key
    Negotiation Type to manual and enter the FreeS/WAN box's external
    IP address for the Remote Gateway IP. Configure a new tunnel with
    a unique SPI. Select 3DES-CBC for Encryption and MD5-HMAC for
    Authentication. Make an Encryption Key and Authentication Key.
    Copy the values and save them for configuration of the FreeS/WAN box.
    Configure a routing policy and any necessary services as you normally
    Here's how I configured FreeS/WAN:
    Modifications to /etc/ipsec.conf:
    Under the "config setup" section, add:
    At the end of the file, add the following connection:
    conn firebox
    The spi used here should match the Firebox's. Note that the Policy Manager
    expects an SPI in decimal, not hexadecimal. The espenckey value should be
    0x and the Encryption Key you're using on the Firebox. Likewise for
    espauthkey and the Authentication Key on the Firebox.
    A user comments:
    Subject: RE: Freeswan
       Date: Wed, 7 Feb 2001
       From: "Patrick Poncet" <>
    It's working!!!
    Voila...  I wish to thank all the FreeS/WAN for putting out such a great
    product out!  And also Philippe PAULEAU who pioneered interoperability
    between FreeS/WAN and Watchguard Firebox II and therefore showed me that my
    efforts would not be wasted!...
    Yes indeed FreeS/WAN to WatchGuard Firebox only works in manual keying mode
    and the best way to generate keys is to have the firebox generate the keys,
    then copy and paste into the ipsec.conf file on the FreeS/WAN side (don't
    forget to prefix the keys with '0x' in your ipsec.conf file.
    Also keep in mind that the SPI is in decimal on the Firebox side and HEX on
    the FreeS/WAN side!!!  We spent 4 hours on fixing this HEX-DEC issue only :)

    Xedia Access Point/QVPN

       Subject: Interoperability result
       Date: Mon, 15 Mar 1999 18:08:12 -0500
      From: Paul Koning <>
    Here's another datapoint for the "FreeS/WAN interoperability
    I tested 0.92 against the Xedia Access Point/QVPN product, using
    dynamic keying (i.e., Pluto at work).
    Results: it works fine so long as you ask for 3DES.  DES and no-crypto 
    modes don't work when Pluto is involved.
    I did limited data testing, which seemed to be fine.  No performance
    numbers yet, could do that if people are interested.
    Any questions, please ask.

    PGP Mac and Windows IPsec Client

    Since version 6.5, the PGP products from PGP Inc. have included an IPsec client program.

    Here is the first message about it to our mailing list, from a senior PGP employee:

       Subject: PGPnet interoperable with FreeSWAN
       Date: Mon, 05 Apr 1999 18:06:13 -0700
       From: Will Price <>
    Network Associates announced PGP 6.5 today.  It includes a new product
    PGPnet which is a full IKE/IPSec client implementation.  This product
    is for Windows and Macintosh.  I just wanted to send a brief note to
    this list that the product was compatibility tested with FreeSWAN
    prior to its release, and the tests were successful!
    - -- 
    Will Price, Architect/Sr. Mgr., PGP Client Products
    Total Network Security Division
    Network Associates, Inc.

    One version is downloadable at no cost for non-commercial use. See our links. That version does not support subnets.

    Several of the user-written HowTos mentioned above cover interoperation between PGPnet and FreeS/WAN.

    A more recent post from the same PGP Inc staff member pointed out:

    Make sure you're using PGP 7.0 or later as the key parser was improved
    in that release.  (PGP 7.0.1 was just released)

    Various users have reported various successes and problems talking to PGPnet with FreeS/WAN. There has also been a fairly complex discussion of some fine points of RFC interpretation between the implementers of the two systems. Check an archive of our mailing list for details.

    A post summarising some of this, from our Pluto programmer:

       Subject: PGPnet 6.5 and freeswan
       Date: Sun, 16 Jan 2000
       From: "D. Hugh Redelmeier" <>
    | From: Yan Seiner
    | OK, I'm stumped.  I am trying to configure IPSEC to support road
    | warriors using PGPnet 6.5.
    | I've set up everything as per the man pages on the ipsec side.
    | I've set up everything on the PGPnet side per the docs for that package.
    | Pluto fails with this:
    | Jan 16 08:14:11 aphrodite Pluto[26401]: "homeusers" #8: no acceptable
    | Oakley Transform
    | and then it terminates the connection.
    As far as I can tell/remember, there are three common ways that PGPnet
    and FreeS/WAN don't get along.
    1. PGPnet proposes a longer lifetime for an SA than Pluto is willing
       to accept.
    2. After rekeying (i.e. after building a new SA bundle because the old
       one is about to expire), FreeS/WAN immediately switches to the new
       one while PGPnet continues using the old
    3. FreeS/WAN defaults to expecting Perfect Forward Secrecy and PGPnet
       does not.
    Perhaps you are bumping into the first.  In any case, look back
    in the log to see why Pluto rejected each transform

    Some advice from the mailing list:

       Subject: Re: Secure Gate Fails- PGPNet & FreeSwan
       Date: Wed, 28 Jun 2000
       From: Andreas Haumer <>
    I have a PGPnet setup running with FreeS/WAN working as secure 
    gateway. It works quite fine, except for a re-negotiation problem 
    I'm currently investigating, and in fact I have it running on some
    test equipment here right now!
    As I tried _several_ different non-working configuration settings 
    I think I know the exact _one_ which works... :-)
    Here's my short "HOWTO":
    FreeS/WAN version: snap1000jun25b
    PGPnet: PGP Personal Privacy, Version 6.5.3
    Linux: 2.2.16 with some patches
    Network setup:
    internal subnet [192.168.x.0/24]
    |        [192.168.x.1]
    secure gateway with FreeS/WAN
    |        [a.b.c.x]
    |        [a.b.c.y]
    router to internet
    |   Internet
    |        [dynamically assigned IP address]
    road-warrior with PGPnet
    Configuration of FreeS/WAN:
    a) /etc/ipsec.conf
    config setup
    conn %default
    conn gw-rw
    conn subnet-rw
    b) /etc/ipsec.secrets
    a.b.c.x "my very secret secret"   
    Note: If you are running ipchains on your secure gateway,
    you have to open the firewall for all the IPsec packets 
    and also for traffic from your ipsec interface!
    Don't masquerade the IPsec traffic!
    Check your logfiles if the firewall is blocking some 
    important packets!
    Configuration of PGPnet:
    (note that there is an excellent description, including
    screenshots of PGPnet, on <>)
    In short, do the following:
    Launch the PGPnet configuration tool and set defaults options
    Start - Program - PGP - PGPnet
    View - Options
    General Panel :
      Expert Mode
      Allow communications with unconfigured hosts
      Require valid authentication key
      Cache passphrases between logins
      IKE Duration : 6h
      IPsec : 6h
    Advanced panel :
      Selected options :
        Ciphers : Tripple DES
        Hashes : MD5
        Diffie-Hellman : 1024 and 1536
        Compression : LZS and Deflate
      Make the IKE proposal :
        Shared-Key - MD5 - 3DES -1024 bits on top of the list (move up)
      Make the IPSec proposal :
        NONE - MD5-TrippleDES -NONE on top of the list (move up)
      Select Perfect Forward Secrecy = 1024 bits
    Press OK
    Create the connection's definition.
    In the Hosts panel, ADD
      Name : Enter a name for the right gateway
      IPaddress : Enter its IP address visible to the internet (a.b.c.x)
      Select Secure Gateway
      Set shared Paraphrase : enter you preshared key
      Identity type : select IP address
      Identity : enter
      Remote Authentication : select Any valid key
    Press Ok
      Select the newly created entry for the right gateway and click ADD,
      Name : Enter a name for the central subnet
      IP address : Enter its network IP address (192.168.x.0)
      Select Insecure Subnet
      Subnet Mask : enter its subnetmask (
    Press OK, YES, YES                             
    This should be it. Note that with this configuration there is
    still this re-keying problem: after 6 hours, the SA is expired
    and the connection fails. You have to re-connect your connection
    with PGPnet.

    and a note from the team's tech support person:

    Date: Thu, 29 Jun 2000
    From: Claudia Schmeing 
    There is a known issue with PGPNet which I don't see mentioned in the docs.
    It's likely related to this one, that you note on the site:
    >2. After rekeying (i.e. after building a new SA bundle because the old
    >   one is about to expire), FreeS/WAN immediately switches to the new
    >   one while PGPnet continues using the old
    The issue is: When taking down and subsequently recreating a connection, 
    it can appear to come up, but it is unusable because PGPNet continues
    to use an old SA, which Linux FreeS/WAN no longer recognizes. The solution is
    to take down the old connection using PGPNet, so that it will then
    use the most recently generated SA.

    IRE Safenet/SoftPK

    IRE have an extensive line of IPsec products, including firewall software with IPsec, and hardware encryption devices for LAN or modem links. Their Soft-PK is a Win 98 and NT client. Quite a few people have used this with FreeS/WAN and, judging by mailing list reports, have had good results.

    SoftRemote is newer product integrating the IPsec client with personal firewall software. As yet, we have few reports on this. One is quoted below.


    Several documents are available:

    Some messages from the mailing list:

    Subject: Re: Identification through other than IP number
    Date:  Fri, 23 Apr 1999
    From:  Tim Miller <>
    Randy Dees writes:
     > Anyone know of a low-cost MS-Win client that interoperates and does not
     > require purchasing a server license to get it?  
            SafeNet/Soft-PK from IRE ( is a low-cost
    client (though I don't have the exact cost available at the moment).
    I've got it running on an NT4 workstation and it interoperates nicely
    (in transport mode, will try tunnel later) with FreeS/WAN.  It's also
    ICSA IPsec certified.
    A later report with some setup details:
    Subject: RE: PGPnet and Freeswan one more time...
       Date: Sat, 16 Dec 2000
      From: "Tim Wilson" <>
    Here are some details about using the IRE SafeNet Soft/PK client with a
    FreeSwan gateway.
    I applied the x509 patch to Pluto according to the instructions. I use the
    "leftcert" and "rightcert" keywords in the ipsec.conf file. This causes
    FreeSwan to read the public keys and identities from the cert files. The
    identities wanted and used by FreeSwan will then be the DNs in the certs.
    I used OpenSSL to generate keys and certs and to sign certs. When generating
    the gateway cert, you should *not* enter an e-mail address because it turns
    out that confuses Soft/PK. Also, Andreas Steffan tells me that you want to
    keep the cert short to avoid fragmentation, so use a 1024-bit RSA key and
    succinct names.
    The FreeSwan gateway cert goes in /etc/ipsec.d/, the gateway private key is
    extracted from the key file using fswcert (part of the x509 patch) and
    pasted into /etc/ipsec.secrets, and a DER version of the gateway cert goes
    in /etc/x509cert.der. This is all according to the instructions that
    accompany the x509 patch.
    The Windows client is of course running Soft/PK in this case. I generated a
    private key and cert for the client on the Linux machine using OpenSSL. I
    created a pkcs12 file containing the client's private key and cert, which I
    put on a floppy and imported into Soft/PK. I also imported the gateway cert
    into Soft/PK (you can either import a self-signed cert for the gateway or
    the self-signed cert for the CA that signed the gateway cert--either works).
    Soft/PK allows you to configure practically everything for the connection.
    Here are the main points to watch for:
    On the first panel you have to set the peer identities. The gateway will
    identify itself using the DN in the gateway cert. So of course you have to
    configure Soft/PK to look for the correct DN. There's no problem with this
    as long as you didn't enter an e-mail address in the gateway cert. Just
    check "Connect using Secure Gateway tunnel", set ID type to "Distinguished
    Name", and enter the correct info in the dialog box.
    In "My identity" just select the client cert that you imported in pcks12
    format. Soft/PK apparently identifies itself with the DN from the cert,
    which is exactly what FreeSwan is looking for.
    In "Security Policy", you want Main mode and make the PFS setting agree with
    whatever FreeSwan is doing (FreeSwan uses PFS by default). If you use PFS I
    believe you must use DH group 2 as FreeSwan doesn't like group 1.
    Phase 1 Authentication must be "RSA signatures" and 3DES plus either MD5 or
    SHA-1 (I used MD5 but I believe FreeSwan accepts either). I left the
    lifetime unspecified. Also you must select DH group 2 because I believe that
    FreeSwan will not accept group 1.
    Phase 2 also must use 3DES and MD5 or SHA-1. I used no compression and only
    ESP and no AH, haven't tried other choices.
    Hope this helps.


    Here is a mailing list message reporting experience with the newer SoftRemote product.
    From: Whit Blauvelt <>
    Subject: Re: [Users] RE: SafeNet SoftRemote 6.1 - FS 1.91 - HOW?
    Date: Fri, 16 Nov 2001
    Things I've learned in getting SoftRemote working with FreeS/WAN:
    Using SoftRemote on dial-in, if there is any configuration adjustment for
    which you stop and start FreeS/WAN, SoftRemote is totally lost until you
    disconnect and dial in again. The SoftRemote "Disconnect All" and subsequent
    "Reload Polilcies" options that show when right-clicking its icon in the
    tray do not fix this - the only thing I've found that works is hanging up
    and then redialing. This makes debugging a total pain, especially if you
    think you're testing your changes, because you're not. 
    Not sure if this affects fixed IP connections from SoftRemote, or what the
    effective equivalent to hanging up and dialing in again would be to clear
    the problem there if it exists.
    Also, as I noted before: In configuring SoftRemote, there are a couple of
    new menu options that Soft-PK didn't have, just in case you're following
    examples given for that. Importantly, in setting the "Remote Party Identity
    and Addressing" choose "IP subnet" rather than "IP", and be sure to provide
    a mask which matches the subnet mask for that conn in ipsec.conf (e.g. and /24). 
    Much of the infornation available above for the earlier SoftPK product should also apply to SoftRemote.



    Subject: ipsec interoperability FYI
       Date: Sun, 02 May 1999
       From: Sean Rooney <>
    we've been doing some basic interoperability testing of the following; 
    PGP NT VPN 6.5 and freeswan both seem to work reasonably well with 
    Borderware 6.0 and freegate 1.3 beta. [as well as eachother] 
    more details coming soon.


       Subject: TimeStep Permit/Gate interop works!
       Date: Thu, 10 Jun 1999
       From: Derick Cassidy <>
    Just a quick note of success.  TimeStep Permit/Gate (2520) and
    Free/Swan (June 4th snapshot) interoperate!
    I have them working in AUTO mode, with IKE.  IPSec SA's are negotiated
    with 3DES and MD5.
    Here are the configs and a diagram for both configs.
    left subnet---| Timestep | --- internet --- | Linux box |
    The left subnet is defined as the red side of the timestep box.
     This network definition MUST exist in the Secure Map.
    On the Linux box:
    conn timestep
    In the TimeStep permit/gate Secure Map
    begin static-map
            target ""
            mode   "ISAKMP-Shared"
            tunnel ""
    In the TimeStep Security Descriptor file
    begin security-descriptor
            Name    "High"
            IPSec   "ESP 3DES MINUTES 300 or ESP 3DES HMAC MD5 MINUTES 300"
                                    or DES MD5 MINUTES 1440"
    The timestep has a shared secret for
    set in the Shared Secret Authentication tab of Permit/Config.

    Shiva/Intel LANrover

    A web page with Shiva compatibility information.

    Sun Solaris

       Subject: Re: FreeS/WAN and Solaris
       Date: Tue, 11 Jan 2000
       From: Peter Onion <>
    Slowaris 8 has a native (in kernel) IPSEC implementation.
    I've not done much interop testing yet, but I seem to rememeber we got a manual
    keyed connection between it and FreeSwan a few months ago.


    Subject: Re: FreeS/WAN and SonicWall
         Date: Mon, 5 Feb 2001
         From: "Dilan Arumainathan" <>
    I know I HAVE TO write the mini-howto - but here is the beginning
    Here is my Sonicwall configuration for my working connection:
    conn testauto
    #You need to set the Unique Firewall Identifier to the parameter that you
    #use as the rightid. 


    Here are some mailing list comments from FreeS/WAN tech support person Claudia Schmeing on this:
    It certainly has been possible to interop between Radguard VPN gateways and
    past Linux FreeS/WAN versions, as is evidenced by, as well as my own interop results
    from San Diego this year. There have been no major changes since SD that 
    I would foresee affecting this.
    The Radguard team said that their VPN gateway will not respond to a peer 
    request with PFS (Perfect Forward Secrecy) on, but it *will* successfully 
    establish such a connection with Linux FreeS/WAN when Radguard is the
    initiator. Since PFS is a desirable feature in terms of cryptographic
    security, this asymmetry may frustrate efforts to provide a connection that 
    is both as reliable as secure as possible.
    While it's not clear that rekeying will present a problem, I suspect that 
    some fine tuning of the key life parameters may be needed. Unfortunately 
    I was unable to do additional tests on this topic.
    Due to the PFS issue, when trying to maintain a connection with PFS,
    you may need to set the rekeying times shorter on the radguard side,
    in order to ensure that it is always the initiator.

    IBM System 390

    IBM offer IPsec for their big mainframes. See this PDF document.

    We do not know of any tests of this with FreeS/WAN. If you do any, please tell us.

    IBM AS 400

    From the mailing list:
    Subject: [Users] AS/400 <-> FreeS/WAN connection
       Date: Mon, 24 Sep 2001 10:28:54 -0600
       From: "Brandon Peterson" <bsp@MCINTOSHSOFTWARE.COM>
    FYI, I got a connection working between my Linux box AS/400. I had to make
    two adjustments to the code...
    1) Ignore the commit flag. (There were some patches floating on the list
    last week)
    2) Make FreeS/WAN start with proposal # 1
    To do this, I modified connections.c and inserted after line 1006...
    /* Hack to make AS/400 happy */
    if (c->policy == 0) c->policy = 1;
    Brandon Peterson
    McIntosh Software Services

    Windows or Mac clients

    If you have Windows 2000 or XP, then you should be able to use the IPsec built into those systems. As far as we know, for all other Windows versions, or for the Mac, you will need a client program.

    Quite a number of client programs for IPsec on Windows are available. Some of the same vendors have Macintosh versions.

    Some of the user-written HowTos have details of configuration for particular clients.

    Most of the relevant vendors are listed in this piece of list mail:

     Subject: Re: Searching Windows95/98 and NT4.0 Clients for FreeS/WAN 
        From: Claudia Schmeing <> 
        Date: Wed, 12 Jul 2000
    F-Secure VPN+
    for Win 95, 98 and NT 4.0
    Checkpoint SecureRemote VPN-1 4.1
    for Win 95, 98 and NT
    Raptor Firewall, Raptor MobileNT 5.0
    Mobile NT is a "Client"* for Win 95, 98 (except SE), First Edition Windows NT 
    up to Service Pack 4. It ships with DES; triple DES may be available as an 
    add-on depending on your location.
    Firewall is for Win NT 4.0 or Win 2000.
    IRE SafeNet SoftPK
    a "Client" for Win 95, 98 and NT 4.0 *
    Xedia's AccessPoint QVPN "Client" or "Builder"
    "Builder" is for NT
    "Client" is for Win 98 *
    * "Client" in this context indicates software that does not support a subnet
    behind its end of the connection.

    That mail omits the PGPnet client because the user asking the question already knew of it. The SSH Sentinel client, released since the above message, is another possibility. Both of those have members of the vendor's staff active on our mailing list, an excellent sign for both interoperability and support.

    We also know of some other Windows IPsec clients:

    No doubt there are others we don't know of.

    NT domains vs. tunnels

    There has been some mailing list discussion of making NT domains work across FreeS/WAN tunnels.

    Robert Cotran asked:

    > I have a VPN setup between two subnets (192.168.1.x and 192.168.2.x).  I
    > would like to be able to join the NT domain on 192.168.1.x from the
    > 192.168.2.x subnet.  Is this possible?  Do I have to forward specific ports
    > to do this?  I've already set up WINS entries for all the machines, so
    > accessing computers by their NetBIOS names works perfectly.  Please let me
    > know about this domain thing.  Thanks!
    Dilan Arumainathan answered:
    All you need to do is to:
    1. Enable NetBIOS over TCP.
    2. Create a "lmhosts" file and enter the address of a BDC or a PDC like
        192.168.1.[x]  [Your PDC/BDC servername] #PRE #DOM:[Your Domain Name]
    3. Reboot if necessary.
    and Sebastien Pfieffer provided a slightly different answer:
    For a trust relationship to work between NT domains in different
    (sub)nets all domain controllers of the 1st domain have to know about
    all controllers of the other domain.
    Either you use the described LMHOSTS entry for every domain controller
    of both domains or consider setting up WINS service(s).
    We suspect that in some cases it may be more complex than that. See the discussion of Linux services and Windows 2000 below and the Interop HowTo documents listed above.

    Windows 2000

    Windows 2000 ships with an IPsec implementation built in.

    There are restrictions. We have had mailing list reports that only the server version will act as a gateway, working with a subnet behind it, and other versions offer only "client" functionality, with no subnet. We have some comment on this "client/server" distinction in an earlier section.

    Some versions of Windows 2000 ship with only weak encryption. You need to upgrade them with the strong encryption pack, available either via the Windows 2000 update service or from Microsoft's web site.

    Windows 2000 IPsec sometimes exhibits remarkably odd behaviour. It will allow you to configure it for 3DES only, then ignore your settings and fall back to single DES in some circumstances. Microsoft have said they will fix this. See this Wired article.

    Other Linux services related to Win 2000

    Windows 2000 also uses a number of other security mechanisms which have Linux equivalents. To integrate well with Windows 2000, you may need to look at several open source projects other than FreeS/WAN:

    Here is a mailing list message, from FreeS/WAN team tech support person Claudia Schmeing, discussing Windows 2000 and L2TP:
    You write,
    > I want some information about IPsec with L2TP.
    > I'm going to build the IPsec tunnel on the L2TP tunnel.
    > Is it strange?
    > Is there any case like this already implemented?
    It's used, but rarely. In many cases, IPSec alone is sufficient. 
    In this thread, Timo Teras reports that he configured the L2TP/IPSec 
    hybrid with Win2k. He gives some pointers.
    See also John P. Eisenmenger's report on his own experiences at:

    FreeS/WAN-to-Win2000 interop

    As for IPsec interoperation between Windows 2000 and FreeS/WAN, there are several web sites listed under Interop HowTo documents above.

    This Microsoft page on Windows 2000 IPsec troubleshooting may also be helpful.

    One user has written a tool to simplify the setup. Here is his description from the mailing list:

    Subject: [Users] Win2K
       Date: Tue, 2 Oct 2001
       From: Marcus Müller <>
    I've written a small Tool for freeswan-win2k Interaction.
    Using this tool you can use a roadwarrior running Win2K
    to connect to Freeswan 1.91 with X509 patch.
    The tool has the following features:
    - FreeSwan like Configuration File
    - It sets up the complete Win2k configuration
    - It reads dynamic RAS/DHCP adresses and updates the IPSec Config
    You only need the following items:
    1. Win2k Client
    2. Win2k Service Pack 2 (for high encryption)
    3. Microsoft ipsecpol tool (included in the resource kit / Downloadable
    from Microsoft)
    4. FreeSwan with working X.509 Patch and X.509 Certificates for your
    5. FreeSwan like Config-File
    6. The small "ipsec.exe" Tool
    I have tested it on several Client PCs in different environment
    I am planning to offer it as Open Source. Right now I don't know the
    right way of distributing my work. Because of the widespread interest,
    I would like to place it on the FreeSwan homepage.
    Rightnow anyone interested should mail me for a Copy, so I get some
    more testers.
    Thank you for the great FreeSwan System !!!

    This tool is now available from the author's web page.

    Performance of FreeS/WAN

    The performance of FreeS/WAN is adequate for most applications. Perhaps because of that, there are few detailed measurements. Those we know of are below.

    The main concern in IPsec overhead is in the encryption, decryption and authentication of the actual IPsec (ESP and/or AH) packets. Tunnel setup and rekeying occur so much less frequently than packet processing that, except in highly unusual circumstances, their overheads are not worth worrying about.

    Published material

    The University of Wales at Aberystwyth has done quite detailed speed tests and put their results on the web.

    Davide Cerri's thesis (in Italian) includes performance results for FreeS/WAN and for TLS. He posted an English summary on the mailing list.

    Steve Bellovin used one of AT&T Research's FreeS/WAN gateways as his data source for an analysis of the cache sizes required for key swapping in IPsec. Available as text or PDF slides for a talk on the topic.

    See also the NAI work mentioned in the next section.

    Estimating CPU overheads

    We can come up with a formula that roughly relates CPU speed to the rate of IPsec processing possible. It is far from exact, but should be usable as a first approximation.

    An analysis of authentication overheads for high-speed networks, including some tests using FreeS/WAN, is on the NAI Labs site. In particular, see figure 3 in this PDF document. Their estimates of overheads, measured in Pentium II cycles per byte processed are:

    IPsecauthentication encryptioncycles/byte
    Linux IP stack alonenonono 5
    IPsec without cryptoyesnono 11
    IPsec, authentication onlyyesSHA-1no24
    IPsec with encryptionyesyesyesnot tested

    Overheads for IPsec with encryption were not tested in the NAI work, but Antoon Bosselaers' web page gives cost for his optimised Triple DES implementation as 928 Pentium cycles per block, or 116 per byte. Adding that to the 24 above, we get 140 cycles per byte for IPsec with encryption.

    At 140 cycles per byte, a 140 MHz machine can handle a megabyte -- 8 megabits -- per second. Speeds for other machines will be proportional to this. To saturate a link with capacity C megabits per second, you need a machine running at C * 140/8 = C * 17.5 MHz.

    However, that estimate is not precise. It ignores the differences between:

    and does not account for some overheads you will almost certainly have: so we suggest using C * 25 to get an estimate with a bit of a built-in safety factor.

    This covers only IP and IPsec processing. If you have other loads on your gateway -- for example if it is also working as a firewall -- then you will need to add your own safety factor atop that.

    This estimate matches empirical data reasonably well. For example, Metheringham's tests, described below, show a 733 topping out between 32 and 36 Mbit/second, pushing data as fast as it can down a 100 Mbit link. Our formula suggests you need at least an 800 to handle a fully loaded 32 Mbit link. The two results are consistent.

    Some examples using this estimation method:

    InterfaceMachine speed in MHz
    TypeMbit per
    Minimum IPSEC gatewayMinimum with other load (e.g. firewall)
    DSL125 MHz whatever you have133, or better if you have it
    cable modem375 MHz
    any link, light load 5125 MHz133200+, almost any surplus machine
    Ethernet10250 MHz surplus 266 or 300500+
    fast link, moderate load 20500 MHz500800+, any current off-the-shelf PC
    T3 or E3451125 MHz 12001500+
    fast Ethernet100 2500 MHz// not feasible in software on current machines //
    OC31553875 MHz

    Such an estimate is far from exact, but should be usable as minimum requirement for planning. The key observations are:

    Some complications: Some work is being done toward support of hardware IPsec acceleration which might extend the range of requirements FreeS/WAN could meet.

    Low-end systems

    Even a 486 can handle a T1 line, according to this mailing list message:
    Subject: Re: linux-ipsec: IPSec Masquerade
       Date: Fri, 15 Jan 1999 11:13:22 -0500
       From: Michael Richardson 
    . . . A 486/66 has been clocked by Phil Karn to do
    10Mb/s encryption.. that uses all the CPU, so half that to get some CPU,
    and you have 5Mb/s. 1/3 that for 3DES and you get 1.6Mb/s....

    and a piece of mail from project technical lead Henry Spencer:

    Oh yes, and a new timing point for Sandy's docs...  A P60 -- yes, a 60MHz
    Pentium, talk about antiques -- running a host-to-host tunnel to another
    machine shows an FTP throughput (that is, end-to-end results with a real
    protocol) of slightly over 5Mbit/s either way.  (The other machine is much
    faster, the network is 100Mbps, and the ether cards are good ones... so
    the P60 is pretty definitely the bottleneck.)

    From the above, and from general user experience as reported on the list, it seems clear that a cheap surplus machine -- a reasonable 486, a minimal Pentium box, a Sparc 5, ... -- can easily handle a home office or a small company connection using any of:

    If available, we suggest using a Pentium 133 or better. This should ensure that, even under maximum load, IPsec will use less than half the CPU cycles. You then have enough left for other things you may want on your gateway -- firewalling, web caching, DNS and such.

    Many tunnels from a single gateway

    FreeS/WAN allows a single gateway machine to build tunnels to many others. There may, however, be some problems for large numbers as indicated in this message from the mailing list:

    Subject: Re: Maximum number of ipsec tunnels?
       Date: Tue, 18 Apr 2000
       From: "John S. Denker" <>
    Christopher Ferris wrote:
    >> What are the maximum number ipsec tunnels FreeS/WAN can handle??
    Henry Spencer wrote:
    >There is no particular limit.  Some of the setup procedures currently
    >scale poorly to large numbers of connections, but there are (clumsy)
    >workarounds for that now, and proper fixes are coming.
    1) "Large" numbers means anything over 50 or so.  I routinely run boxes
    with about 200 tunnels.  Once you get more than 50 or so, you need to worry
    about several scalability issues:
    a) You need to put a "-" sign in syslogd.conf, and rotate the logs daily
    not weekly.
    b) Processor load per tunnel is small unless the tunnel is not up, in which
    case a new half-key gets generated every 90 seconds, which can add up if
    you've got a lot of down tunnels.
    c) There's other bits of lore you need when running a large number of
    tunnels.  For instance, systematically keeping the .conf file free of
    conflicts requires tools that aren't shipped with the standard freeswan
    d) The pluto startup behavior is quadratic.  With 200 tunnels, this eats up
    several minutes at every restart.   I'm told fixes are coming soon.
    2) Other than item (1b), the CPU load depends mainly on the size of the
    pipe attached, not on the number of tunnels.

    It is worth noting that item (1b) applies only to repeated attempts to re-key a data connection (IPsec SA, Phase 2) over an established keying connection (ISAKMP SA, Phase 1). There are two ways to reduce this overhead using settings in ipsec.conf(5):

    The overheads for establishing keying connections (ISAKMP SAs, Phase 1) are lower because for these Pluto does not perform expensive operations before receiving a reply from the peer.

    Measuring KLIPS

    Here is some additional data from the mailing list.
    Subject: FreeSWAN (specically KLIPS) performance measurements
       Date: Thu, 01 Feb 2001
       From: Nigel Metheringham <>
    I've spent a happy morning attempting performance tests against KLIPS 
    (this is due to me not being able to work out the CPU usage of KLIPS so 
    resorting to the crude measurements of maximum throughput to give a 
    baseline to work out loading of a box).
    Measurements were done using a set of 4 boxes arranged in a line, each 
    connected to the next by 100Mbit duplex ethernet.  The inner 2 had an 
    ipsec tunnel between them (shared secret, but I was doing measurements 
    when the tunnel was up and running - keying should not be an issue 
    here).  The outer pair of boxes were traffic generators or traffic sink.
    The crypt boxes are Compaq DL380s - Uniprocessor PIII/733 with 256K 
    cache.  They have 128M main memory.  Nothing significant was running on 
    the boxes other than freeswan.  The kernel was a 2.2.19pre7 patched 
    with freeswan and ext3.
    Without an ipsec tunnel in the chain (ie the 2 inner boxes just being 
    100BaseT routers), throughput (measured with ttcp) was between 10644 
    and 11320 KB/sec
    With an ipsec tunnel in place, throughput was between 3268 and 3402 
    These measurements are for data pushed across a TCP link, so the 
    traffic on the wire between the 2 ipsec boxes would have been higher 
    than this....
    vmstat (run during some other tests, so not affecting those figures) on 
    the encrypting box shows approx 50% system 50% idle CPU - which I 
    don't believe at all.  Interactive feel of the box was significantly 
    I also tried running the kernel profiler (see man readprofile) during 
    test runs.
    A box doing primarily decrypt work showed basically nothing happening - 
    I assume interrupts were off.
    A box doing encrypt work showed the following:-
     Ticks Function                                   Load
     ~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    ~~~~~~
       956 total                                      0.0010
       532 des_encrypt2                               0.1330
       110 MD5Transform                               0.0443
        97 kmalloc                                    0.1880
        39 des_encrypt3                               0.1336
        23 speedo_interrupt                           0.0298
        14 skb_copy_expand                            0.0250
        13 ipsec_tunnel_start_xmit                    0.0009
        13 Decode                                     0.1625
        11 handle_IRQ_event                           0.1019
        11 .des_ncbc_encrypt_end                      0.0229
        10 speedo_start_xmit                          0.0188
         9 satoa                                      0.0225
         8 kfree                                      0.0118
         8 ip_fragment                                0.0121
         7 ultoa                                      0.0365
         5 speedo_rx                                  0.0071
         5 .des_encrypt2_end                          5.0000
         4 _stext                                     0.0140
         4 ip_fw_check                                0.0035
         2 rj_match                                   0.0034
         2 ipfw_output_check                          0.0200
         2 inet_addr_type                             0.0156
         2 eth_copy_and_sum                           0.0139
         2 dev_get                                    0.0294
         2 addrtoa                                    0.0143
         1 speedo_tx_buffer_gc                        0.0024
         1 speedo_refill_rx_buf                       0.0022
         1 restore_all                                0.0667
         1 number                                     0.0020
         1 net_bh                                     0.0021
         1 neigh_connected_output                     0.0076
         1 MD5Final                                   0.0083
         1 kmem_cache_free                            0.0016
         1 kmem_cache_alloc                           0.0022
         1 __kfree_skb                                0.0060
         1 ipsec_rcv                                  0.0001
         1 ip_rcv                                     0.0014
         1 ip_options_fragment                        0.0071
         1 ip_local_deliver                           0.0023
         1 ipfw_forward_check                         0.0139
         1 ip_forward                                 0.0011
         1 eth_header                                 0.0040
         1 .des_encrypt3_end                          0.0833
         1 des_decrypt3                               0.0034
         1 csum_partial_copy_generic                  0.0045
         1 call_out_firewall                          0.0125
    Hope this data is helpful to someone... however the lack of visibility 
    into the decrypt side makes things less clear

    Speed with compression

    Another user reported some results for connections with and without IP compression:
    Subject: [Users] Speed with compression
       Date: Fri, 29 Jun 2001
       From: John McMonagle <>
    Did a couple tests with compression using the new 1.91 freeswan.
    Running between 2 sites with cable modems.  Both  using approximately
    130 mhz pentium.
    Transferred files with ncftp.
    Compressed file was a 6mb compressed  installation file.
    Non compressed was 18mb /var/lib/rpm/packages.rpm
                                Compressed vpn          regular vpn
    Compress file                42.59 kBs               42.08 kBs
    regular file                110.84 kBs               41.66 kBs
    Load  was about 0 either way.
    Ping times were very similar  a bit above 9 ms.
    Compression looks attractive to me.
    Later in the same thread, project technical lead Henry Spencer added:
    > is there a reason not to switch compression on?  I have large gateway boxes
    > connecting 3 connections, one of them with a measly DS1 link...
    Run some timing tests with and without, with data and loads representative
    of what you expect in production.  That's the definitive way to decide. 
    If compression is a net loss, then obviously, leave it turned off.  If it
    doesn't make much difference, leave it off for simplicity and hence
    robustness.  If there's a substantial gain, by all means turn it on. 
    If both ends support compression and can successfully negotiate a
    compressed connection (trivially true if both are FreeS/WAN 1.91), then
    the crucial question is CPU cycles. 
    Compression has some overhead, so one question is whether *your* data
    compresses well enough to save you more CPU cycles (by reducing the volume
    of data going through CPU-intensive encryption/decryption) than it costs
    you.  Last time I ran such tests on data that was reasonably compressible
    but not deliberately contrived to be so, this generally was not true --
    compression cost extra CPU cycles -- so compression was worthwhile only if
    the link, not the CPU, was the bottleneck.  However, that was before the
    slow-compression bug was fixed.  I haven't had a chance to re-run those
    tests yet, but it sounds like I'd probably see a different result. 
    The bug he refers to was a problem with the compression libraries that had us using C code, rather than assembler, for compression. It was fixed before 1.91.

    Methods of measuring

    If you want to measure the loads FreeS/WAN puts on a system, note that tools such as top or measurements such as load average are more-or-less useless for this. They are not designed to measure something that does most of its work inside the kernel.

    Here is a message from FreeS/WAN kernel programmer Richard Guy Briggs on this:

    > I have a batch of boxes doing Freeswan stuff.
    > I want to measure the CPU loading of the Freeswan tunnels, but am 
    > having trouble seeing how I get some figures out...
    >  - Keying etc is in userspace so will show up on the per-process
    >    and load average etc (ie pluto's load)
    >  - KLIPS is in the kernel space, and does not show up in load average
    >    I think also that the KLIPS per-packet processing stuff is running
    >    as part of an interrupt handler so it does not show up in the
    >    /proc/stat system_cpu or even idle_cpu figures
    It is not running in interrupt handler.  It is in the bottom half.
    This is somewhere between user context (careful, this is not
    userspace!) and hardware interrupt context.
    > Is this correct, and is there any means of instrumenting how much the 
    > cpu is being loaded - I don't like the idea of a system running out of 
    > steam whilst still showing 100% idle CPU :-)
    vmstat seems to do a fairly good job, but use a running tally to get a
    good idea.  A one-off call to vmstat gives different numbers than a
    running stat.  To do this, put an interval on your vmstat command
    and another suggestion from the same thread:
    Subject: Re: Measuring the CPU usage of Freeswan
       Date: Mon, 29 Jan 2001
       From: Patrick Michael Kane <>
    The only truly accurate way to accurately track FreeSWAN CPU usage is to use
    a CPU soaker. You run it on an unloaded system as a benchmark, then start up
    FreeSWAN and take the difference to determine how much FreeSWAN is eating.
    I believe someone has done this in the past, so you may find something in
    the FreeSWAN archives.  If not, someone recently posted a URL to a CPU
    soaker benchmark tool on linux-kernel.

    Testing FreeS/WAN

    This document discusses testing FreeS/WAN.

    Not all types of testing are described here. Other parts of the documentation describe some tests:

    installation document
    testing for a successful install
    configuration document
    basic tests for a working configuration
    web links document
    General information on tests for interoperability between various IPsec implementations. This includes links to several test sites.
    interoperation document.
    More specific information on FreeS/WAN interoperation with other implementations.
    performance document
    performance measurements

    The test setups and procedures described here can also be used in other testing, but this document focuses on testing the IPsec functionality of FreeS/WAN.

    Testing with User Mode Linux

    User Mode Linux allows you to run Linux as a user process on another Linux machine.

    As of 1.92, the distribution has a new directory named testing. It contains a collection of test scripts and sample configurations. Using these, you can bring up several copies of Linux in user mode and have them build tunnels to each other. This lets you do some testing of a FreeS/WAN configuration on a single machine.

    You need a moderately well-endowed machine for this to work well. Each UML wants about 16 megs of memory by default, which is plenty for FreeS/WAN usage. Typical regression testing only occasionally uses as many as 4 UMLs. If one is doing nothing else with the machine (in particular, not running X on it), then 128 megs and a 500MHz CPU are fine.

    Documentation on these scripts is here .

    Configuration for a testbed network

    A common test setup is to put a machine with dual Ethernet cards in between two gateways under test. You need at least five machines; two gateways, two clients and a testing machine in the middle.

    The central machine both routes packets and provides a place to run diagnostic software for checking IPsec packets. See next section for discussion of using tcpdump(8) for this.

    This makes things more complicated than if you just connected the two gateway machines directly to each other, but it also makes your test setup much more like the environment you actually use IPsec in. Those environments nearly always involve routing, and quite a few apparent IPsec failures turn out to be problems with routing or with firewalls dropping packets. This approach lets you deal with those problems on your test setup.

    What you end up with looks like:

    Testbed network

          subnet a.b.c.0/24
          eth1 = a.b.c.1
          eth0 = 192.168.p.1
          eth0 = 192.168.p.2
             route/monitor box
          eth1 = 192.168.q.2
          eth0 = 192.168.q.1
          eth1 = x.y.z.1
           subnet x.y.z.0/24

    Where p and q are any convenient values that do not interfere with other routes you may have. The ipsec.conf(5) file then has, among other things:

    conn abc-xyz
    Once that works, you can remove the "route/monitor box", and connect the two gateways to the Internet. The only parameters in ipsec.conf(5) that need to change are the four shown above. You replace them with values appropriate for your Internet connection, and change the eth0 IP addresses and the default routes on both gateways.

    Note that nothing on either subnet needs to change. This lets you test most of your IPsec setup before connecting to the insecure Internet.

    Using packet sniffers in testing

    A number of tools are available for looking at packets. We will discuss using tcpdump(8), a common Linux tool included in most distributions. Alternatives offerring more-or-less the same functionality include:

    Several people on our mailing list report a preference for this over tcpdump.
    a Windows version of tcpdump(8), possibly handy if you have Windows boxes in your network
    A linux sniffer that we don't know much about. If you use it, please comment on our mailing list.

    See also this index of packet sniffers.

    tcpdump(8) may misbehave if run on the gateways themselves. It is designed to look into a normal IP stack and may become confused if you ask it to display data from a stack which has IPsec in play.

    At one point, the problem was quite severe. Recent versions of tcpdump, however, understand IPsec well enough to be usable on a gateway. You can get the latest version from

    Even with a recent tcpdump, some care is required. Here is part of a post from Henry on the topic:

    > a) data from sunset to sunrise or the other way is not being
    > encrypted (I am using tcpdump (ver. 3.4) -x/ping -p to check
    > packages) 
    What *interface* is tcpdump being applied to?  Use the -i option to
    control this.  It matters!  If tcpdump is looking at the ipsecN
    interfaces, e.g. ipsec0, then it is seeing the packets before they are
    encrypted or after they are decrypted, so of course they don't look
    encrypted.  You want to have tcpdump looking at the actual hardware
    interfaces, e.g. eth0. 
    Actually, the only way to be *sure* what you are sending on the wire is to
    have a separate machine eavesdropping on the traffic.  Nothing you can do
    on the machines actually running IPsec is 100% guaranteed reliable in this
    area (although tcpdump is a lot better now than it used to be).
    The most certain way to examine IPsec packets is to look at them on the wire. For security, you need to be certain, so we recommend doing that. To do so, you need a separate sniffer machine located between the two gateways. This machine can be routing IPsec packets, but it must not be an IPsec gateway. Network configuration for such testing is discussed above.

    Here's another mailing list message with advice on using tcpdump(8):

    Subject: RE: [Users] Encrypted???
       Date: Thu, 29 Nov 2001
       From: "Joe Patterson" <>
    tcpdump -nl -i $EXT-IF proto 50
    -nl tells it not to buffer output or resolve names (if you don't do that it
    may confuse you by not outputing anything for a while), -i $EXT-IF (replace
    with your external interface) tells it what interface to listen on, and
    proto 50 is ESP.  Use "proto 51" if for some odd reason you're using AH, and
    "udp port 500" if you want to see the isakmp key exchange/tunnel setup
    You can also run `tcpdump -nl -i ipsec0` to see what traffic is on that
    virtual interface.  Anything you see there *should* be either encrypted or
    dropped (unless you've turned on some strange options in your ipsec.conf
    Another very handy thing is ethereal ( which runs
    on just about anything, has a nice gui interface (or a nice text-based
    interface), and does a great job of protocol  breakdown.  For ESP and AH
    it'll basically just tell you that there's a packet of that protocol, and
    what the spi is, but for isakmp it'll actually show you a lot of the tunnel
    setup information (until it gets to the point in the protocol where isakmp
    is encrypted....)

    Verifying encryption

    The question of how to verify that messages are actually encrypted has been extensively discussed on the mailing list. See this thread.

    If you just want to verify that packets are encrypted, just look at them between the gateways with tcpdump(8). The packets should, except for some of the header information, be utterly unintelligible. The output of good encryption looks exactly like random noise.

    You can put recognizable data into ping packets with something like:

            ping -p feedfacedeadbeef

    "feedfacedeadbeef" is a legal hexadecimal pattern that is easy to pick out of hex dumps.

    For other protocols, you may need to check if you have encrypted data or ASCII text. Encrypted data has approximately equal frequencies for all 256 possible characters. ASCII text has most characters in the printable range 0x20-0x7f, a few control characters less than 0x20, and none at all in the range 0x80-0xff. 0x20, space, is a good character to look for. In normal English text space occurs about once in seven characters, versus about once in 256 for random or encrypted data.

    One thing to watch for: the output of good compression, like that of good encryption, looks just like random noise. In general, it is not possible to tell just by looking at a data stream whether it has been compressed, encrypted, or both. You need a little care not to mistake compressed data for encrypted data in your testing.

    A simple check with tcpdump(8) only tells you that the data you looked at was encrypted. If you have stronger requirements -- for example if your security policy requires verification that plaintext is not leaked during startup or under various anomolous conditions -- then you will need to devise much more thorough tests. If you do that, please post any results or methodological details which your security policy allows you to make public.

    Mailing list pointers

    Additional information on testing can be found in these mailing list messages:

    Kernel configuration for FreeS/WAN

    This section lists many of the options available when configuring a Linux kernel, and explains how they should be set on a FreeS/WAN IPsec gateway.

    Not everyone needs to worry about kernel configuration

    Note that in many cases you do not need to mess with these.

    You may have a Linux distribution which comes with FreeS/WAN installed (see this list). In that case, you need not do a FreeS/WAN installation or a kernel configuration. Of course, you might still want to configure and rebuild your kernel to improve performance or security. This can be done with standard tools described in the Kernel HowTo.

    If you need to install FreeS/WAN, then you do need to configure a kernel. However, you may choose to do that using the simplest procedure:

    This document is for those who choose to configure their FreeS/WAN kernel themselves.

    Assumptions and notation

    Help text for most kernel options is included with the kernel files, and is accessible from within the configuration utilities. We assume you will refer to that, and to the Kernel HowTo, as necessary. This document covers only the FreeS/WAN-specific aspects of the problem.

    To avoid duplication, this document section does not cover settings for the additional IPsec-related kernel options which become available after you have patched your kernel with FreeS/WAN patches. There is help text for those available from within the configuration utility.

    We assume a common configuration in which the FreeS/WAN IPsec gateway is also doing ipchains(8) firewalling for a local network, and possibly masquerading as well.

    Some suggestions below are labelled as appropriate for "a true paranoid". By this we mean they may cause inconvenience and it is not entirely clear they are necessary, but they appear to be the safest choice. Not using them might entail some risk. Of course one suggested mantra for security administrators is: "I know I'm paranoid. I wonder if I'm paranoid enough."

    Labels used

    Six labels are used to indicate how options should be set. We mark the labels with [square brackets]. For two of these labels, you have no choice:

    essential for FreeS/WAN operation.
    incompatible with FreeS/WAN.

    those must be set correctly or FreeS/WAN will not work

    FreeS/WAN should work with any settings of the others, though of course not all combinations have been tested. We do label these in various ways, but these labels are only suggestions.

    useful on most FreeS/WAN gateways
    an unwelcome complication on a FreeS/WAN gateway.
    Your choice. We outline issues you might consider.
    This option has no direct effect on FreeS/WAN and related tools, so you should be able to set it as you please.

    Of course complexity is an enemy in any effort to build secure systems. For maximum security, any feature that can reasonably be turned off should be. "If in doubt, leave it out."

    Kernel options for FreeS/WAN

    Indentation is based on the nesting shown by 'make menuconfig' with a 2.2.16 kernel for the i386 architecture.

    Code maturity and level options
    Prompt for development ... code/drivers
    [optional] If this is no, experimental drivers are not shown in later menus.

    For most FreeS/WAN work, no is the preferred setting. Using new or untested components is too risky for a security gateway.

    However, for some hardware (such as the author's network cards) the only drivers available are marked new/experimental. In such cases, you must enable this option or your cards will not appear under "network device support". A true paranoid would leave this option off and replace the cards.

    Processor type and features
    Loadable module support
    Enable loadable module support
    [optional] A true paranoid would disable this. An attacker who has root access to your machine can fairly easily install a bogus module that does awful things, provided modules are enabled. A common tool for attackers is a "rootkit", a set of tools the attacker uses once he or she has become root on your system. The kit introduces assorted additional compromises so that the attacker will continue to "own" your system despite most things you might do to recovery the situation. For Linux, there is a tool called knark which is basically a rootkit packaged as a kernel module.

    With modules disabled, an attacker cannot install a bogus module. The only way he can achieve the same effects is to install a new kernel and reboot. This is considerably more likely to be noticed.

    Many FreeS/WAN gateways run with modules enabled. This simplifies some administrative tasks and some ipchains features are available only as modules. Once an enemy has root on your machine your security is nil, so arguably defenses which come into play only in that situation are pointless.

    Set version information ....
    [optional] This provides a check to prevent loading modules compiled for a different kernel.
    Kernel module loader
    [disable] It gives little benefit on a typical FreeS/WAN gate and entails some risk.
    General setup
    We list here only the options that matter for FreeS/WAN.
    Networking support
    Sysctl interface
    [optional] If this option is turned on and the /proc filesystem installed, then you can control various system behaviours by writing to files under /proc/sys. For example:
            echo 1 > /proc/sys/net/ipv4/ipforward
    turns IP forwarding on.

    Disabling this option breaks many firewall scripts. A true paranoid would disable it anyway since it might conceivably be of use to an attacker.

    Plug and Play support
    Block devices
    Networking options
    Packet socket
    [optional] This kernel feature supports tools such as tcpdump(8) which communicate directly with network hardware, bypassing kernel protocols. This is very much a two-edged sword:
    • such tools can be very useful to the firewall admin, especially during initial testing
    • should an evildoer breach your firewall, such tools could give him or her a great deal of information about the rest of your network
    We recommend disabling this option on production gateways.
    Kernel/User netlink socket
    [optional] Required if you want to use advanced router features.
    Routing messages
    Netlink device emulation
    Network firewalls
    [recommended] You need this if the IPsec gateway also functions as a firewall.

    Even if the IPsec gateway is not your primary firewall, we suggest setting this so that you can protect the gateway with at least basic local packet filters.

    Socket filtering
    [disable] This enables an older filtering interface. We suggest using ipchains(8) instead. To do that, set the "Network firewalls" option just above, and not this one.
    Unix domain sockets
    [required] These sockets are used for communication between the ipsec(8) commands and the ipsec_pluto(8) daemon.
    TCP/IP networking
    IP: multicasting
    IP: advanced router
    [optional] This gives you policy routing, which some people have used to good advantage in their scripts for FreeS/WAN gateway management. It is not used in our distributed scripts, so not required unless you want it for custom scripts. It requires the netlink interface between kernel code and the iproute2(8) command.
    IP: kernel level autoconfiguration
    [disable] It gives little benefit on a typical FreeS/WAN gate and entails some risk.
    IP: firewall packet netlink device
    IP: transparent proxy support
    [optional] This is required in some firewall configurations, but should be disabled unless you have a definite need for it.
    IP: masquerading
    [optional] Required if you want to use non-routable private IP addresses for your local network.
    IP: Optimize as router not host
    IP: tunneling
    IP: GRE tunnels over IP
    IP: aliasing support
    IP: ARP daemon support (EXPERIMENTAL)
    Not required on most systems, but might prove useful on heavily-loaded gateways.
    IP: TCP syncookie support
    [recommended] It provides a defense against a denial of service attack which uses bogus TCP connection requests to waste resources on the victim machine.
    IP: Reverse ARP
    IP: large window support
    [recommended] unless you have less than 16 meg RAM
    [optional] FreeS/WAN does not currently support IPv6, though work on integrating FreeS/WAN with the Linux IPv6 stack has begun. Details.

    It should be possible to use IPv4 FreeS/WAN on a machine which also does IPv6. This combination is not yet well tested. We would be quite interested in hearing results from anyone expermenting with it, via the mailing list.

    We do not recommend using IPv6 on production FreeS/WAN gateways until more testing has been done.

    Novell IPX
    [disable] Quite a few Linux installations use IP but also have some other protocol, such as Appletalk or IPX, for communication with local desktop machines. In theory it should be possible to configure IPsec for the IP side of things without interfering with the second protocol.

    We do not recommend this. Keep the software on your gateway as simple as possible. If you need a Linux-based Appletalk or IPX server, use a separate machine.

    Telephony support
    SCSI support
    I2O device support
    Network device support
    [anything] should work, but there are some points to note.

    The development team test almost entirely on 10 or 100 megabit Ethernet and modems. In principle, any device that can do IP should be just fine for IPsec, but in the real world any device that has not been well-tested is somewhat risky. By all means try it, but don't bet your project on it until you have solid test results.

    If you disabled experimental drivers in the Code maturity section above, then those drivers will not be shown here. Check that option before going off to hunt for missing drivers.

    If you want Linux to automatically find more than one ethernet interface at boot time, you need to:

    Having Linux find the cards this way is not necessary, but is usually more convenient than loading modules in your boot scripts.
    Amateur radio support
    IrDA (infrared) support
    ISDN subsystem
    Old CDROM drivers
    Character devices
    The only required character device is:
    [required] This is a source of random numbers which are required for many cryptographic protocols, including several used in IPsec.

    If you are comfortable with C source code, it is likely a good idea to go in and adjust the #define lines in /usr/src/linux/drivers/char/random.c to ensure that all sources of randomness are enabled. Relying solely on keyboard and mouse randomness is dubious procedure for a gateway machine. You could also increase the randomness pool size from the default 512 bytes (128 32-bit words).

    [anything] should work, but we suggest limiting a gateway machine to the standard Linux ext2 filesystem in most cases.
    Network filesystems
    [disable] These systems are an unnecessary risk on an IPsec gateway.
    Console drivers
    [anything] should work, but we suggest enabling sound only if you plan to use audible alarms for firewall problems.
    Kernel hacking
    [disable] This might be enabled on test machines, but should not be on production gateways.

    Other configuration possibilities

    This document describes various options for FreeS/WAN configuration which are less used or more complex (often both) than the standard cases described in our basic configuration document.

    Choosing connection types

    The first major decision you need to make before configuring additional connections is what type or types of connections you will use. There are several options, and you can use more than one concurrently.

    Manual vs. automatic keying

    IPsec allows two types of connections, with manual or automatic keying. FreeS/WAN starts them with commands such as:

            ipsec manual --start name
            ipsec auto --up name

    The difference is in how they are keyed.

    Manually keyed connections
    use keys stored in ipsec.conf .
    Automatically keyed connections
    use keys automatically generated by the Pluto key negotiation daemon. The key negotiation protocol, IKE, must authenticate the other system. (It is vulnerable to a man-in-the-middle attack if used without authentication.) We currently support two authentication methods:

    Manually keyed connections provide weaker security than automatically keyed connections. An opponent who gets a key gets all data encrypted by it. We discuss using manual keying in production below, but this is not recommended except in special circumstances, such as needing to communicate with some implementation that offers no auto-keyed mode compatible with FreeS/WAN. Manual keying is useful for testing.

    With automatically-(re)-keyed connections, the keys change often so an opponent who gets one key does not get a large amount of data. An opponent who gets a shared secret, or your private key if public key authentication is used, does not automatically gain access to any encryption keys or any data. Once your authentication mechanism has been subverted you have no way to prevent the attacker getting keys and data, but the attacker still has to work for them.

    Authentication methods for auto-keying

    The IKE protocol which Pluto uses to negotiate connections between gateways must use some form of authentication of peers. A gateway must know who it is talking to before it can create a secure connection. We currently support two methods for this authentication:

    See our links section for information on user-contributed patches which provide a third mechanism:

    As a long-term goal, FreeS/WAN plans to support distribution of public keys for authentication via secure DNS. This would allow us to support opportunistic encryption . Any two FreeS/WAN gateways could provide secure communication, without either of them having any preset information about the other.

    Advantages of public key methods

    Authentication with a public key method such as RSA has some important advantages over using shared secrets.

  • does not require fixed IP addresses
  • There is also a disadvantage:

    This is partly counterbalanced by the fact that the key is never transmitted and remains under your control at all times. It is likely necessary, however, to take account of this in setting security policy. For example, you should change gateway keys when an administrator leaves the company, and should change them periodically in any case.

    Overall, public key methods are more secure, more easily managed and more flexible. We recommend that they be used for all connections, unless there is a compelling reason to do otherwise.

    Using shared secrets in production

    Generally, public key methods are preferred for reasons given above, but shared secrets can be used with no loss of security, just more work and perhaps more need to take precautions.

    Putting secrets in ipsec.secrets(5)

    If shared secrets are to be used to authenticate communication for the Diffie-Hellman key exchange in the IKE protocol, then those secrets must be stored in /etc/ipsec.secrets. For details, see the ipsec.secrets(5) man page.

    A few considerations are vital:

    Each line has the IP addresses of the two gateways plus the secret. It should look something like this:

   : PSK "jxTR1lnmSjuj33n4W51uW3kTR55luUmSmnlRUuWnkjRj3UuTV4T3USSu23Uk55nWu5TkTUnjT"

    PSK indicates the use of a pre-s hared key. The quotes and the whitespace shown are required.

    You can use any character string as your secret. For security, it should be both long and extremely hard to guess. We provide a utility to generate such strings, ipsec_ranbits(8).

    You want the same secret on the two gateways used, so you create a line with that secret and the two gateway IP addresses. The installation process supplies an example secret, useful only for testing. You must change it for production use.

    File security

    You must deliver this file, or the relevant part of it, to the other gateway machine by some secure means. Don't just FTP or mail the file! It is vital that the secrets in it remain secret. An attacker who knew those could easily have all the data on your "secure" connection.

    This file must be owned by root and should have permissions rw-------.

    Shared secrets for road warriors

    You can use a shared secret to support a single road warrior connecting to your gateway, and this is a reasonable thing to do in some circumstances. Public key methods have advantages, discussed above, but they are not critical in this case.

    To do this, the line in ipsec.secrets(5) is something like:

   : PSK "jxTR1lnmSjuj33n4W51uW3kTR55luUmSmnlRUuWnkjRj3UuTV4T3USSu23Uk55nWu5TkTUnjT"
    where the means that any IP address is acceptable.

    For more than one road warrior, shared secrets are not recommended. If shared secrets are used, then when the responder needs to look up the secret, all it knows about the sender is an IP address. This is fine if the sender is at a fixed IP address specified in the config file. It is also fine if only one road warrior uses the wildcard address. However, if you have more than one road warrior using shared secret authentication, then they must all use that wildcard and therefore all road warriors using PSK autentication must use the same secret. Obviously, this is insecure.

    For multiple road warriors, use public key authentication. Each roadwarrior can then have its own identity (our leftid= or rightid= parameters), its own public/private key pair, and its own secure connection.

    Using manual keying in production

    Generally, automatic keying is preferred over manual keying for production use because it is both easier to manage and more secure. Automatic keying frees the admin from much of the burden of managing keys securely, and can provide perfect forward secrecy.

    However, it is possible to use manual keying in production if that is what you want to do. This might be necessary, for example, in order to interoperate with some device that either does not provide automatic keying or provides it in some version we cannot talk to.

    Note that with manual keying all security rests with the keys. If an adversary acquires your keys, you've had it. He or she can read everything ever sent with those keys, including old messages he or she may have archived. You need to be really paranoid about keys if you're going to rely on manual keying for anything important.

    Linux FreeS/WAN provides some facilities to help with this. In particular, it is good policy to keep keys in separate files so you can edit configuration information in /etc/ipsec.conf without exposing keys to "shoulder surfers" or network snoops. We support this with the also= and include syntax in ipsec.conf(5).

    See the last example in our examples file. In the /etc/ipsec.conf conn samplesep section, it has the line:


    which tells the "ipsec manual" script to insert the configuration description labelled "samplesep-keys" if it can find it. The /etc/ipsec.conf file must also have a line such as:

    include ipsec.*.conf

    which tells it to read other files. One of those other files then might contain the additional data:

    conn samplesep-keys

    The first line matches the label in the "also=" line, so the indented lines are inserted. The net effect is exactly as if the inserted lines had occurred in the original file in place of the "also=" line.

    Variables set here are:

    A number needed by the manual keying code. Any 3-digit hex number will do, but if you have more than one manual connection then spi must be different for each connection.
    Options for ESP (Encapsulated Security Payload), the usual IPsec encryption mode. Settings here are for encryption using triple DES and authentication using MD5. Note that encryption without authentication should not be used; it is insecure.
    Key for ESP encryption. Here, a 192-bit hex number for triple DES.
    Key for ESP authentication. Here, a 128-bit hex number for MD5.

    Note that the example keys we supply are intended only for testing. For real use, you should go to automatic keying. If that is not possible, create your own keys for manual mode and keep them secret

    Of course, any files containing keys must have 600 permissions and be owned by root.

    If you connect in this way to multiple sites, we recommend that you keep keys for each site in a separate file and adopt some naming convention that lets you pick them all up with a single "include" line. This minimizes the risk of losing several keys to one error or attack and of accidentally giving another site admin keys which he or she has no business knowing.

    Also note that if you have multiple manually keyed connections on a single machine, then the spi parameter must be different for each one. Any 3-digit hex number is OK, provided they are different for each connection. We reserve the range 0x100 to 0xfff for manual connections. Pluto assigns SPIs from 0x1000 up for automatically keyed connections.

    If ipsec.conf(5) contains keys for manual mode connections, then it too must have permissions rw-------. We recommend instead that, if you must manual keying in production, you keep the keys in separate files.

    Note also that ipsec.conf is installed with permissions rw-r--r--. If you plan to use manually keyed connections for anything more than initial testing, you must:

    We recommend the latter method for all but the simplest configurations.

    Creating keys with ranbits

    You can create new random keys with the ranbits(8) utility. For example, the commands:

          umask 177
          ipsec ranbits 192  > temp
          ipsec ranbits 128 >> temp

    create keys in the sizes needed for our default algorithms:

    If you want to use SHA instead of MD5, that requires a 160-bit key

    Note that any temporary files used must be kept secure since they contain keys. That is the reason for the umask command above. The temporary file should be deleted as soon as you are done with it. You may also want to change the umask back to its default value after you are finished working on keys.

    The ranbits utility may pause for a few seconds if not enough entropy is available immediately. See ipsec_ranbits(8) and random(4) for details. You may wish to provide some activity to feed entropy into the system. For example, you might move the mouse around, type random characters, or do du /usr > /dev/null in the background.

    Setting up connections at boot time

    You can tell the system to set up connections automatically at boot time by putting suitable stuff in /etc/ipsec.conf on both systems. The relevant section of the file is labelled by a line reading config setup.

    Details can be found in the ipsec.conf(5) man page. We also provide a file of example configurations.

    The most likely options are something like:

    interfaces="ipsec0=eth0 ipsec1=ppp0"
    Tells KLIPS which interfaces to use. Up to four interfaces numbered ipsec[0-3] are supported. Each interface can support an arbitrary number of tunnels.

    Note that for PPP, you give the ppp[0-9] device name here, not the underlying device such as modem (or eth1 if you are using PPPoE).

    Alternative setting, useful in simple cases. KLIPS will pick up both its interface and the next hop information from the settings of the Linux default route.
    Normally "no". Set to "yes" if the IP forwarding option is disabled in your network configuration. (This can be set as a kernel configuration option or later. e.g. on Redhat, it's in /etc/sysconfig/network and on SuSE you can adjust it with Yast.) Linux FreeS/WAN will then enable forwarding when starting up and turn it off when going down. This is used to ensure that no packets will be forwarded before IPsec comes up and takes control.
    Used in messages to the system logging daemon (syslogd) to specify what type of software is sending the messages. If the settings are "daemon.error" as in our example, then syslogd treats the messages as error messages from a daemon.

    Note that Pluto does not currently pay attention to this variable. The variable controls setup messages only.

    Debug settings for KLIPS.
    Debug settings for Pluto.
    ... for both the above DEBUG settings
    Normally, leave empty as shown above for no debugging output.
    Use "all" for maximum information.
    See ipsec_klipsdebug(8) and ipsec_pluto(8) man page for other options. Beware that if you set /etc/ipsec.conf to enable debug output, your system's log files may get large quickly.
    Normally, programs started by ipsec setup don't crash. If they do, by default, no core dump will be produced because such dumps would contain secrets. If you find you need to debug such crashes, you can set dumpdir to the name of a directory in which to collect the core file.
    List of manually keyed connections to be automatically started at boot time. Useful for testing, but not for long term use. Connections which are automatically started should also be automatically re-keyed.
    Whether to start Pluto when ipsec startup is done.
    This parameter is optional and defaults to "yes" if not present.

    "yes" is strongly recommended for production use so that the keying daemon (Pluto) will automatically re-key the connections regularly. The ipsec-auto parameters ikelifetime, ipseclifetime and reykeywindow give you control over frequency of rekeying.

    plutoload="reno-van reno-adam reno-nyc"
    List of tunnels (by name, e.g. fred-susan or reno-van in our examples) to be loaded into Pluto's internal database at startup. In this example, Pluto loads three tunnels into its database when it is started.

    If plutoload is "%search", Pluto will load any connections whose description includes "auto=add" or "auto=start".

    plutostart="reno-van reno-adam reno-nyc"
    List of tunnels to attempt to negotiate when Pluto is started.

    If plutostart is "%search", Pluto will start any connections whose description includes "auto=start".

    Note that, for a connection intended to be permanent, both gateways should be set try to start the tunnel. This allows quick recovery if either gateway is rebooted or has its IPsec restarted. If only one gateway is set to start the tunnel and the other gateway restarts, the tunnel may not be rebuilt.

    Controls whether Pluto waits for one tunnel to be established before starting to negotiate the next. You might set this to "yes" For a busy and resource-laden production gateway, you likely want "no" so that connections are brought up in parallel and the whole process takes less time.

    The example assumes you are at the Reno office and will use IPsec to Vancouver, New York City and Amsterdam.

    Multiple tunnels between the same two gateways

    Consider a pair of subnets, each with a security gateway, connected via the Internet:

              left subnet
             North Gateway
                 left next hop
                 right next hop
             South gateway
              right subnet

    A tunnel specification such as:

    conn northnet-southnet
    will allow machines on the two subnets to talk to each other. You might test this by pinging from polarbear ( to penguin (

    However, this does not cover other traffic you might want to secure. To handle all the possibilities, you might also want these connection descriptions:

    conn northgate-southnet
    conn northnet-southgate

    Without these, neither gateway can do IPsec to the remote subnet. There is no IPsec tunnel or eroute set up for the traffic.

    In our example, with the non-routable 192.168.* addresses used, packets would simply be discarded. In a different configuration, with routable addresses for the remote subnet, they would be sent unencrypted since there would be no IPsec eroute and there would be a normal IP route.

    You might also want:

    conn northgate-southgate

    This is required if you want the two gateways to speak IPsec to each other.

    This requires a lot of duplication of details. Judicious use of also= and include can reduce this problem.

    Note that, while FreeS/WAN supports all four tunnel types, not all implementations do. In particular, some versions of Windows 2000 and the freely downloadable version of PGP provide only "client" functionality. You cannot use them as gateways with a subnet behind them. To get that functionality, you must upgrade to Windows 2000 server or the commercially available PGP products.

    One tunnel plus advanced routing

    It is also possible to use the new routing features in 2.2 and later kernels to avoid most needs for multple tunnels. Here is one mailing list message on the topic:
    Subject: Re: linux-ipsec: IPSec packets not entering tunnel?
       Date: Mon, 20 Nov 2000
       From: Justin Guyett <>
    On Mon, 20 Nov 2000, Claudia Schmeing wrote:
    > Right                                                         Left
    >                      "home"                "office"
    > ---- ========= ----
    > I've created all four tunnels, and can ping to test each of them,
    > *except* homegate-officenet.
    I keep wondering why people create all four tunnels.  Why not route
    traffic generated from home to out ipsec0 with iproute2?
    And 99% of the time you don't need to access "office" directly, which
    means you can eliminate all but the subnet<->subnet connection.
    and FreeS/WAN technical lead Henry Spencer's comment:
    > I keep wondering why people create all four tunnels.  Why not route
    > traffic generated from home to out ipsec0 with iproute2?
    This is feasible, given some iproute2 attention to source addresses, but
    it isn't something we've documented yet... (partly because we're still
    making some attempt to support 2.0.xx kernels, which can't do this, but
    mostly because we haven't caught up with it yet).
    > And 99% of the time you don't need to access "office" directly, which
    > means you can eliminate all but the subnet<->subnet connection.
    Correct in principle, but people will keep trying to ping to or from the
    gateways during testing, and sometimes they want to run services on the
    gateway machines too.

    Extruded Subnets

    What we call extruded subnets are a special case of VPNs.

    If your buddy has some unused IP addresses, in his subnet far off at the other side of the Internet, he can loan them to you... provided that the connection between you and him is fast enough to carry all the traffic between your machines and the rest of the Internet. In effect, he "extrudes" a part of his address space over the network to you, with your Internet traffic appearing to originate from behind his Internet gateway.

    Suppose your friend has a.b.c.0/24 and wants to give you a.b.c.240/28. The initial situation is:

        subnet           gateway          Internet
      a.b.c.0/24    a.b.c.1    p.q.r.s
    where anything from the Internet destined for any machine in a.b.c.0/24 is routed via p.q.r.s and that gateway knows what to do from there.

    Of course it is quite normal for various smaller subnets to exist behind your friend's gateway. For example, your friend's company might have a.b.c.16/28=development, a.b.c.32/28=marketing and so on. The Internet neither knows not cares about this; it just delivers packets to the p.q.r.s and lets the gateway do whatever needs to be done from there.

    What we want to do is take a subnet, perhaps a.b.c.240/28, out of your friend's physical location while still having your friend's gateway route to it. As far as the Internet is concerned, you remain behind that gateway.

        subnet           gateway          Internet       your gate  extruded
      a.b.c.0/24   a.b.c.1     p.q.r.s              d.e.f.g         a.b.c.240/28                
                               ========== tunnel ==========

    The extruded addresses have to be a complete subnet.

    In our example, the friend's security gateway is also his Internet gateway, but this is not necessary. As long as all traffic from the Internet to his addresses passes through the Internet gate, the security gate could be a machine behind that. The IG would need to route all traffic for the extruded subnet to the SG, and the SG could handle the rest.

    First, configure your subnet using the extruded addresses. Your security gateway's interface to your subnet needs to have an extruded address (possibly using a Linux virtual interface , if it also has to have a different address). Your gateway needs to have a route to the extruded subnet, pointing to that interface. The other machines at your site need to have addresses in that subnet, and default routes pointing to your gateway.

    If any of your friend's machines need to talk to the extruded subnet, they need to have a route for the extruded subnet, pointing at his gateway.

    Then set up an IPsec subnet-to-subnet tunnel between your gateway and his, with your subnet specified as the extruded subnet, and his subnet specified as "". Do it with manual keying first for testing, and then with automatic keying for production use.

    The tunnel description should be:

    conn extruded

    If either side was doing firewalling for the extruded subnet before the IPsec connection is set up, ipsec_manual and ipsec_auto need to know about that (via the {left|right}firewall parameters) so that it can be overridden for the duration of the connection.

    And it all just works. Your SG routes traffic for -- that is, the whole Internet -- through the tunnel to his SG, which then sends it onward as if it came from his subnet. When traffic for the extruded subnet arrives at his SG, it gets sent through the tunnel to your SG, which passes it to the right machine.

    Remember that when ipsec_manual or ipsec_auto takes a connection down, it does not undo the route it made for that connection. This lets you take a connection down and bring up a new one, or a modified version of the old one, without having to rebuild the route it uses and without any risk of packets which should use IPsec accidentally going out in the clear. Because the route always points into KLIPS, the packets will always go there. Because KLIPS temporarily has no idea what to do with them (no eroute for them), they will be discarded.

    If you do want to take the route down, this is what the "unroute" operation in manual and auto is for. Just do an unroute after doing the down.

    Note that the route for a connection may have replaced an existing non-IPsec route. Nothing in Linux FreeS/WAN will put that pre-IPsec route back. If you need it back, you have to create it with the route command.

    Road Warrior with virtual IP address

    Here is a mailing list message about another way to configure for road warrior support:

    Subject: Re: linux-ipsec: understanding the vpn
       Date: Thu, 28 Oct 1999 10:43:22 -0400
       From: Irving Reid <>
    >  local-------linux------internet------mobile
    >  LAN        box                         user
    >  ...
    >  now when the mobile user connects to the linux box
    >  it is given a virtual IP address, i have configured it to
    >  be in the 10.x.x.x range. mobile user and linux box 
    >  have a tunnel between them with these IP addresses.
    >   Uptil this all is fine.
    If it is possible to configure your mobile client software *not* to
    use a virtual IP address, that will make your life easier. It is easier
    to configure FreeS/WAN to use the actual address the mobile user gets
    from its ISP.
    Unfortunately, some Windows clients don't let you choose.
    >  what i would like to know is that how does the mobile
    >  user communicate with other computers on the local
    >  LAN , of course with the vpn ?
    >   what IP address should the local LAN 
    >  computers have ? I guess their default gateway 
    >  should be the linux box ? and does the linux box need
    >  to be a 2 NIC card box or one is fine.
    As someone else stated, yes, the Linux box would usually be the default
    IP gateway for the local lan.
    If you mobile user has software that *must* use a virtual IP address,
    the whole picture changes. Nobody has put much effort into getting
    FreeS/WAN to play well in this environment, but here's a sketch of one
    Local Lan
        +- Linux FreeS/WAN
    Mobile User
          Virtual Address:
    Note that the Local Lan network (1.0.0.x) can be registered, routable
    Now, the Mobile User sets up an IPSec security association with the
    Linux box (; it should ESP encapsulate all traffic to the
    network 1.0.0.x **EXCEPT** UDP port 500. 500/udp is required for the key
    negotiation, which needs to work outside of the IPSec tunnel.
    On the Linux side, there's a bunch of stuff you need to do by hand (for
    now). FreeS/WAN should correctly handle setting up the IPSec SA and
    routes, but I haven't tested it so this may not work...
    The FreeS/WAN conn should look like:
    conn mobile
            left=  # The infamous "road warrior"
    Note that the left subnet contains *only* the remote host's virtual
    Hopefully the routing table on the FreeS/WAN box ends up looking like
    % netstat -rn
    Kernel IP routing table
    Destination     Gateway      Genmask         Flags   MSS Window  irtt Iface   U      1500 0          0 eth0       U      3584 0          0 lo         UG     1500 0          0 eth0 UG     1433 0          0 ipsec0
    So, if anybody sends a packet for to the Linux box, it should
    get bundled up and sent through the tunnel. To get the packets for to the Linux box in the first place, you need to use "proxy
    How this works is: when a host or router on the local Ethernet segment
    wants to send a packet to, it sends out an Ethernet level
    broadcast "ARP request". If was on the local LAN, it would
    reply, saying "send IP packets for to my Ethernet address".
    Instead, you need to set up the Linux box so that _it_ answers ARP
    requests for, even though that isn't its IP address. That
    convinces everyone else on the lan to send packets to the Linux
    box, where the usual FreeS/WAN processing and routing take over.
    % arp -i eth0 -s -D eth0 pub
    This says, if you see an ARP request on interface eth0 asking for, respond with the Ethernet address of interface eth0.
    Now, as I said at the very beginning, if it is *at all* possible to
    configure your client *not* to use the virtual IP address, you can avoid
    this whole mess.

    Dynamic Network Interfaces

    Sometimes you have to cope with a situation where the network interface(s) aren't all there at boot. The common example is notebooks with PCMCIA.


    The key issue here is that the config setup section of the /etc/ipsec.conf configuration file lists the connection between ipsecN and hardware interfaces, in the interfaces= variable. At any time when ipsec setup start or ipsec setup restart is run this variable must correspond to the current real situation. More precisely, it must not mention any hardware interfaces which don't currently exist. The difficulty is that an ipsec setup start command is normally run at boot time so interfaces that are not up then are mis-handled.

    Boot Time

    Normally, an ipsec setup start is run at boot time. However, if the hardware situation at boot time is uncertain, one of two things must be done.

    Change Time

    When the hardware *is* in place, IPsec has to be made aware of it. Someday there may be a nice way to do this.

    Right now, the way to do it is to fix the /etc/ipsec.conf file appropriately, so interfaces reflects the new situation, and then restart the IPsec subsystem. This does break any existing IPsec connections.

    If IPsec wasn't brought up at boot time, do

            ipsec setup start
    while if it was, do
            ipsec setup restart
    which won't be as quick.

    If some of the hardware is to be taken out, before doing that, amend the configuration file so interfaces no longer includes it, and do

            ipsec setup restart

    Again, this breaks any existing connections.

    Unencrypted tunnels

    Sometimes you might want to create a tunnel without encryption. Often this is a bad idea, even if you have some data which need not be private. See this discussion.

    The IPsec protocols provide two ways to do build such tunnels:

    using ESP with null encryption
    not supported by FreeS/WAN
    using AH without ESP
    supported for manually keyed connections
    possible with explicit commands via ipsec_whack(8) (see this list message)
    not supported in the ipsec_auto(8) scripts.
    One situation in which this comes up is when otherwise some data would be encrypted twice. Alice wants a secure tunnel from her machine to Bob's. Since she's behind one security gateway and he's behind another, part of the tunnel that they build passes through the tunnel that their site admins have built between the gateways. All of Alice and Bob's messages are encrypted twice.

    There are several ways to handle this.

    Note that if Alice and Bob want end-to-end security, they must build a tunnel end-to-end between their machines or use some other end-to-end tool such as PGP or SSL that suits their data. The only question is whether the admins build some special unencrypted tunnel for those already-encrypted packets.

    Linux FreeS/WAN background

    This section discusses a number of issues which have three things in common:

    Grouping them here lets us provide the explanations some users will need without unduly complicating the main text.

    Some DNS background

    Opportunistic encryption requires that the gateway systems be able to fetch public keys, and other IPsec-related information, from each other's DNS (Domain Name Service) records.

    DNS is a distributed database that maps names to IP addresses and vice versa.

    Much good reference material is available for DNS, including:

    We give only a brief overview here, intended to help you use DNS for FreeS/WAN purposes.

    Forward and reverse maps

    Although the implementation is distributed, it is often useful to speak of DNS as if it were just two enormous tables:

    Both maps can optionally contain additional data. For opportunistic encryption, we insert the data need for IPsec authentication.

    A system named with IP address should have at least two DNS records, one in each map: IN A
    used to look up the name and get an IP address IN PTR
    used for reverse lookups, looking up an address to get the associated name. Notice that the digits here are in reverse order; the actual address is but we use here.

    For both maps there is a hierachy of DNS servers and a system of delegating authority so that, for example:

    Returning to the example records: IN A IN PTR
    some syntactic details are: The capitalised strings after IN indicate the type of record. Possible types include: To set up for opportunistic encryption, you add some KEY and TXT records to your DNS data. Details are in our quickstart document.

    Problems with packet fragmentation

    It seems, from mailing list reports, to be moderately common for problems to crop up in which small packets pass through the IPsec tunnels just fine but larger packets fail.

    These problems are caused by various devices along the way mis-handling either packet fragments or path MTU discovery.

    IPsec makes packets larger by adding an ESP or AH header. This can tickle assorted bugs in fragment handling in routers and firewalls, or in path MTU discovery mechanisms, and cause a variety of symptoms which are both annoying and, often, quite hard to diagnose.

    An explanation from project technical lead Henry Spencer:

    The problem is IP fragmentation; more precisely, the problem is that the
    second, third, etc. fragments of an IP packet are often difficult for
    filtering mechanisms to classify.
    Routers cannot rely on reassembling the packet, or remembering what was in
    earlier fragments, because the fragments may be out of order or may even
    follow different routes.  So any general, worst-case filtering decision
    pretty much has to be made on each fragment independently.  (If the router
    knows that it is the only route to the destination, so all fragments
    *must* pass through it, reassembly would be possible... but most routers
    don't want to bother with the complications of that.)
    All fragments carry roughly the original IP header, but any higher-level
    header is (for IP purposes) just the first part of the packet data... so
    only the first fragment carries that.  So, for example, on examining the
    second fragment of a TCP packet, you could tell that it's TCP, but not
    what port number it is destined for -- that information is in the TCP
    header, which appears in the first fragment only. 
    The result of this classification difficulty is that stupid routers and
    over-paranoid firewalls may just throw fragments away.  To get through
    them, you must reduce your MTU enough that fragmentation will not occur.
    (In some cases, they might be willing to attempt reassembly, but have very
    limited resources to devote to it, meaning that packets must be small and
    fragments few in number, leading to the same conclusion:  smaller MTU.)
    In addition to the problem Henry describes, you may also have trouble with path MTU discovery.

    By default, FreeS/WAN uses a large MTU for the ipsec device. This avoids some problems, but may complicate others. Here's an explanation from Claudia:

    Here are a couple of pieces of background information. Apologies if you
    have seen these already. An excerpt from one of my old posts:
        An MTU of 16260 on ipsec0 is usual. The IPSec device defaults to this 
        high MTU so that it does not fragment incoming packets before encryption 
        and encapsulation. If after IPSec processing packets are larger than 1500,
        [ie. the mtu of eth0] then eth0 will fragment them. 
        Adding IPSec headers adds a certain number of bytes to each packet. 
        The MTU of the IPSec interface refers to the maximum size of the packet
        before the IPSec headers are added. In some cases, people find it helpful 
        to set ipsec0's MTU to 1500-(IPSec header size), which IIRC is about 1430.
        That way, the resulting encapsulated packets don't exceed 1500. On most 
        networks, packets less than 1500 will not need to be fragmented.
    and... (from Henry Spencer)
        The way it *ought* to work is that the MTU advertised by the ipsecN
        interface should be that of the underlying hardware interface, less a
        pinch for the extra headers needed. 
        Unfortunately, in certain situations this breaks many applications.
        There is a widespread implicit assumption that the smallest MTUs are 
        at the ends of paths, not in the middle, and another that MTUs are 
        never less than 1500.  A lot of code is unprepared to handle paths 
        where there is an "interior minimum" in the MTU, especially when it's 
        less than 1500. So we advertise a big MTU and just let the resulting 
        big packets fragment.
    This usually works, but we do get bitten in cases where some intermediate
    point can't handle all that fragmentation.  We can't win on this one.
    The MTU can be changed with an overridemtu= statement in the config setup section of ipsec.conf.5.

    Here is an example of the difficulty of diagnosing an MTU-related problem, from the mailing list:

    Date: Mon, 3 Apr 2000
    From: "Michael H. Warfield" <>
    Paul Koning wrote:
    >  Chris>  It appears that the Osicom router discards IP
    >  Chris> fragments...
    > Amazing.  A device that discards fragments isn't a router, it's at
    > best a boat anchor.
            It may not be exactly what it appears.  I ran into a similar problem
    with an ISDN link a while ago giving similar symptoms.  Turned out that
    the device was negotiating an MTU that it really couldn't handle and the
    device in front of it (a Linux box with always defragment enabled) was
    defragmenting the huge IPSec datagrams and then refragmenting them into
    hunks that the ISDN PPP thought it could handle but couldn't.  Problem was
    solved by manually capping the MTU on the ISDN link to a smaller value.
            I gave the FreeSwan guys a hard time until tracking it down since
    FreeSwan was the only thing that appeared to be able to tickle the bug.
    Nothing else seemed to be broken.  What it really was that MTU discovery
    was avoiding the problem on normal links and it was only the IPsec tunnels
    that were creating huge datagrams that went through the defragment/refragment
            Point here is that it "appeared" as though the ISDN link was
    failing to handle fragments when it was really a configuration error and
    a software bug resulting in a bad MTU that was really the culprit.  So
    it may not be that the router is not handling fragments.  It may be that
    it's missconfigured or has some other bug that only FreeSwan is tripping

    Network address translation (NAT)

    Network Address T ranslation is a service provided by some gateway machines. Calling it NAPT (adding the word Port) would be more precise, but we will follow the widespread usage.

    A gateway doing NAT rewrites the headers of packets it is forwarding, changing one or more of:

    On Linux 2.4, NAT services are provided by the netfilter(8) firewall code. There are several Netfilter HowTos including one on NAT.

    For older versions of Linux, this was referred to as "IP masquerade" and different tools were used. See this resource page.

    NAT to non-routable addresses

    The most common application of NAT uses private non-routable addresses.

    Often a home or small office network will have:

    Of course this poses a problem since several machines cannot use one address. The best solution might be to obtain more addresses, but often this is impractical or uneconomical.

    A common solution is to have:

    The client machines are set up with reserved non-routable IP addresses defined in RFC 1918. The masquerading gateway, the machine with the actual link to the Internet, rewrites packet headers so that all packets going onto the Internet appear to come from one IP address, that of its Internet interface. It then gets all the replies, does some table lookups and more header rewriting, and delivers the replies to the appropriate client machines.

    As far as anyone else on the Internet is concerned, the systems behind the gateway are completely hidden. Only one machine with one IP address is visible.

    For IPsec on such a gateway, you can entirely ignore the NAT in:

    Those can be set up exactly as they would be if your gateway had no other systems behind it.

    You do, however, have to take account of the NAT in firewall rules which affect packet forwarding.

    NAT to routable addresses

    NAT to routable addresses is also possible, but is less common and may make for rather tricky routing problems. We will not discuss it here. See the Netfilter HowTos.

    FreeS/WAN script examples

    This file is intended to hold a collection of user-written example scripts or configuration files for use with FreeS/WAN.

    So far it has only one entry.

    Poltorak's Firewall script

    From: Poltorak Serguei <>
    Subject: [Users] Using FreeS/WAN
    Date: Tue, 16 Oct 2001
    I'm using FreeS/WAN IPsec for half a year. I learned a lot of things about
    it and I think it would be interesting for someone to see the result of my
    experiments and usage of FreeS/WAN. If you find a mistake in this
    file, please e-mail me. And excuse me for my english... I'm learning.. :)
    I'll talk about vary simple configuration:
    addresses prefix = 192.168
        lan1          sgw1     .0.0/24 (Internet)       sgw2            lan2
      .1.0/24---[ .1.1 ; .0.1 ]===================[ .0.10 ; . 2.10 ]---.2.0/24
    We need to let lan1 see lan2 across Internet like it is behind sgw1. The
    same for lan2. And we need to do IPX bridge for Novel Clients and NDS
    my config:
    ------------------- ipsec.conf -------------------
    conn lan1-lan2
    --------------- end of ipsec.conf ----------------
    ping .2.x from .1.y   (y != 1)
    It works?? Fine. Let's continue...
    Why y != 1 ?? Because kernel of sgw1 have 2 IP addresses and it will choose
    the first IP (which is used to go to Internet) .0.1 and the packet won't go
    through IPsec tunnel :(  But if do ping on .1.1 kernel will respond from
    that address (.1.1) and the packet will be tunneled. The same problem occurred then
    .2.x sends a packet to .1.2 which is down at the moment. What happens? .1.1
    sends ARP requesting .1.2... after 3 tries it send to .2.x an destunreach,
    but from his "natural" IP or .0.1 . So the error message won't be delivered!
    It's a big problem...
    Resolution... One can manipulate with ipsec0 or ipsec0:0 to solve the
    problem (if ipsec0 has .1.1 kernel will send packets correctly), but there
    are powerful and elegant iproute2 :) We simply need to change source address
    of packet that goes to other secure lan. This is done with
    ip route replace via dev ipsec0 src
    Cool!! Now it works!!
    The second step. We want install firewall on sgw1 and sgw2. Encryption of 
    traffic without security isn't a good idea. I don't use {left|right}firewall, 
    because I'm running firewall from init scripts.
    We want IPsec data between lan1-lan2, some ICMP errors (destination
    unreachable, TTL exceeded, parameter problem and source quench), replying on 
    pings from both lans and Internet, ipxtunnel data for IPX and of course SSH
    between sgw1 and sgw2 and from/to one specified host.
    I'm using ipchains. With iptables there are some changes.
    ---------------- rc.firewall ---------------------
    # Firewall for IPsec lan1-lan2
    # left
    # right
    # SSH from and to this host
    # this is for left. exchange these values for right.
    $IPC -F
    $IPC -P input DENY
    $IPC -P forward DENY
    $IPC -P output DENY
    # Loopback traffic
    $IPC -A input -i lo -j ACCEPT
    $IPC -A output -i lo -j ACCEPT
    # for IPsec SGW1-SGW2
    ## IKE
    $IPC -A input -p udp -s $PEER_EXT 500 -d $MY_EXT 500 -i $EXT_IF -j ACCEPT
    $IPC -A output -p udp -s $MY_EXT 500 -d $PEER_EXT 500 -i $EXT_IF -j ACCEPT
    ## ESP
    $IPC -A input -p 50 -s $PEER_EXT -d $MY_EXT -i $EXT_IF -j ACCEPT
    ### we don't need this line ### $IPC -A output -p 50 -s $MY_EXT -d $PEER_EXT -i $EXT_IF -j ACCEPT
    ## forward LAN1-LAN2
    $IPC -A forward -s $MY_LAN -d $PEER_LAN -i $IPSEC_IF -j ACCEPT
    $IPC -A forward -s $PEER_LAN -d $MY_LAN -i $INT_IF -j ACCEPT
    $IPC -A output -s $PEER_LAN -d $MY_LAN -i $INT_IF -j ACCEPT
    $IPC -A input -s $PEER_LAN -d $MY_LAN -i $IPSEC_IF -j ACCEPT
    $IPC -A input -s $MY_LAN -d $PEER_LAN -i $INT_IF -j ACCEPT
    $IPC -A output -s $MY_LAN -d $PEER_LAN -i $IPSEC_IF -j ACCEPT
    # ICMP
    ## Dest unreachable
    ### from/to Internet
    $IPC -A input -p icmp --icmp-type destination-unreachable -s $ANY -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type destination-unreachable -s $MY_EXT -d $ANY -i $EXT_IF -j ACCEPT
    ### from/to Lan
    $IPC -A input -p icmp --icmp-type destination-unreachable -s $ANY -d $MY_INT -i $INT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type destination-unreachable -s $MY_INT -d $ANY -i $INT_IF -j ACCEPT
    ### from/to Peer Lan
    $IPC -A input -p icmp --icmp-type destination-unreachable -s $ANY -d $MY_INT -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type destination-unreachable -s $MY_INT -d $ANY -i $IPSEC_IF -j ACCEPT
    ## Source quench
    ### from/to Internet
    $IPC -A input -p icmp --icmp-type source-quench -s $ANY -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type source-quench -s $MY_EXT -d $ANY -i $EXT_IF -j ACCEPT
    ### from/to Lan
    $IPC -A input -p icmp --icmp-type source-quench -s $ANY -d $MY_INT -i $INT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type source-quench -s $MY_INT -d $ANY -i $INT_IF -j ACCEPT
    ### from/to Peer Lan
    $IPC -A input -p icmp --icmp-type source-quench -s $ANY -d $MY_INT -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type source-quench -s $MY_INT -d $ANY -i $IPSEC_IF -j ACCEPT
    ## Parameter problem
    ### from/to Internet
    $IPC -A input -p icmp --icmp-type parameter-problem -s $ANY -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type parameter-problem -s $MY_EXT -d $ANY -i $EXT_IF -j ACCEPT
    ### from/to Lan
    $IPC -A input -p icmp --icmp-type parameter-problem -s $ANY -d $MY_INT -i $INT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type parameter-problem -s $MY_INT -d $ANY -i $INT_IF -j ACCEPT
    ### from/to Peer Lan
    $IPC -A input -p icmp --icmp-type parameter-problem -s $ANY -d $MY_INT -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type parameter-problem -s $MY_INT -d $ANY -i $IPSEC_IF -j ACCEPT
    ## Time To Live exceeded
    ### from/to Internet
    $IPC -A input -p icmp --icmp-type time-exceeded -s $ANY -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type time-exceeded -s $MY_EXT -d $ANY -i $EXT_IF -j ACCEPT
    ### to Lan
    $IPC -A input -p icmp --icmp-type time-exceeded -s $ANY -d $MY_INT -i $INT_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type time-exceeded -s $MY_INT -d $ANY -i $INT_IF -j ACCEPT
    ### to Peer Lan
    $IPC -A input -p icmp --icmp-type time-exceeded -s $ANY -d $MY_INT -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p icmp --icmp-type time-exceeded -s $MY_INT -d $ANY -i $IPSEC_IF -j ACCEPT
    # ICMP PINGs
    ## from Internet
    $IPC -A input -p icmp -s $ANY -d $MY_EXT --icmp-type echo-request  -i $EXT_IF -j ACCEPT
    $IPC -A output -p icmp -s $MY_EXT -d $ANY --icmp-type echo-reply  -i $EXT_IF -j ACCEPT
    ## from LAN
    $IPC -A input -p icmp -s $ANY -d $MY_INT --icmp-type echo-request -i $INT_IF -j ACCEPT
    $IPC -A output -p icmp -s $MY_INT -d $ANY --icmp-type echo-reply  -i $INT_IF -j ACCEPT
    ## from Peer LAN
    $IPC -A input -p icmp -s $ANY -d $MY_INT --icmp-type echo-request -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p icmp -s $MY_INT -d $ANY --icmp-type echo-reply  -i $IPSEC_IF -j ACCEPT
    # SSH
    ## from SSH_PEER_HOST
    $IPC -A input -p tcp -s $SSH_PEER_HOST -d $MY_EXT 22 -i $EXT_IF -j ACCEPT
    $IPC -A output -p tcp \! -y -s $MY_EXT 22 -d $SSH_PEER_HOST -i $EXT_IF -j ACCEPT
    ## to SSH_PEER_HOST
    $IPC -A input -p tcp \! -y -s $SSH_PEER_HOST 22 -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p tcp -s $MY_EXT -d $SSH_PEER_HOST 22 -i $EXT_IF -j ACCEPT
    ## from PEER
    $IPC -A input -p tcp -s $PEER_EXT -d $MY_EXT 22 -i $EXT_IF -j ACCEPT
    $IPC -A output -p tcp \! -y -s $MY_EXT 22 -d $PEER_EXT -i $EXT_IF -j ACCEPT
    ## to PEER
    $IPC -A input -p tcp \! -y -s $PEER_EXT 22 -d $MY_EXT -i $EXT_IF -j ACCEPT
    $IPC -A output -p tcp -s $MY_EXT -d $PEER_EXT 22 -i $EXT_IF -j ACCEPT
    # ipxtunnel
    $IPC -A input -p udp -s $PEER_INT 2005 -d $MY_INT 2005 -i $IPSEC_IF -j ACCEPT
    $IPC -A output -p udp -s $MY_INT 2005 -d $PEER_INT 2005 -i $IPSEC_IF -j ACCEPT
    ---------------- end of rc.firewall ----------------------
    To understand this we need to look on this scheme:
               || ipsec0                                             |
               \/                                                    |
     eth0  +--------+    /---------/ yes  /---------/ yes +-----------------------+
    ------>| INPUT  |-->/ ?local? /----->/ ?IPsec? /----->| decrypt decapsulate |
     eth1  +--------+  /---------/      /---------/       +-----------------------+
                           || no            || no
                           \/               \/
                      +----------+      +---------+        +-------+
                      | routing  |      |  local  |        | local |
                      | decision |      | deliver |        | send  |
                      +----------+      +---------+        +-------+
                           ||                                 ||
                           \/                                 \/
                       +---------+                       +----------+
                       | forward |                       | routing  |
                       +---------+                       | decision |
                           ||                            +----------+
                           ||                                  ||
                       +--------+ eth0
                       | OUTPUT | eth1
                       +--------+ ipsec0
                       /---------/ yes +-----------------------+
                      / ?IPsec? /----->| encrypt encapsulate |
                     /---------/       +-----------------------+
                          || no                    ||
                          ||                       ||
                          ||                       \/   eth0, eth1
    This explain how a packet traverse TCP/IP stack in IPsec capable kernel.
    FIX ME, please, if there are any errors
    Test the new firewall now.
    Now about IPX. I tried 3 programs for tunneling IPX: tipxd, SIB and ipxtunnel
    tipxd didn't send packets.. :(
    SIB and ipxtunnel worked fine :)
    With ipxtunnel there was a little problem. In sources there are an error.
    --------------------- in main.c ------------------------
    <       bytes += p.len;
    >       bytes += len;
    After this FIX everything goes right...
    ------------------- /etc/ipxtunnel.conf ----------------
    port    2005
    remote    2005
    interface eth1
    --------------- end of /etc/ipxtunnel.conf -------------
    I use IPX tunnel between .1.1 and .2.10 so we don't need to encrypt nor
    authenticate encapsulated IPX packets, it is done with IPsec.
    If you don't wont to use iproute2 to change source IP you need to use SIB
    (it is able to bind local address) or establish tunnel between .0.1 and
    .0.10 (external IPs, you need to do encryption in the program, but it isn't
    For now I'm using ipxtunnel.
    I think that's all for the moment. If there are any error, please e-mail me: . It would be cool if someone puts the scheme of TCP/IP in
    kernel and firewall example on FreeS/WAN's manual pages.

    History and politics of cryptography

    Cryptography has a long and interesting history, and has been the subject of considerable political controversy.



    The classic book on the history of cryptography is David Kahn's The Codebreakers. It traces codes and codebreaking from ancient Egypt to the 20th century.

    Diffie and Landau Privacy on the Line: The Politics of Wiretapping and Encryption covers the history from the First World War to the 1990s, with an emphasis on the US.

    World War II

    During the Second World War, the British "Ultra"project achieved one of the greatest intelligence triumphs in the history of warfare, breaking many Axis codes. One major target was the Enigma cipher machine, a German device whose users were convinced it was unbreakable. The American "Magic" project had some similar triumphs against Japanese codes.

    There are many books on this period. See our bibliography for a few, or try a (web or library) search on "Ultra" and "Enigma". Two books I particularly like are:

    Bletchley Park, where much of the Ultra work was done, now has a museum and a web site.

    The Ultra work introduced three major innovations.

    So by the end of the war, Allied code-breakers were expert at large-scale mechanised code-breaking. The payoffs were enormous.

    Postwar and Cold War

    The wartime innovations were enthusiastically adopted by post-war and Cold War signals intelligence agencies. Presumably many nations now have some agency capable of sophisticated attacks on communications security, and quite a few engage in such activity on a large scale.

    America's NSA, for example, is said to be both the world's largest employer of mathematicians and the world's largest purchaser of computer equipment. Such claims may be somewhat exaggerated, but beyond doubt the NSA -- and similar agencies in other countries -- have some excellent mathematicians, lots of powerful computers, sophisticated software, and the organisation and funding to apply them on a large scale. Details of the NSA budget are secret, but there are some published estimates.

    Changes in the world's communications systems since WW II have provided these agencies with new targets. Cracking the codes used on an enemy's military or diplomatic communications has been common practice for centuries. Extensive use of radio in war made large-scale attacks such as Ultra possible. Modern communications make it possible to go far beyond that. Consider listening in on cell phones, or intercepting electronic mail, or tapping into the huge volumes of data on new media such as fiber optics or satellite links. None of these targets existed in 1950. All of them can be attacked today, and almost certainly are being attacked.

    The Ultra story was not made public until the 1970s. Much of the recent history of codes and code-breaking has not been made public, and some of it may never be. Two important books are:

    Note that these books cover only part of what is actually going on, and then only the activities of nations open and democratic enough that (some of) what they are doing can be discovered. A full picture, including:

    might be really frightening.

    Recent history -- the crypto wars

    Until quite recently, cryptography was primarily a concern of governments, especially of the military, of spies, and of diplomats. Much of it was extremely secret.

    In recent years, that has changed a great deal. With computers and networking becoming ubiquitous, cryptography is now important to almost everyone. Among the developments since the 1970s:

    This has led to a complex ongoing battle between various mainly government groups wanting to control the spread of crypto and various others, notably the computer industry and the cypherpunk crypto advocates, wanting to encourage widespread use.

    Steven Levy has written a fine history of much of this, called Crypto: How the Code rebels Beat the Government -- Saving Privacy in the Digital Age.

    The FreeS/WAN project is to a large extent an outgrowth of cypherpunk ideas. Our reasons for doing the project can be seen in these quotes from the Cypherpunk Manifesto:

    Privacy is necessary for an open society in the electronic age. ...

    We cannot expect governments, corporations, or other large, faceless organizations to grant us privacy out of their beneficence. It is to their advantage to speak of us, and we should expect that they will speak. ...

    We must defend our own privacy if we expect to have any. ...

    Cypherpunks write code. We know that someone has to write software to defend privacy, and since we can't get privacy unless we all do, we're going to write it. We publish our code so that our fellow Cypherpunks may practice and play with it. Our code is free for all to use, worldwide. We don't much care if you don't approve of the software we write. We know that software can't be destroyed and that a widely dispersed system can't be shut down.

    Cypherpunks deplore regulations on cryptography, for encryption is fundamentally a private act. ...

    For privacy to be widespread it must be part of a social contract. People must come and together deploy these systems for the common good. ...

    To quote project leader John Gilmore:
    We are literally in a race between our ability to build and deploy technology, and their ability to build and deploy laws and treaties. Neither side is likely to back down or wise up until it has definitively lost the race.

    If FreeS/WAN reaches its goal of making opportunistic encryption widespread so that secure communication can become the default for a large part of the net, we will have struck a major blow.


    The political problem is that nearly all governments want to monitor their enemies' communications, and some want to monitor their citizens. They may be very interested in protecting some of their own communications, and often some types of business communication, but not in having everyone able to communicate securely. They therefore attempt to restrict availability of strong cryptography as much as possible.

    Things various governments have tried or are trying include:

    Of course governments are by no means the only threat to privacy and security on the net. Other threats include:

    One study enumerates threats and possible responses for small and medium businesses. VPNs are a key part of the suggested strategy.

    We consider privacy a human right. See the UN's Universal Declaration of Human Rights, article twelve:

    No one shall be subjected to arbitrary interference with his privacy, family, home or correspondence, nor to attacks upon his honor and reputation. Everyone has the right to the protection of the law against such interference or attacks.
    Our objective is to help make privacy possible on the Internet using cryptography strong enough not even those well-funded government agencies are likely to break it. If we can do that, the chances of anyone else breaking it are negliible.


    Many groups are working in different ways to defend privacy on the net and elsewhere. Please consider contributing to one or more of these groups:

    For more on these issues see:

    There are several collections of crypto quotes on the net.

    See also the bibliography and our list of web references on cryptography law and policy.

    Outline of this section

    The remainder of this section includes two pieces of writing by our project leader

    and discussions of:

    and a section on press coverage of FreeS/WAN.

    From our project leader

    FreeS/WAN project founder John Gilmore wrote a web page about why we are doing this. The version below is slightly edited, to fit this format and to update some links. For a version without these edits, see his home page.

    Swan: Securing the Internet against Wiretapping

    My project for 1996 was to secure 5% of the Internet traffic against passive wiretapping. It didn't happen in 1996, so I'm still working on it in 1997, 1998, and 1999! If we get 5% in 1999 or 2000, we can secure 20% the next year, against both active and passive attacks; and 80% the following year. Soon the whole Internet will be private and secure. The project is called S/WAN or S/Wan or Swan for Secure Wide Area Network; since it's free software, we call it FreeSwan to distinguish it from various commercial implementations. RSA came up with the term "S/WAN". Our main web site is at Want to help?

    The idea is to deploy PC-based boxes that will sit between your local area network and the Internet (near your firewall or router) which opportunistically encrypt your Internet packets. Whenever you talk to a machine (like a Web site) that doesn't support encryption, your traffic goes out "in the clear" as usual. Whenever you connect to a machine that does support this kind of encryption, this box automatically encrypts all your packets, and decrypts the ones that come in. In effect, each packet gets put into an "envelope" on one side of the net, and removed from the envelope when it reaches its destination. This works for all kinds of Internet traffic, including Web access, Telnet, FTP, email, IRC, Usenet, etc.

    The encryption boxes are standard PC's that use freely available Linux software that you can download over the Internet or install from a cheap CDROM.

    This wasn't just my idea; lots of people have been working on it for years. The encryption protocols for these boxes are called IPSEC (IP Security). They have been developed by the IP Security Working Group of the Internet Engineering Task Force, and will be a standard part of the next major version of the Internet protocols ( IPv6). For today's (IP version 4) Internet, they are an option.

    The Internet Architecture Board and Internet Engineering Steering Group have taken a strong stand that the Internet should use powerful encryption to provide security and privacy. I think these protocols are the best chance to do that, because they can be deployed very easily, without changing your hardware or software or retraining your users. They offer the best security we know how to build, using the Triple-DES, RSA, and Diffie-Hellman algorithms.

    This "opportunistic encryption box" offers the "fax effect". As each person installs one for their own use, it becomes more valuable for their neighbors to install one too, because there's one more person to use it with. The software automatically notices each newly installed box, and doesn't require a network administrator to reconfigure it. Instead of "virtual private networks" we have a "REAL private network"; we add privacy to the real network instead of layering a manually-maintained virtual network on top of an insecure Internet.

    Deployment of IPSEC

    The US government would like to control the deployment of IP Security with its crypto export laws. This isn't a problem for my effort, because the cryptographic work is happening outside the United States. A foreign philanthropist, and others, have donated the resources required to add these protocols to the Linux operating system. Linux is a complete, freely available operating system for IBM PC's and several kinds of workstation, which is compatible with Unix. It was written by Linus Torvalds, and is still maintained by a talented team of expert programmers working all over the world and coordinating over the Internet. Linux is distributed under the GNU Public License, which gives everyone the right to copy it, improve it, give it to their friends, sell it commercially, or do just about anything else with it, without paying anyone for the privilege.

    Organizations that want to secure their network will be able to put two Ethernet cards into an IBM PC, install Linux on it from a $30 CDROM or by downloading it over the net, and plug it in between their Ethernet and their Internet link or firewall. That's all they'll have to do to encrypt their Internet traffic everywhere outside their own local area network.

    Travelers will be able to run Linux on their laptops, to secure their connection back to their home network (and to everywhere else that they connect to, such as customer sites). Anyone who runs Linux on a standalone PC will also be able to secure their network connections, without changing their application software or how they operate their computer from day to day.

    There will also be numerous commercially available firewalls that use this technology. RSA Data Security is coordinating the S/Wan (Secure Wide Area Network) project among more than a dozen vendors who use these protocols. There's a compatability chart that shows which vendors have tested their boxes against which other vendors to guarantee interoperatility.

    Eventually it will also move into the operating systems and networking protocol stacks of major vendors. This will probably take longer, because those vendors will have to figure out what they want to do about the export controls.

    Current status

    My initial goal of securing 5% of the net by Christmas '96 was not met. It was an ambitious goal, and inspired me and others to work hard, but was ultimately too ambitious. The protocols were in an early stage of development, and needed a lot more protocol design before they could be implemented. As of April 1999, we have released version 1.0 of the software ( freeswan-1.0.tar.gz), which is suitable for setting up Virtual Private Networks using shared secrets for authentication. It does not yet do opportunistic encryption, or use DNSSEC for authentication; those features are coming in a future release.

    The low-level encrypted packet formats are defined. The system for publishing keys and providing secure domain name service is defined. The IP Security working group has settled on an NSA-sponsored protocol for key agreement (called ISAKMP/Oakley), but it is still being worked on, as the protocol and its documentation is too complex and incomplete. There are prototype implementations of ISAKMP. The protocol is not yet defined to enable opportunistic encryption or the use of DNSSEC keys.
    Linux Implementation
    The Linux implementation has reached its first major release and is ready for production use in manually-configured networks, using Linux kernel version 2.0.36.
    Domain Name System Security
    There is now a release of BIND 8.2 that includes most DNS Security features.

    The first prototype implementation of Domain Name System Security was funded by DARPA as part of their Information Survivability program. Trusted Information Systems wrote a modified version of BIND, the widely-used Berkeley implementation of the Domain Name System.

    TIS, ISC, and I merged the prototype into the standard version of BIND. The first production version that supports KEY and SIG records is bind-4.9.5. This or any later version of BIND will do for publishing keys. It is available from the Internet Software Consortium. This version of BIND is not export-controlled since it does not contain any cryptography. Later releases starting with BIND 8.2 include cryptography for authenticating DNS records, which is also exportable. Better documentation is needed.


    Because I can. I have made enough money from several successful startup companies, that for a while I don't have to work to support myself. I spend my energies and money creating the kind of world that I'd like to live in and that I'd like my (future) kids to live in. Keeping and improving on the civil rights we have in the United States, as we move more of our lives into cyberspace, is a particular goal of mine.

    What You Can Do

    Install the latest BIND at your site.
    You won't be able to publish any keys for your domain, until you have upgraded your copy of BIND. The thing you really need from it is the new version of named, the Name Daemon, which knows about the new KEY and SIG record types. So, download it from the Internet Software Consortium and install it on your name server machine (or get your system administrator, or Internet Service Provider, to install it). Both your primary DNS site and all of your secondary DNS sites will need the new release before you will be able to publish your keys. You can tell which sites this is by running the Unix command "dig MYDOMAIN ns" and seeing which sites are mentioned in your NS (name server) records.
    Set up a Linux system and run a 2.0.x kernel on it
    Get a machine running Linux (say the 5.2 release from Red Hat). Give the machine two Ethernet cards.
    Install the Linux IPSEC (Freeswan) software
    If you're an experienced sysadmin or Linux hacker, install the freeswan-1.0 release, or any later release or snapshot. These releases do NOT provide automated "opportunistic" operation; they must be manually configured for each site you wish to encrypt with.
    Get on the linux-ipsec mailing list
    The discussion forum for people working on the project, and testing the code and documentation, is: To join this mailing list, send email to containing a line of text that says "subscribe linux-ipsec". (You can later get off the mailing list the same way -- just send "unsubscribe linux-ipsec").

    Check back at this web page every once in a while
    I update this page periodically, and there may be new information in it that you haven't seen. My intent is to send email to the mailing list when I update the page in any significant way, so subscribing to the list is an alternative.

    Would you like to help? I can use people who are willing to write documentation, install early releases for testing, write cryptographic code outside the United States, sell pre-packaged software or systems including this technology, and teach classes for network administrators who want to install this technology. To offer to help, send me email at Tell me what country you live in and what your citizenship is (it matters due to the export control laws; personally I don't care). Include a copy of your resume and the URL of your home page. Describe what you'd like to do for the project, and what you're uniquely qualified for. Mention what other volunteer projects you've been involved in (and how they worked out). Helping out will require that you be able to commit to doing particular things, meet your commitments, and be responsive by email. Volunteer projects just don't work without those things.

    Related projects

    IPSEC for NetBSD
    This prototype implementation of the IP Security protocols is for another free operating system. Download BSDipsec.tar.gz.
    IPSEC for OpenBSD
    This prototype implementation of the IP Security protocols is for yet another free operating system. It is directly integrated into the OS release, since the OS is maintained in Canada, which has freedom of speech in software.

    Stopping wholesale monitoring

    From a message project leader John Gilmore posted to the mailing list:

    John Denker wrote:
    > Indeed there are several ways in which the documentation overstates the 
    > scope of what this project does -- starting with the name 
    > FreeS/WAN.  There's a big difference between having an encrypted IP tunnel 
    > versus having a Secure Wide-Area Network.  This software does a fine job of 
    > the former, which is necessary but not sufficient for the latter.
    The goal of the project is to make it very hard to tap your wide area
    communications.  The current system provides very good protection
    against passive attacks (wiretapping and those big antenna farms).
    Active attacks, which involve the intruder sending packets to your
    system (like packets that break into sendmail and give them a root
    shell :-) are much harder to guard against.  Active attacks that
    involve sending people (breaking into your house and replacing parts
    of your computer with ones that transmit what you're doing) are also
    much harder to guard against.  Though we are putting effort into
    protecting against active attacks, it's a much bigger job than merely
    providing strong encryption.  It involves general computer security,
    and general physical security, which are two very expensive problems
    for even a site to solve, let alone to build into a whole society.
    The societal benefit of building an infrastructure that protects
    well against passive attacks is that it makes it much harder to do
    undetected bulk monitoring of the population.  It's a defense against
    police-states, not against policemen.
    Policemen can put in the effort required to actively attack sites that
    they have strong suspicions about.  But police states won't be able to
    build systems that automatically monitor everyone's communications.
    Either they will be able to monitor only a small subset of the
    populace (by targeting those who screwed up their passive security),
    or their monitoring activities will be detectable by those monitored
    (active attacks leave packet traces or footprints), which can then be
    addressed through the press and through political means if they become
    too widespread.
    FreeS/WAN does not protect very well against traffic analysis, which
    is a kind of widespread police-state style monitoring that still
    reveals significant information (who's talking to who) without
    revealing the contents of what was said.  Defenses against traffic
    analysis are an open research problem.  Zero Knowledge Systems is
    actively deploying a system designed to thwart it, designed by Ian
    Goldberg.  The jury is out on whether it actually works; a lot more
    experience with it will be needed.

    Notes on things mentioned in that message:

    Government promotion of weak crypto

    Various groups, especially governments and especially the US government, have a long history of advocating various forms of bogus security.

    We regard bogus security as extremely dangerous. If users are deceived into relying on bogus security, then they may be exposed to large risks. They would be better off having no security and knowing it. At least then they would be careful about what they said.

    Avoiding bogus security is a key design criterion for everything we do in FreeS/WAN. The most conspicuous example is our refusal to support single DES. Other IPsec "features" which we do not implement are discussed in our compatibility document.

    Escrowed encryption

    Various governments have made persistent attempts to encourage or mandate "escrowed encrytion", also called "key recovery", or GAK for "government access to keys". The idea is that cryptographic keys be held by some third party and turned over to law enforcement or security agencies under some conditions.

      Mary had a little key - she kept it in escrow,
      and every thing that Mary said,
      the feds were sure to know.
    A crypto quotes page attributes this to Sam Simpson.

    There is an excellent paper available on Risks of Escrowed Encryption, from a group of cryptographic luminaries which included our project leader.

    Like any unnecessary complication, GAK tends to weaken security of any design it infects. For example:

    FreeS/WAN does not support escrowed encryption, and never will.

    Limited key lengths

    Various governments, and some vendors, have also made persistent attempts to convince people that:

    This is nonsense.

    Weak systems touted include:

    The notion that choice of ciphers or keysize should be determined by a trade-off between security requirements and overheads is pure bafflegab.

    In short, there has never been any technical reason to use inadequate ciphers. The only reason there has ever been for anyone to use such ciphers is that government agencies want weak ciphers used so that they can crack them. The alleged savings are simply propaganda.

       Mary had a little key (It's all she could export),
       and all the email that she sent was opened at the Fort.
    A crypto quotes page attributes this to Ron Rivest. NSA headquarters is at Fort Meade, Maryland.

    Of course, making systems secure does involve costs, and trade-offs can be made between cost and security. There can be substantial hardware and software costs. There are almost always substantial staff or contracting costs:

    Compared to those costs, cipher overheads are an insignificant factor in the cost of security. Note, however, that choosing an insecure cipher can cause all your other investment to be wasted.

    Our policy in FreeS/WAN is to use only cryptographic components with adequate keylength and no known weaknesses.

    These decisions imply that we cannot fully conform to the IPsec RFCs, since those have DES as the only required cipher and Group 1 as the only required DH group. (In our view, the standards were subverted into offerring bogus security.) Fortunately, we can still interoperate with most other IPsec implementations since nearly all implementers provide at least 3DES and Group 2 as well.

    We hope that eventually the RFCs will catch up with our (and others') current practice and reject dubious components. Some of our team and a number of others are working on this in IETF working groups.

    Cryptography Export Laws

    Many nations restrict the export of cryptography and some restrict its use by their citizens or others within their borders.

    US Law

    US laws, as currently interpreted by the US government, forbid export of most cryptographic software from the US in machine-readable form without government permission. In general, the restrictions apply even if the software is widely-disseminated or public-domain and even if it came from outside the US originally. Cryptography is legally a munition and export is tightly controlled under the EAR Export Administration Regulations.

    If you are a US citizen, your brain is considered US territory no matter where it is physically located at the moment. The US believes that its laws apply to its citizens everywhere, not just within the US. Providing technical assistance or advice to foreign "munitions" projects is illegal. The US government has very little sense of humor about this issue and does not consider good intentions to be sufficient excuse. Beware.

    The official website for these regulations is run by the Commerce Department's Bureau of Export Administration (BXA).

    The Bernstein case challenges the export restrictions on Constitutional grounds. Code is speech so restrictions on export of code violate the First Amendment's free speech provisions. This argument has succeeded in two levels of court so far. It is quite likely to go on to the Supreme Court.

    The regulations were changed substantially in January 2000, apparently as a government attempt to get off the hook in the Bernstein case. It is now legal to export public domain source code for encryption, provided you notify the BXA.

    There are, however, still restrictions in force. article. --> Moreover, the regulations can still be changed again whenever the government chooses to do so. Short of a Supreme Court ruling (in the Berstein case or another) that overturns the regulations completely, the problem of export regulation is not likely to go away in the forseeable future.

    US contributions to FreeS/WAN

    The FreeS/WAN project cannot accept software contributions, not even small bug fixes, from US citizens or residents. We want it to be absolutely clear that our distribution is not subject to US export law. Any contribution from an American might open that question to a debate we'd prefer to avoid. It might also put the contributor at serious legal risk.

    Of course Americans can still make valuable contributions (many already have) by reporting bugs, or otherwise contributing to discussions, on the project mailing list. Since the list is public, this is clearly constitutionally protected free speech.

    Note, however, that the export laws restrict Americans from providing technical assistance to foreign "munitions" projects. The government might claim that private discussions or correspondence with FreeS/WAN developers were covered by this. It is not clear what the courts would do with such a claim, so we strongly encourage Americans to use the list rather than risk the complications.

    What's wrong with restrictions on cryptography

    Some quotes from prominent cryptography experts:

    The real aim of current policy is to ensure the continued effectiveness of US information warfare assets against individuals, businesses and governments in Europe and elsewhere.
    Ross Anderson, Cambridge University
    If the government were honest about its motives, then the debate about crypto export policy would have ended years ago.
    Bruce Schneier, Counterpane Systems
    The NSA regularly lies to people who ask it for advice on export control. They have no reason not to; accomplishing their goal by any legal means is fine by them. Lying by government employees is legal.
    John Gilmore.

    The Internet Architecture Board (IAB) and the Internet Engineering Steering Group (IESG) made a strong statement in favour of worldwide access to strong cryptography. Essentially the same statement is in the appropriately numbered RFC 1984. Two critical paragraphs are:

    ... various governments have actual or proposed policies on access to cryptographic technology ...

    (a) ... export controls ...
    (b) ... short cryptographic keys ...
    (c) ... keys should be in the hands of the government or ...
    (d) prohibit the use of cryptology ...

    We believe that such policies are against the interests of consumers and the business community, are largely irrelevant to issues of military security, and provide only a marginal or illusory benefit to law enforcement agencies, ...

    The IAB and IESG would like to encourage policies that allow ready access to uniform strong cryptographic technology for all Internet users in all countries.

    Our goal in the FreeS/WAN project is to build just such "strong cryptographic technology" and to distribute it "for all Internet users in all countries".

    More recently, the same two bodies (IESG and IAB) have issued RFC 2804 on why the IETF should not build wiretapping capabilities into protocols for the convenience of security or law enforcement agenicies. The abstract from that document is:

    The Internet Engineering Task Force (IETF) has been asked to take a position on the inclusion into IETF standards-track documents of functionality designed to facilitate wiretapping.

    This memo explains what the IETF thinks the question means, why its answer is "no", and what that answer means.

    A quote from the debate leading up to that RFC:

    We should not be building surveillance technology into standards. Law enforcement was not supposed to be easy. Where it is easy, it's called a police state.
    Jeff Schiller of MIT, in a discussion of FBI demands for wiretap capability on the net, as quoted by Wired.
    The Raven mailing list was set up for this IETF discussion.

    Our goal is to go beyond that RFC and prevent Internet wiretapping entirely.

    The Wassenaar Arrangement

    Restrictions on the export of cryptography are not just US policy, though some consider the US at least partly to blame for the policies of other nations in this area.

    A number of countries:

    Argentina, Australia, Austria, Belgium, Bulgaria, Canada, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Luxembourg, Netherlands, New Zealand, Norway, Poland, Portugal, Republic of Korea, Romania, Russian Federation, Slovak Republic, Spain, Sweden, Switzerland, Turkey, Ukraine, United Kingdom and United States

    have signed the Wassenaar Arrangement which restricts export of munitions and other tools of war. Cryptographic sofware is covered there.

    Wassenaar details are available from the Wassenaar Secretariat, and elsewhere in a more readable HTML version.

    For a critique see the GILC site:

    The Global Internet Liberty Campaign (GILC) has begun a campaign calling for the removal of cryptography controls from the Wassenaar Arrangement.

    The aim of the Wassenaar Arrangement is to prevent the build up of military capabilities that threaten regional and international security and stability . . .

    There is no sound basis within the Wassenaar Arrangement for the continuation of any export controls on cryptographic products.

    We agree entirely.

    An interesting analysis of Wassenaar can be found on the site.

    Export status of Linux FreeS/WAN

    We believe our software is entirely exempt from these controls since the Wassenaar General Software Note says:

    The Lists do not control "software" which is either:
    1. Generally available to the public by . . . retail . . . or
    2. "In the public domain".

    There is a note restricting some of this, but it is a sub-heading under point 1, so it appears not to apply to public domain software.

    Their glossary defines "In the public domain" as:

    . . . "technology" or "software" which has been made available without restrictions upon its further dissemination.

    N.B. Copyright restrictions do not remove "technology" or "software" from being "in the public domain".

    We therefore believe that software freely distributed under the GNU Public License, such as Linux FreeS/WAN, is exempt from Wassenaar restrictions.

    Most of the development work is being done in Canada. Our understanding is that the Canadian government accepts this interpretation.

    Recent copies of the freely modifiable and distributable source code exist in many countries. Citizens all over the world participate in its use and evolution, and guard its ongoing distribution. Even if Canadian policy were to change, the software would continue to evolve in countries which do not restrict exports, and would continue to be imported from there into unfree countries. "The Net culture treats censorship as damage, and routes around it."

    Help spread IPsec around

    You can help. If you don't know of a Linux FreeS/WAN archive in your own country, please download it now to your personal machine, and consider making it publicly accessible if that doesn't violate your own laws. If you have the resources, consider going one step further and setting up a mirror site for the whole munitions Linux crypto software archive.

    If you make Linux CD-ROMs, please consider including this code, in a way that violates no laws (in a free country, or in a domestic-only CD product).

    Please send a note about any new archive mirror sites or CD distributions to so we can update the documentation.

    Lists of current mirror sites and of distributions which include FreeS/WAN are in our introduction section.

    DES is Not Secure

    DES, the Data Encryption S tandard, can no longer be considered secure. While no major flaws in its innards are known, it is fundamentally inadequate because its 56-bit key is too short. It is vulnerable to brute-force search of the whole key space, either by large collections of general-purpose machines or even more quickly by specialized hardware. Of course this also applies to any other cipher with only a 56-bit key. The only reason anyone could have for using a 56 or 64-bit key is to comply with various export laws intended to ensure the use of breakable ciphers.

    Non-government cryptologists have been saying DES's 56-bit key was too short for some time -- some of them were saying it in the 70's when DES became a standard -- but the US government has consistently ridiculed such suggestions.

    A group of well-known cryptographers looked at key lengths in a 1996 paper. They suggested a minimum of 75 bits to consider an existing cipher secure and a minimum of 90 bits for new ciphers. More recent papers, covering both symmetric and public key systems are at and For all algorithms, the minimum keylengths recommended in such papers are significantly longer than the maximums allowed by various export laws.

    In a 1998 ruling, a German court described DES as "out-of-date and not safe enough" and held a bank liable for using it.

    Dedicated hardware breaks DES in a few days

    The question of DES security has now been settled once and for all. In early 1998, the Electronic Frontier Foundation built a DES-cracking machine. It can find a DES key in an average of a few days' search. The details of all this, including complete code listings and complete plans for the machine, have been published in Cracking DES, by the Electronic Frontier Foundation.

    That machine cost just over $200,000 to design and build. "Moore's Law" is that machines get faster (or cheaper, for the same speed) by roughly a factor of two every 18 months. At that rate, their $200,000 in 1998 becomes $50,000 in 2001.

    However, Moore's Law is not exact and the $50,000 estimate does not allow for the fact that a copy based on the published EFF design would of course cost far less than the original. We cannot say exactly what such a cracker would cost today, but it would likely be somewhere between $10,000 and $100,000.

    A large corporation could build one of these out of petty cash. The cost is low enough for a senior manager to hide it in a departmental budget and avoid having to announce or justify the project. Any government agency, from a major municipal police force up, could afford one. Or any other group with a respectable budget -- criminal organisations, political groups, labour unions, religious groups, ... Or any millionaire with an obsession or a grudge, or just strange taste in toys.

    One might wonder if a private security or detective agency would have one for rent. They wouldn't need many clients to pay off that investment.

    Spooks may break DES faster yet

    As for the security and intelligence agencies of various nations, some of them may have had DES crackers for years. Possibly very fast ones! Cipher-cracking is one of the few known applications which is easy to speed up by just adding more processors and memory. Within very broad limits, you can make it as fast as you like if you have the budget. The EFF's $200,000 machine breaks DES in a few days. An aviation website gives the cost of a B1 bomber as $200,000,000. Spending that much, an intelligence agency could expect to break DES in an average time of six and a half minutes.

    That estimate assumes they use the EFF's 1998 technology and just spend more money. They may have an attack that is superior to brute force, they quite likely have better chip technology (Moore's law, a bigger budget, and whatever secret advances they may have made) and of course they may have spent the price of an aircraft carrier, not just one aircraft.

    In short, we have no idea how quickly these organisations can break DES. Unless they're grossly incompetent, they can certainly do it more quickly than the users of the cipher would like, but beyond that we can't say. Pick any time unit between days and milliseconds. None of these is entirely unbelievable. More to the point, none of them is of any comfort if you don't want such organisations reading your communications.

    Note that this may be a concern even if nothing you do is a threat to anyone's national security. An intelligence agency might well consider it to be in their national interest for certain companies to do well. If you're competing against such companies in a world market and that agency can read your secrets, you have a serious problem.

    One might wonder about technology the former Soviet Union and its allies developed for cracking DES during the Cold War. They must have tried; the cipher was an American standard and widely used. How well did they succeed? Is their technology now for sale or rent?

    Networks break DES in a few weeks

    Before the definitive EFF effort, DES had been cracked several times by people using many machines. See this press release for example.

    A major corporation, university, or government department could break DES by using spare cycles on their existing collection of computers, by dedicating a group of otherwise surplus machines to the problem, or by combining the two approaches. It might take them weeks or months, rather than the days required for the EFF machine, but they could do it.

    What about someone working alone, without the resources of a large organisation? For them, cracking DES will not be easy, but it may be possible. A few thousand dollars buys a lot of surplus workstations. A pile of such machines will certainly heat your garage nicely and might break DES in a few months or years. Or enroll at a university and use their machines. Or use an employer's machines. Or crack security somewhere and steal the resources to crack a DES key. Or write a virus that steals small amounts of resources on many machines. Or . . .

    None of these approaches are easy or break DES really quickly, but an attacker only needs to find one that is feasible and breaks DES quickly enough to be dangerous. How much would you care to bet that this will be impossible if the attacker is clever and determined? How valuable is your data? Are you authorised to risk it on a dubious bet?

    We disable DES

    In short, it is now absolutely clear that DES is not secure against

    That is why Linux FreeS/WAN disables all transforms which use plain DES for encryption.

    DES is in the source code, because we need DES to implement our default encryption transform, Triple DES. We urge you not to use single DES. We do not provide any easy way to enable it in FreeS/WAN, and our policy is to provide no assistance to anyone wanting to do so.

    40-bits is laughably weak

    The same is true, in spades, of ciphers -- DES or others -- crippled by 40-bit keys, as many ciphers were required to be until recently under various export laws. A brute force search of such a cipher's keyspace is 216 times faster than a similar search against DES. The EFF's machine can do a brute-force search of a 40-bit key space in seconds. One contest to crack a 40-bit cipher was won by a student using a few hundred idle machines at his university. It took only three and half hours.

    We do not, and will not, implement any 40-bit cipher.

    Triple DES is almost certainly secure

    Triple DES, usually abbreviated 3DES, applies DES three times, with three different keys. DES seems to be basically an excellent cipher design; it has withstood several decades of intensive analysis without any disastrous flaws being found. It's only major flaw is that the small keyspace allows brute force attacks to succeeed. Triple DES enlarges the key space to 168 bits, making brute-force search a ridiculous impossibility.

    3DES is currently the only block cipher implemented in FreeS/WAN. 3DES is, unfortunately, about 1/3 the speed of DES, but modern CPUs still do it at quite respectable speeds. Some speed measurements for our code are available.

    AES in IPsec

    The AES project has recently chosen a replacement for DES, a new standard cipher for use in non-classified US government work and in regulated industries such as banking. This cipher will almost certainly become widely used for many applications, including IPsec, but perhaps not quickly.

    The winner, announced in October 2000 after several years of analysis and discussion, was the Rijndael cipher from two Belgian designers.

    It is likely that many IPsec implementations will add Rijndael support over the next few months or years. FreeS/WAN will almost certainly do so, but it is not high on the priority list. This might be an excellent project for a volunteer.

    Press coverage of Linux FreeS/WAN:

    FreeS/WAN 1.0 press

    Press release for version 1.0

            Strong Internet Privacy Software Free for Linux Users Worldwide
    Toronto, ON, April 14, 1999 - 
    The Linux FreeS/WAN project today released free software to protect
    the privacy of Internet communications using strong encryption codes.
    FreeS/WAN automatically encrypts data as it crosses the Internet, to
    prevent unauthorized people from receiving or modifying it.  One
    ordinary PC per site runs this free software under Linux to become a
    secure gateway in a Virtual Private Network, without having to modify
    users' operating systems or application software.  The project built
    and released the software outside the United States, avoiding US
    government regulations which prohibit good privacy protection.
    FreeS/WAN version 1.0 is available immediately for downloading at
    "Today's FreeS/WAN release allows network administrators to build
    excellent secure gateways out of old PCs at no cost, or using a cheap
    new PC," said John Gilmore, the entrepreneur who instigated the
    project in 1996.  "They can build operational experience with strong
    network encryption and protect their users' most important
    communications worldwide."
    "The software was written outside the United States, and we do not
    accept contributions from US citizens or residents, so that it can be
    freely published for use in every country," said Henry Spencer, who
    built the release in Toronto, Canada.  "Similar products based in the
    US require hard-to-get government export licenses before they can be
    provided to non-US users, and can never be simply published on a Web
    site.  Our product is freely available worldwide for immediate
    downloading, at no cost."
    FreeS/WAN provides privacy against both quiet eavesdropping (such as
    "packet sniffing") and active attempts to compromise communications
    (such as impersonating participating computers).  Secure "tunnels" carry
    information safely across the Internet between locations such as a
    company's main office, distant sales offices, and roaming laptops.  This
    protects the privacy and integrity of all information sent among those
    locations, including sensitive intra-company email, financial transactions
    such as mergers and acquisitions, business negotiations, personal medical
    records, privileged correspondence with lawyers, and information about
    crimes or civil rights violations.  The software will be particularly
    useful to frequent wiretapping targets such as private companies competing
    with government-owned companies, civil rights groups and lawyers,
    opposition political parties, and dissidents. 
    FreeS/WAN provides privacy for Internet packets using the proposed
    standard Internet Protocol Security (IPSEC) protocols.  FreeS/WAN
    negotiates strong keys using Diffie-Hellman key agreement with 1024-bit
    keys, and encrypts each packet with 168-bit Triple-DES (3DES).  A modern
    $500 PC can set up a tunnel in less than a second, and can encrypt
    6 megabits of packets per second, easily handling the whole available
    bandwidth at the vast majority of Internet sites.  In preliminary testing,
    FreeS/WAN interoperated with 3DES IPSEC products from OpenBSD, PGP, SSH,
    Cisco, Raptor, and Xedia.  Since FreeS/WAN is distributed as source code,
    its innards are open to review by outside experts and sophisticated users,
    reducing the chance of undetected bugs or hidden security compromises.
    The software has been in development for several years.  It has been
    funded by several philanthropists interested in increased privacy on
    the Internet, including John Gilmore, co-founder of the Electronic
    Frontier Foundation, a leading online civil rights group.
    Press contacts:
    Hugh Daniel,   +1 408 353 8124,
    Henry Spencer, +1 416 690 6561,
    * FreeS/WAN derives its name from S/WAN, which is a trademark of RSA Data
      Security, Inc; used by permission.

    The IPsec protocols

    This section provides details of the IPsec protocols which FreeS/WAN implements

    The basic idea of IPsec is to provide security functions, authentication and encryption, at the IP (Internet Protocol) level. This requires a higher-level protocol (IKE) to set things up for the IP-level services (ESP and AH).

    Three protocols are used in an IPsec implementation:

    ESP, Encapsulating Security Payload
    Encrypts and/or authenticates data
    AH, Authentication Header
    Provides a packet authentication service
    IKE, Internet Key Exchange
    Negotiates connection parameters, including keys, for the other two

    The term "IPsec" (also written as IPSEC) is slightly ambiguous. In some contexts, it includes all three of the above but in other contexts it refers only to AH and ESP.

    Applying IPsec

    Authentication and encryption functions for network data can, of course, be provided at other levels. Many security protocols work at levels above IP.

    and so on. Other techniques work at levels below IP. For example, data on a communications circuit or an entire network can be encrypted by specialised hardware. This is common practice in high-security applications.

    Advantages of IPsec

    There are, however, advantages to doing it at the IP level instead of, or as well as, at other levels.

    IPsec is the most general way to provide these services for the Internet.

    IPsec, however, can protect any protocol running above IP and any medium which IP runs over. More to the point, it can protect a mixture of application protocols running over a complex combination of media. This is the normal situation for Internet communication; IPsec is the only general solution.

    IPsec can also provide some security services "in the background", with no visible impact on users. To use PGP encryption and signatures on mail, for example, the user must at least:

    These systems can be designed so that the burden on users is not onerous, but any system will place some requirements on users. No such system can hope to be secure if users are sloppy about meeting those requirements. The author has seen username and password stuck on terminals with post-it notes in an allegedly secure environment, for example.

    Limitations of IPsec

    IPsec is designed to secure IP links between machines. It does that well, but it is important to remember that there are many things it does not do. Some of the important limitations are:

    IPsec cannot be secure if your system isn't
    System security on IPsec gateway machines is an essential requirement if IPsec is to function as designed. No system can be trusted if the underlying machine has been subverted. See books on Unix security such as Garfinkel and Spafford or our web references for Linux security or more general computer security.

    Of course, there is another side to this. IPsec can be a powerful tool for improving system and network security. For example, requiring packet authentication makes various spoofing attacks harder and IPsec tunnels can be extremely useful for secure remote administration of various things.

    IPsec is not end-to-end
    IPsec cannot provide the same end-to-end security as systems working at higher levels. IPsec encrypts an IP connection between two machines, which is quite a different thing than encrypting messages between users or between applications.

    For example, if you need mail encrypted from the sender's desktop to the recipient's desktop and decryptable only by the recipient, use PGP or another such system. IPsec can encrypt any or all of the links involved -- between the two mail servers, or between either server and its clients. It could even be used to secure a direct IP link from the sender's desktop machine to the recipient's, cutting out any sort of network snoop. What it cannot ensure is end-to-end user-to-user security. If only IPsec is used to secure mail, then anyone with appropriate privileges on any machine where that mail is stored (at either end or on any store-and-forward servers in the path) can read it.

    In another common setup, IPsec encrypts packets at a security gateway machine as they leave the sender's site and decrypts them on arrival at the gateway to the recipient's site. This does provide a useful security service -- only encrypted data is passed over the Internet -- but it does not even come close to providing an end-to-end service. In particular, anyone with appropriate privileges on either site's LAN can intercept the message in unencrypted form.

    IPsec cannot do everything
    IPsec also cannot provide all the functions of systems working at higher levels of the protocol stack. If you need a document electronically signed by a particular person, then you need his or her digital signature and a public key cryptosystem to verify it with.

    Note, however, that IPsec authentication of the underlying communication can make various attacks on higher-level protocols more difficult. In particular, authentication prevents man-in-the-middle attacks.

    IPsec authenticates machines, not users
    IPsec uses strong authentication mechanisms to control which messages go to which machines, but it does not have the concept of user ID, which is vital to many other security mechansims and policies. This means some care must be taken in fitting the various security mechansims on a network together. For example, if you need to control which users access your database server, you need some non-IPsec mechansim for that. IPsec can control which machines connect to the server, and can ensure that data transfer to those machines is done securely, but that is all. Either the machines themselves must control user access or there must be some form of user authentication to the database, independent of IPsec.
    IPsec does not stop denial of service attacks
    Denial of service attacks aim at causing a system to crash, overload, or become confused so that legitimate users cannot get whatever services the system is supposed to provide. These are quite different from attacks in which the attacker seeks either to use the service himself or to subvert the service into delivering incorrect results.

    IPsec shifts the ground for DoS attacks; the attacks possible against systems using IPsec are different than those that might be used against other systems. It does not, however, eliminate the possibility of such attacks.

    IPsec does not stop traffic analysis
    Traffic analysis is the attempt to derive intelligence from messages without regard for their contents. In the case of IPsec, it would mean analysis based on things visible in the unencrypted headers of encrypted packets -- source and destination gateway addresses, packet size, et cetera. Given the resources to acquire such data and some skill in analysing it (both of which any national intelligence agency should have), this can be a very powerful technique.

    IPsec is not designed to defend against this. Partial defenses are certainly possible, and some are described below, but it is not clear that any complete defense can be provided.

    IPsec is a general mechanism for securing IP

    While IPsec does not provide all functions of a mail encryption package, it can encrypt your mail. In particular, it can ensure that all mail passing between a pair or a group of sites is encrypted. An attacker looking only at external traffic, without access to anything on or behind the IPsec gateway, cannot read your mail. He or she is stymied by IPsec just as he or she would be by PGP.

    The advantage is that IPsec can provide the same protection for anything transmitted over IP. In a corporate network example, PGP lets the branch offices exchange secure mail with head office. SSL and SSH allow them to securely view web pages, connect as terminals to machines, and so on. IPsec can support all those applications, plus database queries, file sharing (NFS or Windows), other protocols encapsulated in IP (Netware, Appletalk, ...), phone-over-IP, video-over-IP, ... anything-over-IP. The only limitation is that IP Multicast is not yet supported, though there are Internet Draft documents for that.

    IPsec creates secure tunnels through untrusted networks . Sites connected by these tunnels form VPNs, Virtual Private Networks.

    IPsec gateways can be installed wherever they are required.

    Which of these, or of the many other possible variants, to use is up to you. IPsec provides mechanisms; you provide the policy .

    No end user action is required for IPsec security to be used; they don't even have to know about it. The site administrators, of course, do have to know about it and to put some effort into making it work. Poor administration can compromise IPsec as badly as the post-it notes mentioned above. It seems reasonable, though, for organisations to hope their system administrators are generally both more security-conscious than end users and more able to follow computer security procedures. If not, at least there are fewer of them to educate or replace.

    IPsec can be, and often should be, used with along with security protocols at other levels. If two sites communicate with each other via the Internet, then IPsec is the obvious way to protect that communication. If two others have a direct link between them, either link encryption or IPsec would make sense. Choose one or use both. Whatever you use at and below the IP level, use other things as required above that level. Whatever you use above the IP level, consider what can be done with IPsec to make attacks on the higher levels harder. For example, man-in-the-middle attacks on various protocols become difficult if authentication at packet level is in use on the potential victims' communication channel.

    Using authentication without encryption

    Where appropriate, IPsec can provide authentication without encryption. One might do this, for example:

    Authentication has lower overheads than encryption.

    The protocols provide four ways to build such connections, using either an AH-only connection or ESP using null encryption, and in either manually or automatically keyed mode. FreeS/WAN supports only one of these, manually keyed AH-only connections, and we do not recommend using that. Our reasons are discussed under Resisting traffic analysis a few sections further along.

    Encryption without authentication is dangerous

    Originally, the IPsec encryption protocol ESP didn't do integrity checking. It only did encryption. Steve Bellovin found many ways to attack ESP used without authentication. See his paper Problem areas for the IP Security Protocols. To make a secure connection, you had to add an AH Authentication Header as well as ESP. Rather than incur the overhead of several layers (and rather than provide an ESP layer that didn't actually protect the traffic), the IPsec working group built integrity and replay checking directly into ESP.

    Today, typical usage is one of:

    Other variants are allowed by the standard, but not much used:

    ESP encryption without authentication
    Bellovin has demonstrated fatal flaws in this. Do not use.
    ESP encryption with AH authentication
    This has higher overheads than using the authentication in ESP, and no obvious benefit in most cases. The exception might be a network where AH authentication was widely or universally used. If you're going to do AH to conform with network policy, why authenticate again in the ESP layer?
    Authenticate twice, with AH and with ESP
    Why? Of course, some folk consider "belt and suspenders" the sensible approach to security. If you're among them, you might use both protocols here. You might also use both to satisfy different parts of a security policy. For example, an organisation might require AH authentication everywhere but two users within the organisation might use ESP as well.
    ESP authentication without encryption
    The standard allows this, calling it "null encryption". FreeS/WAN does not support it. We recommend that you use AH instead if authentication is all you require. AH authenticates parts of the IP header, which ESP-null does not do.
    Some of these variants cannot be used with FreeS/WAN because we do not support ESP-null and do not support automatic keying of AH-only connections.

    There are fairly frequent suggestions that AH be dropped entirely from the IPsec specifications since ESP and null encryption can handle that situation. It is not clear whether this will occur. My guess is that it is unlikely.

    Multiple layers of IPsec processing are possible

    The above describes combinations possible on a single IPsec connection. In a complex network you may have several layers of IPsec in play, with any of the above combinations at each layer.

    For example, a connection from a desktop machine to a database server might require AH authentication. Working with other host, network and database security measures, AH might be just the thing for access control. You might decide not to use ESP encryption on such packets, since it uses resources and might complicate network debugging. Within the site where the server is, then, only AH would be used on those packets.

    Users at another office, however, might have their whole connection (AH headers and all) passing over an IPsec tunnel connecting their office to the one with the database server. Such a tunnel should use ESP encryption and authentication. You need authentication in this layer because without authentication the encryption is vulnerable and the gateway cannot verify the AH authentication. The AH is between client and database server; the gateways aren't party to it.

    In this situation, some packets would get multiple layers of IPsec applied to them, AH on an end-to-end client-to-server basis and ESP from one office's security gateway to the other.

    Resisting traffic analysis

    Traffic analysis is the attempt to derive useful intelligence from encrypted traffic without breaking the encryption.

    Is your CEO exchanging email with a venture capital firm? With bankruptcy trustees? With an executive recruiting agency? With the holder of some important patents? If an eavesdropper learns about any of those, then he has interesting intelligence on your company, whether or not he can read the messages themselves.

    Except in the simplest cases, traffic analysis is hard to do well. It requires both considerable resources and considerable analytic skill. However, intelligence agencies of various nations have been doing it for centuries and many of them are likely quite good at it by now. Various commercial organisations, especially those working on "targeted marketing" may also be quite good at analysing certain types of traffic.

    In general, defending against traffic analysis is also difficult. Inventing a really good defense could get you a PhD and some interesting job offers.

    IPsec is not designed to stop traffic analysis and we know of no plausible method of extending it to do so. That said, there are ways to make traffic analysis harder. This section describes them.

    Using "unnecessary" encryption

    One might choose to use encryption even where it appears unnecessary in order to make analysis more difficult. Consider two offices which pass a small volume of business data between them using IPsec and also transfer large volumes of Usenet news. At first glance, it would seem silly to encrypt the newsfeed, except possibly for any newsgroups that are internal to the company. Why encrypt data that is all publicly available from many sites?

    However, if we encrypt a lot of news and send it down the same connection as our business data, we make traffic analysis much harder. A snoop cannot now make inferences based on patterns in the volume, direction, sizes, sender, destination, or timing of our business messages. Those messages are hidden in a mass of news messages encapsulated in the same way.

    If we're going to do this we need to ensure that keys change often enough to remain secure even with high volumes and with the adversary able to get plaintext of much of the data. We also need to look at other attacks this might open up. For example, can the adversary use a chosen plaintext attack, deliberately posting news articles which, when we receive and encrypt them, will help break our encryption? Or can he block our business data transmission by flooding us with silly news articles? Or ...

    Also, note that this does not provide complete protection against traffic analysis. A clever adversary might still deduce useful intelligence from statistical analysis (perhaps comparing the input newsfeed to encrypted output, or comparing the streams we send to different branch offices), or by looking for small packets which might indicate establishment of TCP connections, or ...

    As a general rule, though, one can improve resistance to traffic analysis by encrypting as much traffic as possible rather than only as much as seems necessary.

    Using multiple encryption

    This also applies to using multiple layers of encryption. If you have an IPsec tunnel between two branch offices, it might appear silly to send PGP-encrypted email through that tunnel. However, if you suspect someone is snooping your traffic, then it does make sense: Similar arguments apply for SSL-encrypted web traffic or SSH-encrypted remote login sessions, even for end-to-end IPsec tunnels between systems in the two offices.

    Using fewer tunnels

    It may also help to use fewer tunnels. For example, if all you actually need encrypted is connections between:

    You might build one tunnel per mail server and one per remote database user, restricting traffic to those applications. This gives the traffic analyst some information, however. He or she can distinguish the tunnels by looking at information in the ESP header and, given that distinction and the patterns of tunnel usage, might be able to figure out something useful. Perhaps not, but why take the risk?

    We suggest instead that you build one tunnel per branch office, encrypting everything passing from head office to branches. This has a number of advantages:

    Of course you might also want to add additional tunnels. For example, if some of the database data is confidential and should not be exposed even within the company, then you need protection from the user's desktop to the database server. We suggest you do that in whatever way seems appropriate -- IPsec, SSH or SSL might fit -- but, whatever you choose, pass it between locations via a gateway-to-gateway IPsec tunnel to provide some resistance to traffic analysis.

    Cryptographic components

    IPsec combines a number of cryptographic techniques, all of them well-known and well-analyzed. The overall design approach was conservative; no new or poorly-understood components were included.

    This section gives a brief overview of each technique. It is intended only as an introduction. There is more information, and links to related topics, in our glossary. See also our bibliography and cryptography web links.

    Block ciphers

    The encryption in the ESP encapsulation protocol is done with a block cipher .

    We do not implement single DES. It is insecure. Our default, and currently only, block cipher is triple DES.

    The Rijndael block cipher has just won the AES competition to choose a relacement for DES. It will almost certainly be added to FreeS/WAN and to other IPsec implementations.

    Hash functions

    The HMAC construct

    IPsec packet authentication is done with the HMAC construct. This is not just a hash of the packet data, but a more complex operation which uses both a hashing algorithm (MD5 or SHA) and a key. It therefore does more than a simple hash would. A simple hash would only tell you that the packet data was not changed in transit, or that whoever changed it also regenerated the hash. An HMAC also tells you that the sender knew the HMAC key.

    For IPsec HMAC, the output of the hash algorithm is truncated to 96 bits. This saves some space in the packets. More important, it prevents an attacker from seeing all the hash output bits and perhaps creating some sort of attack based on that knowledge.

    Diffie-Hellman key agreement

    The Diffie-Hellman key agreement protocol allows two parties (A and B or Alice and Bob) to agree on a key in such a way that an eavesdropper who intercepts the entire conversation cannot learn the key.

    The protocol is based on the discrete logarithm problem and is therefore thought to be secure. Mathematicians have been working on that problem for years and seem no closer to a solution, though there is no proof that an efficient solution is impossible.

    RSA authentication

    The RSA algorithm (named for its inventors -- Rivest, Shamir and Adleman) is a very widely used public key cryptographic technique. It is used in IPsec as one method of authenticating gateways for Diffie-Hellman key negotiation.

    Structure of IPsec

    There are three protocols used in an IPsec implementation:

    ESP, Encapsulating Security Payload
    Encrypts and/or authenticates data
    AH, Authentication Header
    Provides a packet authentication service
    IKE, Internet Key Exchange
    Negotiates connection parameters, including keys, for the other two

    The term "IPsec" is slightly ambiguous. In some contexts, it includes all three of the above but in other contexts it refers only to AH and ESP.

    IKE (Internet Key Exchange)

    The IKE protocol sets up IPsec (ESP or AH) connections after negotiating appropriate parameters (algorithms to be used, keys, connection lifetimes) for them. This is done by exchanging packets on UDP port 500 between the two gateways.

    IKE (RFC 2409) was the outcome of a long, complex process in which quite a number of protocols were proposed and debated. Oversimplifying mildly, IKE combines:

    ISAKMP (RFC 2408)
    The Internet Security A ssociation and Key Management Protocol manages negotiation of connections and defines SAs (Security Associations) as a means of describing connection properties.
    IPsec DOI for ISAKMP (RFC 2407)
    A Domain Of I nterpretation fills in the details necessary to turn the rather abstract ISAKMP protocol into a more tightly specified protocol, so it becomes applicable in a particular domain.
    Oakley key determination protocol (RFC 2412)
    Oakley creates keys using the Diffie-Hellman key agreement protocol.

    For all the details, you would need to read the four RFCs just mentioned (over 200 pages) and a number of others. We give a summary below, but it is far from complete.

    Phases of IKE

    IKE negotiations have two phases.

    Phase one
    The two gateways negotiate and set up a two-way ISAKMP SA which they can then use to handle phase two negotiations. One such SA between a pair of gateways can handle negotiations for multiple tunnels.
    Phase two
    Using the ISAKMP SA, the gateways negotiate IPsec (ESP and/or AH) SAs as required. IPsec SAs are unidirectional (a different key is used in each direction) and are always negotiated in pairs to handle two-way traffic. There may be more than one pair defined between two gateways.
    Both of these phases use the UDP protocol and port 500 for their negotiations.

    After both IKE phases are complete, you have IPsec SAs to carry your encrypted data. These use the ESP or AH protocols. These protocols do not have ports; ports apply only to UDP or TCP.

    The IKE protocol is designed to be extremely flexible. Among the things that can be negotiated (separately for each SA) are:

    The protocol also allows implementations to add their own encryption algorithms, authentication algorithms or Diffie-Hellman groups. We do not support any such extensions.

    There are a number of complications:

    These complications can of course lead to problems, particularly when two different implementations attempt to interoperate. For example, we have seen problems such as:

    Despite this, we do interoperate successfully with many implementations, including both Windows 2000 and PGPnet. Details are in our interoperability document.

    Structure of IKE messages

    Here is our Pluto developer explaining some of this on the mailing list:

    When one IKE system (for example, Pluto) is negotiating with another
    to create an SA, the Initiator proposes a bunch of choices and the
    Responder replies with one that it has selected.
    The structure of the choices is fairly complicated.  An SA payload
    contains a list of lists of "Proposals".  The outer list is a set of
    choices: the selection must be from one element of this list.
    Each of these elements is a list of Proposals.  A selection must be
    made from each of the elements of the inner list.  In other words,
    *all* of them apply (that is how, for example, both AH and ESP can
    apply at once).
    Within each of these Proposals is a list of Transforms.  For each
    Proposal selected, one Transform must be selected (in other words,
    each Proposal provides a choice of Transforms).
    Each Transform is made up of a list of Attributes describing, well,
    attributes.  Such as lifetime of the SA.  Such as algorithm to be
    used.  All the Attributes apply to a Transform.
    You will have noticed a pattern here: layers alternate between being
    disjunctions ("or") and conjunctions ("and").
    For Phase 1 / Main Mode (negotiating an ISAKMP SA), this structure is
    cut back.  There must be exactly one Proposal.  So this degenerates to
    a list of Transforms, one of which must be chosen.

    IPsec Services, AH and ESP

    IPsec offers two services, authentication and encryption. These can be used separately but are often used together.

    Packet-level authentication allows you to be confident that a packet came from a particular machine and that its contents were not altered en route to you. No attempt is made to conceal or protect the contents, only to assure their integrity.

    Packet authentication can be provided separately using an Authentication Header, described just below, or it can be included as part of the ESP (Encapsulated Security Payload) service, described in the following section. That service offers encryption as well as authentication.

    Encryption allows you to conceal the contents of a message from eavesdroppers.

    In IPsec this is done using a block cipher (normally Triple DES for Linux). In the most used setup, keys are automatically negotiated, and periodically re-negotiated, using the IKE (Internet Key Exchange) protocol. In Linux FreeS/WAN this is handled by the Pluto Daemon.

    The IPsec protocol offering encryption is ESP, Encapsulated Security Payload. It can also include a packet authentication service.

    Note that encryption should always be used with some packet authentication service. Unauthenticated encryption is vulnerable to man-in-the-middle attacks. Also note that encryption does not necessarily prevent traffic analysis.

    The Authentication Header (AH)

    Packet authentication can be provided separately from encryption by adding an authentication header (AH) after the IP header but before the other headers on the packet. This is the subject of this section. Details are in RFC 2402.

    Each of the several headers on a packet header contains a "next protocol" field telling the system what header to look for next. IP headers generally have either TCP or UDP in this field. When IPsec authentication is used, the packet IP header has AH in this field, saying that an Authentication Header comes next. The AH header then has the next header type -- usually TCP, UDP or encapsulated IP.

    IPsec packet authentication can be added in transport mode, as a modification of standard IP transport. This is shown in this diagram from the RFC:

                      BEFORE APPLYING AH
          IPv4  |orig IP hdr  |     |      |
                |(any options)| TCP | Data |
                      AFTER APPLYING AH
          IPv4  |orig IP hdr  |    |     |      |
                |(any options)| AH | TCP | Data |
                     except for mutable fields

    Athentication can also be used in tunnel mode, encapsulating the underlying IP packet beneath AH and an additional IP header.

    IPv4  | new IP hdr* |    | orig IP hdr*  |    |      |
          |(any options)| AH | (any options) |TCP | Data |
          |           in the new IP hdr                  |

    This would normally be used in a gateway-to-gateway tunnel. The receiving gateway then strips the outer IP header and the AH header and forwards the inner IP packet.

    The mutable fields referred to are things like the time-to-live field in the IP header. These cannot be included in authentication calculations because they change as the packet travels.

    Keyed MD5 and Keyed SHA

    The actual authentication data in the header is typically 96 bits and depends both on a secret shared between sender and receiver and on every byte of the data being authenticated.

    The algorithms involved are the MD5 Message Digest Algorithm or SHA, the Secure Hash Algorithm. For details on their use in this application, see RFCs 2403 and 2404 respectively.

    For descriptions of the algorithms themselves, see RFC 1321 for MD5 and FIPS (Federal Information Processing Standard) number 186 from NIST, the US National Institute of Standards and Technology for SHA. Applied Cryptography covers both in some detail, MD5 starting on page 436 and SHA on 442.

    These algorithms are intended to make it nearly impossible for anyone to alter the authenticated data in transit. The sender calculates a digest or hash value from that data and includes the result in the authentication header. The recipient does the same calculation and compares results. For unchanged data, the results will be identical. The hash algorithms are designed to make it extremely difficult to change the data in any way and still get the correct hash.

    Since the shared secret key is also used in both calculations, an interceptor cannot simply alter the authenticated data and change the hash value to match. Without the key, he or she (or even the dreaded They) cannot produce a usable hash.

    Sequence numbers

    The authentication header includes a sequence number field which the sender is required to increment for each packet. The receiver can ignore it or use it to check that packets are indeed arriving in the expected sequence.

    This provides partial protection against replay attacks in which an attacker resends intercepted packets in an effort to confuse or subvert the receiver. Complete protection is not possible since it is necessary to handle legitmate packets which are lost, duplicated, or delivered out of order, but use of sequence numbers makes the attack much more difficult.

    The RFCs require that sequence numbers never cycle, that a new key always be negotiated before the sequence number reaches 2^32-1. This protects both against replays attacks using packets from a previous cyclce and against birthday attacks on the the packet authentication algorithm.

    In Linux FreeS/WAN, the sequence number is ignored for manually keyed connections and checked for automatically keyed ones. In automatic mode, we do that. In manual mode, there is no way to negotiate a new key, or to recover from a sequence number problem, so we don't use sequence numbers.

    Encapsulated Security Payload (ESP)

    The ESP protocol is defined in RFC 2406. It provides one or both of encryption and packet authentication. It may be used with or without AH packet authentication.

    Note that some form of packet authentication should always be used whenever data is encrypted. Without authentication, the encryption is vulnerable to active attacks which may allow an enemy to break the encryption. ESP should always either include its own authentication or be used with AH authentication.

    The RFCs require support for only two mandatory encryption algorithms -- DES, and null encryption -- and for two authentication methods -- keyed MD5 and keyed SHA. Implementers may choose to support additional algorithms in either category.

    The authentication algorithms are the same ones used in the IPsec authentication header.

    We do not implement single DES since DES is insecure. Instead we provide triple DES or 3DES . This is currently the only encryption algorithm supported.

    We do not implement null encryption since it is obviously insecure.

    IPsec modes

    IPsec can connect in two modes. Transport mode is a host-to-host connection involving only two machines. In tunnel mode, the IPsec machines act as gateways and trafiic for any number of client machines may be carried.

    Tunnel mode

    Security gateways are required to support tunnel mode connections. In this mode the gateways provide tunnels for use by client machines behind the gateways. The client machines need not do any IPsec processing; all they have to do is route things to gateways.

    Transport mode

    Host machines (as opposed to security gateways) with IPsec implementations must also support transport mode. In this mode, the host does its own IPsec processing and routes some packets via IPsec.

    FreeS/WAN parts

    KLIPS: Kernel IPsec Support

    KLIPS is KerneL IP SEC Support, the modifications necessary to support IPsec within the Linux kernel. KILPS does all the actual IPsec packet-handling, including

    KLIPS also checks all non-IPsec packets to ensure they are not bypassing IPsec security policies.

    The Pluto daemon

    Pluto(8) is a daemon which implements the IKE protocol. It

    Pluto is controlled mainly by the ipsec.conf(5) configuration file.

    The ipsec(8) command

    The ipsec(8) command is a front end that allows control over IPsec activity.

    Linux FreeS/WAN configuration file

    The configuration file for Linux FreeS/WAN is

    For details see the ipsec.conf(5) manual page and our Configuration section.

    Key management

    There are several ways IPsec can manage keys. Not all are implemented in Linux FreeS/WAN.

    Currently Implemented Methods

    Manual keying

    IPsec allows keys to be manually set. In Linux FreeS/WAN, such keys are stored with the connection definitions in /etc/ipsec.conf.

    Manual keying is useful for debugging since it allows you to test the KLIPS kernel IPsec code without the Pluto daemon doing key negotiation.

    In general, however, automatic keying is preferred because it is more secure.

    Automatic keying

    In automatic keying, the Pluto daemon negotiates keys using the IKE Internet Key Exchange protocol. Connections are automatically re-keyed periodically.

    This is considerably more secure than manual keying. In either case an attacker who acquires a key can read every message encrypted with that key, but automatic keys can be changed every few hours or even every few minutes without breaking the connection or requiring intervention by the system administrators. Manual keys can only be changed manually; you need to shut down the connection and have the two admins make changes. Moreover, they have to communicate the new keys securely, perhaps with PGP or SSH . This may be possible in some cases, but as a general solution it is expensive, bothersome and unreliable. Far better to let Pluto handle these chores; no doubt the administrators have enough to do.

    Also, automatic keying is inherently more secure against an attacker who manages to subvert your gateway system. If manual keying is in use and an adversary acquires root privilege on your gateway, he reads your keys from /etc/ipsec.conf and then reads all messages encrypted with those keys.

    If automatic keying is used, an adversary with the same privileges can read /etc/ipsec.secrets, but this does not contain any keys, only the secrets used to authenticate key exchanges. Having an adversary able to authenticate your key exchanges need not worry you overmuch. Just having the secrets does not give him any keys. You are still secure against passive attacks. This property of automatic keying is called perfect forward secrecy, abbreviated PFS.

    Unfortunately, having the secrets does allow an active attack, specifically a man-in-the-middle attack. Losing these secrets to an attacker may not be quite as disastrous as losing the actual keys, but it is still a serious security breach. These secrets should be guarded as carefully as keys.

    Methods not yet implemented

    Unauthenticated key exchange

    It would be possible to exchange keys without authenticating the players. This would support opportunistic encryption -- allowing any two systems to encrypt their communications without requiring a shared PKI or a previously negotiated secret -- and would be secure against passive attacks. It would, however, be highly vulnerable to active man-in-the-middle attacks. RFC 2408 therefore specifies that all ISAKMP key management interactions must be authenticated.

    There is room for debate here. Should we provide immediate security against passive attacks and encourage widespread use of encryption, at the expense of risking the more difficult active attacks? Or should we wait until we can implement a solution that can both be widespread and offer security against active attacks?

    So far, we have chosen the second course, complying with the RFCs and waiting for secure DNS (see below) so that we can do opportunistic encryption right.

    Key exchange using DNS

    The IPsec RFCs allow key exchange based on authentication services provided by Secure DNS. Once Secure DNS service becomes widely available, we expect to make this the primary key management method for Linux FreeS/WAN. It is the best way we know of to support opportunistic encryption, allowing two systems without a common PKI or previous negotiation to secure their communication.

    As of FreeS/WAN 1.4, we have experimental code to acquire RSA keys from DNS but do not yet have code to validate Secure DNS signatures.

    Key exchange using a PKI

    The IPsec RFCs allow key exchange based on authentication services provided by a PKI or Public Key Infrastructure. With many vendors selling such products and many large organisations building these infrastructures, this will clearly be an important application of IPsec and one Linux FreeS/WAN will eventually support.

    On the other hand, this is not as high a priority for Linux FreeS/WAN as solutions based on secure DNS. We do not expect any PKI to become as universal as DNS.

    Some patches to handle authentication with X.509 certificates, which most PKIs use, are available.


    Photuris is another key management protocol, an alternative to IKE and ISAKMP, described in RFCs 2522 and 2523 which are labelled "experimental". Adding Photuris support to Linux FreeS/WAN might be a good project for a volunteer. The likely starting point would be the OpenBSD photurisd code.


    SKIP is yet another key management protocol, developed by Sun. At one point it was fairly widely used, but our current impression is that it is moribund, displaced by IKE. Sun now (as of Solaris 8.0) ship an IPsec implementation using IKE. We have no plans to implement SKIP.

    Mailing lists and newsgroups

    Mailing lists about FreeS/WAN

    The project mailing lists

    The Linux FreeS/WAN project has several email lists for user support, bug reports and software development discussions.

    We had a single list on for several years (Thanks, folks!), then one list on, but now we've split into several lists:

    To subscribe to any of these, you can:

    Archives of these lists are available via the web interface.

    Which list should I use?

    For most questions, please check the FAQ first, and if that does not have an answer, ask on the users list. "My configuration doesn't work." does not belong on the bugs list, and "Can FreeS/WAN do such-and-such" or "How do I configure it to..."do not belong in design discussions.

    Cross-posting the same message to two or more of these lists is discouraged. Quite a few people read more than one list and getting multiple copies is annoying.

    List policies

    US citizens or residents are asked not to post code to the lists, not even one-line bug fixes. The project cannot accept code which might entangle it in US export restrictions .

    Non-subscribers can post to some of these lists. This is necessary; someone working on a gateway install who encounters a problem may not have access to a subscribed account.

    Some spam turns up on these lists from time to time. For discussion of why we do not attempt to filter it, see the FAQ. Please do not clutter the lists with complaints about this.

    Archives of the lists

    Searchable archives of the old single list have existed for some time. At time of writing, it is not yet clear how they will change for the new multi-list structure.

    Note that these use different search engines. Try both.

    Archives of the new lists are available via the web interface.

    Indexes of mailing lists

    PAML is the standard reference for Publicly Accessible Mailing Lists. When we last checked, it had over 7500 lists on an amazing variety of topics. It also has FAQ information and a search engine.

    There is an index of Linux mailing lists available.

    A list of computer security mailing lists, with descriptions.

    Lists for related software and topics

    Most links in this section point to subscription addresses for the various lists. Send the one-line message "subscribe list_name " to subscribe to any of them.

    Linux mailing lists

    Each of the scure distribution projects also has its own web site and mailing list. Some of the sites are:

    Lists for IETF working groups

    Each IETF working group has an associated mailing list where much of the work takes place.

    Other mailing lists

    Usenet newsgroups

    Web links

    The Linux FreeS/WAN Project

    The main project web site is

    Links to other project-related sites are provided in our introduction section.

    Add-ons and patches for FreeS/WAN

    Some user-contributed patches gave been integrated into the FreeS/WAN distribution. For a variety of reasons, those listed below have not.

    Patches believed current at time of writing (March 2001, just before 1.9 release):

    Before using these, check the mailing lists for news of newer versions and to see whether they have been incorporated into more recent versions of FreeS/WAN.

    A set of PKIX patches were recently announced on the mailing list:

       Subject: a different PKIX patch.
       Date: Mon, 5 Mar 2001
       From: Luc Lanthier <>
    I'd like to invite volunteers to use the now-complete PKIX project I've
    been working on since about August. Because of this, the patch is for
    FreeSWAN 1.5, not 1.8... I haven't really felt the need to update it since
    I don't use IPV6 nor DNSSec.
    This is similar, but different than Andreas Steffen's pkix
    implementation. I've based this work on Neil Dunbar's openssl-pkix patch
    for FreeSWAN 1.1. I've updated it to run on FreeSWAN 1.5 correctly, and
    added support for ID_DER_ASN1_DN ID packet support. It will do LDAP
    certificate lookups no problem, as well as local flatfile, directory, or
    DB lookup for testing or speed.
    IE: It's a full CA-compatible client, capable of looking up, checking the
    CRL for expiry and such. It will not only do the classic PSK and RSASIG
    freeswan methods just fine, but also does PKIX's RSASIG, PKE and
    RPKE. I've spent a lot of time adding RoadWarrior support for these last
    IKE exchange methods.
    The patch can be found as:
    There are also freeswan-1.5 - kernel 2.4 patches for those who need them.
    Let me know. Feedback is appreciated.
    A more recent URL has patches for FreeS/WAN versions up to 1.91.

    Older patches:

    These patches are for older versions of FreeS/WAN and will likely not work with the current version. Older versions of FreeS/WAN may be available on some of the distribution sites, but we recommend using the current release.

    Note: At one point the way PGP generates RSA keys and the way FreeS/WAN checks them for validity before using them were slightly different, so quite a few PGP-generated keys would be rejected by FreeS/WAN, confusing users no end. This is fixed in 1.9.

    VPN masquerade patches

    Finally, there are some patches to other code that may be useful with FreeS/WAN: Note that this is not required if the same machine does IPsec and masquerading, only if you want a to locate your IPsec gateway on a masqueraded network. See our firewalls document for discussion of why this is problematic.

    At last report, this patch could not co-exist with FreeS/WAN on the same machine.

    Distributions including FreeS/WAN

    The introductory section of our document set lists several Linux distributions which include FreeS/WAN.

    Things FreeS/WAN uses or could use

    Other approaches to VPNs for Linux

    There is a list of Linux VPN software in the Linux Security Knowledge Base.

    The IPsec Protocols

    General IPsec or VPN information

    IPsec overview documents or slide sets

    IPsec information in languages other than English

    RFCs and other reference documents

    Analysis and critiques of IPsec protocols

    Background information on IP

    IPsec Implementations

    Linux products

    Vendors using FreeS/WAN in turnkey firewall or VPN products are listed in our introduction.

    Other vendors have Linux IPsec products which, as far as we know, do not use FreeS/WAN

    IPsec in router products

    All the major router vendors support IPsec, at least in some models.

    IPsec in firewall products

    Many firewall vendors offer IPsec, either as a standard part of their product, or an optional extra. A few we know about are:

    Vendors using FreeS/WAN in turnkey firewall products are listed in our introduction.

    Operating systems with IPsec support

    All the major open source operating systems support IPsec. See below for details on BSD-derived Unix variants.

    Among commercial OS vendors, IPsec players include:

    Open source IPsec implementations

    Other Linux IPsec implementations

    We like to think of FreeS/WAN as the Linux IPsec implementation, but it is not the only one. Others we know of are:

    IPsec for BSD Unix

    IPsec for other systems


    The IPsec protocols are designed so that different implementations should be able to work together. As they say "the devil is in the details". IPsec has a lot of details, but considerable success has been achieved.

    Interoperability results

    Linux FreeS/WAN has been tested for interoperability with many other IPsec implementations. Results to date are in our interoperability section.

    Various other sites have information on interoperability between various IPsec implementations:

    Interoperability test sites

    Linux links

    Basic and tutorial Linux information

    General Linux sites


    Nearly any Linux documentation you are likely to want can be found at the Linux Documentation Project or LDP.

    You may not need to go to the LDP to get this material. Most Linux distributions include the HowTos on their CDs and several include the Guides as well. Also, most of the Guides and some collections of HowTos are available in book form from various publishers.

    Much of the LDP material is also available in languages other than English. See this LDP page.

    Advanced routing

    The Linux IP stack has some new features in 2.4 kernels. Some HowTos have been written:

    Security for Linux

    See also the LDP material above.

    Linux firewalls

    Our FreeS/WAN and firewalls document includes links to several sets of scripts known to work with FreeS/WAN.

    Other information sources:

    Miscellaneous Linux information

    Crypto and security links

    Crypto and security resources

    The standard link collections

    Two enormous collections of links, each the standard reference in its area:

    Gene Spafford's COAST hotlist
    Computer and network security.
    Peter Gutmann's Encryption and Security-related Resources

    Frequently Asked Question (FAQ) documents


    See also the interesting papers section below.

    Crypto and security standards

    Crypto quotes

    There are several collections of cryptographic quotes on the net:

    Cryptography law and policy

    Surveys of crypto law

    Organisations opposing crypto restrictions

    Other information on crypto policy

    See also our documentation section on the history and politics of cryptography.

    Cryptography technical information

    Collections of crypto links

    Lists of online cryptography papers

    Particularly interesting papers

    These papers emphasize important issues around the use of cryptography, and the design and management of secure systems.

    Computer and network security

    Security links

    Firewall links

    VPN links

    Security tools

    Links to home pages

    David Wagner at Berkeley provides a set of links to home pages of cryptographers, cypherpunks and computer security people.

    Glossary for the Linux FreeS/WAN project

    Entries are in alphabetical order. Some entries are only one line or one paragraph long. Others run to several paragraphs. I have tried to put the essential information in the first paragraph so you can skip the other paragraphs if that seems appropriate.

    Jump to a letter in the glossary

    numeric A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

    Other glossaries

    Other glossaries which overlap this one include:

    Several Internet glossaries are available as RFCs:

    More general glossary or dictionary information:


    3DES (Triple DES)
    Using three DES encryptions on a single data block, with at least two different keys, to get higher security than is available from a single DES pass. The three-key version of 3DES is the default encryption algorithm for Linux FreeS/WAN.

    IPsec always does 3DES with three different keys, as required by RFC 2451. For an explanation of the two-key variant, see two key triple DES. Both use an EDE encrypt-decrypt-encrpyt sequence of operations.

    Single DES is insecure .

    Double DES is ineffective. Using two 56-bit keys, one might expect an attacker to have to do 2112 work to break it. In fact, only 257 work is required with a meet-in-the-middle attack, though a large amount of memory is also required. Triple DES is vulnerable to a similar attack, but that just reduces the work factor from the 2168 one might expect to 2 112. That provides adequate protection against brute force attacks, and no better attack is known.

    3DES can be somewhat slow compared to other ciphers. It requires three DES encryptions per block. DES was designed for hardware implementation and includes some operations which are difficult in software. However, the speed we get is quite acceptable for many uses. See our performance document for details.

    Active attack
    An attack in which the attacker does not merely eavesdrop (see passive attack) but takes action to change, delete, reroute, add, forge or divert data. Perhaps the best-known active attack is man-in-the-middle. In general, authentication is a useful defense against active attacks.
    The Advanced Encryption Standard, a new block cipher standard to replace DES being developed by NIST, the US National Institute of Standards and Technology. DES used 64-bit blocks and a 56-bit key. AES ciphers use a 128-bit block and are required to support 128, 192 and 256-bit keys. Some of them support other sizes as well. The larger block size helps resist birthday attacks while the large key size prevents brute force attacks.

    Fifteen proposals meeting NIST's basic criteria were submitted in 1998 and subjected to intense discussion and analysis, "round one" evaluation. In August 1999, NIST narrowed the field to five "round two" candidates:

    Three of the five finalists -- Rijndael, Serpent and Twofish -- have completely open licenses.

    In October 2000, NIST announced the winner -- Rijndael.

    For more information, see:

    Adding one or more AES ciphers to Linux FreeS/WAN would be a useful undertaking. Likely one would add all three of the Round Two candidates with good licenses. A complication is that our code is built for a 64-bit block cipher and AES uses a 128-bit block. Volunteers via the mailing lists would be welcome.

    The IPsec Authentication Header, added after the IP header. For details, see our IPsec document and/or RFC 2402.
    Alice and Bob
    A and B, the standard example users in writing on cryptography and coding theory. Carol and Dave join them for protocols which require more players.

    Bruce Schneier extends these with many others such as Eve the Eavesdropper and Victor the Verifier. His extensions seem to be in the process of becoming standard as well. See page 23 of Applied Cryptography

    Alice and Bob have an amusing biography on the web.

    see DARPA
    Australian Security Intelligence Organisation.
    Asymmetric cryptography
    See public key cryptography.
    Ensuring that a message originated from the expected sender and has not been altered on route. IPsec uses authentication in two places:

    Outside IPsec, passwords are perhaps the most common authentication mechanism. Their function is essentially to authenticate the person's identity to the system. Passwords are generally only as secure as the network they travel over. If you send a cleartext password over a tapped phone line or over a network with a packet sniffer on it, the security provided by that password becomes zero. Sending an encrypted password is no better; the attacker merely records it and reuses it at his convenience. This is called a replay attack.

    A common solution to this problem is a challenge-response system. This defeats simple eavesdropping and replay attacks. Of course an attacker might still try to break the cryptographic algorithm used, or the random number generator.

    Automatic keying
    A mode in which keys are automatically generated at connection establisment and new keys automaically created periodically thereafter. Contrast with manual keying in which a single stored key is used.

    IPsec uses the Diffie-Hellman key exchange protocol to create keys. An authentication mechansim is required for this. FreeS/WAN normally uses RSA for this. Other methods supported are discussed in our advanced configuration document.

    Having an attacker break the authentication is emphatically not a good idea. An attacker that breaks authentication, and manages to subvert some other network entities (DNS, routers or gateways), can use a man-in-the middle attack to break the security of your IPsec connections.

    However, having an attacker break the authentication in automatic keying is not quite as bad as losing the key in manual keying.

    That said, the secrets used for authentication, stored in ipsec.secrets(5), should still be protected as tightly as cryptographic keys.

    Bay Networks
    A vendor of routers, hubs and related products, now a subsidiary of Nortel. Interoperation between their IPsec products and Linux FreeS/WAN was problematic at last report; see our interoperation section.
    Our default block cipher, triple DES, is slower than many alternate ciphers that might be used. Speeds achieved, however, seem adequate for many purposes. For example, the assembler code from the LIBDES library we use encrypts 1.6 megabytes per second on a Pentium 200, according to the test program supplied with the library.

    For more detail, see our document on FreeS/WAN performance.

    Berkeley Internet Name Daemon, a widely used implementation of DNS (Domain Name Service). See our bibliography for a useful reference. See the BIND home page for more information and the latest version.
    Birthday attack
    A cryptographic attack based on the mathematics exemplified by the birthday paradox. This math turns up whenever the question of two cryptographic operations producing the same result becomes an issue:

    Resisting such attacks is part of the motivation for:

    Birthday paradox
    Not really a paradox, just a rather counter-intuitive mathematical fact. In a group of 23 people, the chance of a least one pair having the same birthday is over 50%.

    The second person has 1 chance in 365 (ignoring leap years) of matching the first. If they don't match, the third person's chances of matching one of them are 2/365. The 4th, 3/365, and so on. The total of these chances grows more quickly than one might guess.

    Block cipher
    A symmetric cipher which operates on fixed-size blocks of plaintext, giving a block of ciphertext for each. Contrast with stream cipher. Block ciphers can be used in various modes when multiple block are to be encrypted.

    DES is among the the best known and widely used block ciphers, but is now obsolete. Its 56-bit key size makes it highly insecure today. Triple DES is the default block cipher for Linux FreeS/WAN.

    The current generation of block ciphers -- such as Blowfish, CAST-128 and IDEA -- all use 64-bit blocks and 128-bit keys. The next generation, AES, uses 128-bit blocks and supports key sizes up to 256 bits.

    The Block Cipher Lounge web site has more information.

    A block cipher using 64-bit blocks and keys of up to 448 bits, designed by Bruce Schneier and used in several products.

    This is not required by the IPsec RFCs and not currently used in Linux FreeS/WAN.

    Brute force attack (exhaustive search)
    Breaking a cipher by trying all possible keys. This is always possible in theory (except against a one-time pad), but it becomes practical only if the key size is inadequate. For an important example, see our document on the insecurity of DES with its 56-bit key. For an analysis of key sizes required to resist plausible brute force attacks, see this paper.

    Longer keys protect against brute force attacks. Each extra bit in the key doubles the number of possible keys and therefore doubles the work a brute force attack must do. A large enough key defeats any brute force attack.

    For example, the EFF's DES Cracker searches a 56-bit key space in an average of a few days. Let us assume an attacker that can find a 64-bit key (256 times harder) by brute force search in a second (a few hundred thousand times faster). For a 96-bit key, that attacker needs 232 seconds, about 135 years. Against a 128-bit key, he needs 232 times that, over 500,000,000,000 years. Your data is then obviously secure against brute force attacks. Even if our estimate of the attacker's speed is off by a factor of a million, it still takes him over 500,000 years to crack a message.

    This is why

    Inadequate keylength always indicates a weak cipher but it is important to note that adequate keylength does not necessarily indicate a strong cipher. There are many attacks other than brute force, and adequate keylength only guarantees resistance to brute force. Any cipher, whatever its key size, will be weak if design or implementation flaws allow other attacks.

    Also, once you have adequate keylength (somewhere around 90 or 100 bits), adding more key bits make no practical difference , even against brute force. Consider our 128-bit example above that takes 500,000,000,000 years to break by brute force. We really don't care how many zeroes there are on the end of that, as long as the number remains ridiculously large. That is, we don't care exactly how large the key is as long as it is large enough.

    There may be reasons of convenience in the design of the cipher to support larger keys. For example Blowfish allows up to 448 bits and RC4 up to 2048, but beyond 100-odd bits it makes no difference to practical security.

    Bureau of Export Administration
    see BXA
    The US Commerce Department's Bureau of Export A dministration which administers the EAR Export Administration Regulations controling the export of, among other things, cryptography.
    Certification Authority, an entity in a public key infrastructure that can certify keys by signing them. Usually CAs form a hierarchy. The top of this hierarchy is called the root CA.

    See Web of Trust for an alternate model.

    A block cipher using 64-bit blocks and 128-bit keys, described in RFC 2144 and used in products such as Entrust and recent versions of PGP.

    This is not required by the IPsec RFCs and not currently used in Linux FreeS/WAN.

    Entrust's candidate cipher for the AES standard, largely based on the CAST-128 design.
    CBC mode
    Cipher Block Chaining mode , a method of using a block cipher in which for each block except the first, the result of the previous encryption is XORed into the new block before it is encrypted. CBC is the mode used in IPsec.

    An initialisation vector (IV) must be provided. It is XORed into the first block before encryption. The IV need not be secret but should be different for each message and unpredictable.

    Certification Authority
    see CA
    Cipher Modes
    Different ways of using a block cipher when encrypting multiple blocks.

    Four standard modes were defined for DES in FIPS 81. They can actually be applied with any block cipher.

    ECBElectronic CodeBook encrypt each block independently
    CBCCipher Block Chaining
    XOR previous block ciphertext into new block plaintext before encrypting new block
    CFBCipher FeedBack
    OFBOutput FeedBack

    IPsec uses CBC mode since this is only marginally slower than ECB and is more secure. In ECB mode the same plaintext always encrypts to the same ciphertext, unless the key is changed. In CBC mode, this does not occur.

    Various other modes are also possible, but none of them are used in IPsec.

    Challenge-response authentication
    An authentication system in which one player generates a random number, encrypts it and sends the result as a challenge. The other player decrypts and sends back the result. If the result is correct, that proves to the first player that the second player knew the appropriate secret, required for the decryption. Variations on this technique exist using public key or symmetric cryptography. Some provide two-way authentication, assuring each player of the other's identity.

    This is more secure than passwords against two simple attacks:

    A challenge-response system never sends a password, either cleartext or encrypted. An attacker cannot record the response to one challenge and use it as a response to a later challenge. The random number is different each time.

    Of course an attacker might still try to break the cryptographic algorithm used, or the random number generator.

    The encrypted output of a cipher, as opposed to the unencrypted plaintext input.
    A vendor of routers, hubs and related products. Their IPsec products interoperate with Linux FreeS/WAN; see our interop section.
    This term has at least two distinct uses in discussing IPsec:

    We generally use the term in the first sense. Vendors of Windows IPsec solutions often use it in the second.

    Conventional cryptography
    See symmetric cryptography
    Collision resistance
    The property of a message digest algorithm which makes it hard for an attacker to find or construct two inputs which hash to the same output.
    see GNU General Public License
    Communications Security Establishment, the Canadian organisation for signals intelligence.
    DARPA (sometimes just ARPA)
    The US government's Defense Advanced Research Projects Agency. Projects they have funded over the years have included the Arpanet which evolved into the Internet, the TCP/IP protocol suite (as a replacement for the original Arpanet suite), the Berkeley 4.x BSD Unix projects, and Secure DNS.

    For current information, see their web site.

    Denial of service (DoS) attack
    An attack that aims at denying some service to legitimate users of a system, rather than providing a service to the attacker.

    The two example attacks discussed were both quite effective when first discovered, capable of crashing or disabling many operating systems. They were also well-publicised, and today far fewer systems are vulnerable to them.

    The Data Encryption Standard, a block cipher with 64-bit blocks and a 56-bit key. Probably the most widely used symmetric cipher ever devised. DES has been a US government standard for their own use (only for unclassified data), and for some regulated industries such as banking, since the late 70's.

    DES is seriously insecure against current attacks.

    Linux FreeS/WAN does not include DES, even though the RFCs specify it. We strongly recommend that single DES not be used.

    See also 3DES and DESX, stronger ciphers based on DES.

    An improved DES suggested by Ron Rivest of RSA Data Security. It XORs extra key material into the text before and after applying the DES cipher.

    This is not required by the IPsec RFCs and not currently used in Linux FreeS/WAN. DESX would be the easiest additional transform to add; there would be very little code to write. It would be much faster than 3DES and almost certainly more secure than DES. However, since it is not in the RFCs other IPsec implementations cannot be expected to have it.

    see Diffie-Hellman
    Diffie-Hellman (DH) key exchange protocol
    A protocol that allows two parties without any initial shared secret to create one in a manner immune to eavesdropping. Once they have done this, they can communicate privately by using that shared secret as a key for a block cipher or as the basis for key exchange.

    The protocol is secure against all passive attacks, but it is not at all resistant to active man-in-the-middle attacks. If a third party can impersonate Bob to Alice and vice versa, then no useful secret can be created. Authentication of the participants is a prerequisite for safe Diffie-Hellman key exchange. IPsec can use any of several authentication mechanisims. Those supported by FreeS/WAN are discussed in our configuration section.

    The Diffie-Hellman key exchange is based on the discrete logarithm problem and is secure unless someone finds an efficient solution to that problem.

    Given a prime p and generator g (explained under discrete log below), Alice:

    Meanwhile Bob:

    Now Alice and Bob can both calculate the shared secret s = g^(ab). Alice knows a and B, so she calculates s = B^a. Bob knows A and b so he calculates s = A^b.

    An eavesdropper will know p and g since these are made public, and can intercept A and B but, short of solving the discrete log problem, these do not let him or her discover the secret s.

    Digital signature


    If the public-key system is secure and the verification succeeds, then the receiver knows

    Such an encrypted message digest can be treated as a signature since it cannot be created without both the document and the private key which only the sender should possess. The legal issues are complex, but several countries are moving in the direction of legal recognition for digital signatures.

    discrete logarithm problem
    The problem of finding logarithms in a finite field. Given a field defintion (such definitions always include some operation analogous to multiplication) and two numbers, a base and a target, find the power which the base must be raised to in order to yield the target.

    The discrete log problem is the basis of several cryptographic systems, including the Diffie-Hellman key exchange used in the IKE protocol. The useful property is that exponentiation is relatively easy but the inverse operation, finding the logarithm, is hard. The cryptosystems are designed so that the user does only easy operations (exponentiation in the field) but an attacker must solve the hard problem (discrete log) to crack the system.

    There are several variants of the problem for different types of field. The IKE/Oakley key determination protocol uses two variants, either over a field modulo a prime or over a field defined by an elliptic curve. We give an example modulo a prime below. For the elliptic curve version, consult an advanced text such as Handbook of Applied Cryptography.

    Given a prime p, a generator g for the field modulo that prime, and a number x in the field, the problem is to find y such that g^y = x.

    For example, let p = 13. The field is then the integers from 0 to 12. Any integer equals one of these modulo 13. That is, the remainder when any integer is divided by 13 must be one of these.

    2 is a generator for this field. That is, the powers of two modulo 13 run through all the non-zero numbers in the field. Modulo 13 we have:

              y      x
            2^0  ==  1
            2^1  ==  2
            2^2  ==  4
            2^3  ==  8
            2^4  ==  3 that is, the remainder from 16/13 is 3
            2^5  ==  6          the remainder from 32/13 is 6
            2^6  == 12 and so on
            2^7  == 11
            2^8  ==  9
            2^9  ==  5
            2^10 == 10
            2^11 ==  7
            2^12 ==  1

    Exponentiation in such a field is not difficult. Given, say, y = 11, calculating x = 7 is straightforward. One method is just to calculate 2^11 = 2048 , then 2048 mod 13 == 7. When the field is modulo a large prime (say a few 100 digits) you need a silghtly cleverer method and even that is moderately expensive in computer time, but the calculation is still not problematic in any basic way.

    The discrete log problem is the reverse. In our example, given x = 7, find the logarithm y = 11. When the field is modulo a large prime (or is based on a suitable elliptic curve), this is indeed problematic. No solution method that is not catastrophically expensive is known. Quite a few mathematicians have tackled this problem. No efficient method has been found and mathematicians do not expect that one will be. It seems likely no efficient solution to either of the main variants the discrete log problem exists.

    Note, however, that no-one has proven such methods do not exist. If a solution to either variant were found, the security of any crypto system using that variant would be destroyed. This is one reason IKE supports two variants. If one is broken, we can switch to the other.

    Domain Name Service, a distributed database through which names are associated with numeric addresses and other information in the Internet Protocol Suite. See also BIND, the Berkeley Internet Name Daemon which implements DNS services and Secure DNS. See our bibliography for a useful reference on both.
    DOS attack
    see Denial Of Service attack
    The US government's Export Administration R egulations, administered by the Bureau of Export Administration. These have replaced the earlier ITAR regulations as the controls on export of cryptography.
    ECB mode
    Electronic CodeBook mode, the simplest way to use a block cipher. See Cipher Modes.
    The sequence of operations normally used in either the three-key variant of triple DES used in IPsec or the two-key variant used in some other systems.

    The sequence is:

    For the two-key version, key1=key3.

    The "advantage" of this EDE order of operations is that it makes it simple to interoperate with older devices offering only single DES. Set key1=key2=key3 and you have the worst of both worlds, the overhead of triple DES with the security of single DES. Since single DES is insecure, this is an extremely dubious "advantage".

    The EDE two-key variant can also interoperate with the EDE three-key variant used in IPsec; just set k1=k3.

    A Canadian company offerring enterprise PKI products using CAST-128 symmetric crypto, RSA public key and X.509 directories. Web site
    Electronic Frontier Foundation, an advocacy group for civil rights in cyberspace.
    Techniques for converting a readable message ( plaintext) into apparently random material ( ciphertext) which cannot be read if intercepted. A key is required to read the message.

    Major variants include symmetric encryption in which sender and receiver use the same secret key and public key methods in which the sender uses one of a matched pair of keys and the receiver uses the other. Many current systems, including IPsec, are hybrids combining the two techniques.

    Encapsulated Security Payload, the IPsec protocol which provides encryption. It can also provide authentication service and may be used with null encryption (which we do not recommend). For details see our IPsec document and/or RFC 2406.
    Extruded subnet
    A situation in which something IP sees as one network is actually in two or more places.

    For example, the Internet may route all traffic for a particular company to that firm's corporate gateway. It then becomes the company's problem to get packets to various machines on their subnets in various departments. They may decide to treat a branch office like a subnet, giving it IP addresses "on" their corporate net. This becomes an extruded subnet.

    Packets bound for it are delivered to the corporate gateway, since as far as the outside world is concerned, that subnet is part of the corporate network. However, instead of going onto the corporate LAN (as they would for, say, the accounting department) they are then encapsulated and sent back onto the Internet for delivery to the branch office.

    For information on doing this with Linux FreeS/WAN, look in our advanced configuration section.

    Exhaustive search
    See brute force attack.
    Federal Information Processing S tandard, the US government's standards for products it buys. These are issued by NIST. Among other things, DES and SHA are defined in FIPS documents. NIST have a FIPS home page.
    Free Software Foundation (FSF)
    An organisation to promote free software, free in the sense of these quotes from their web pages
    "Free software" is a matter of liberty, not price. To understand the concept, you should think of "free speech", not "free beer."

    "Free software" refers to the users' freedom to run, copy, distribute, study, change and improve the software.

    See also GNU, GNU General Public License, and the FSF site.

    see Linux FreeS/WAN
    see Free software Foundation
    Government Communications Headquarters, the British organisation for signals intelligence.
    generator of a prime field
    see discrete logarithm problem
    Global Internet Liberty Campaign, an international organisation advocating, among other things, free availability of cryptography. They have a campaign to remove cryptographic software from the Wassenaar Arrangement.
    Global Internet Liberty Campaign
    see GILC.
    Global Trust Register
    An attempt to create something like a root CA for PGP by publishing both as a book and on the web the fingerprints of a set of verified keys for well-known users and organisations.
    The GNU Multi-Precision library code, used in Linux FreeS/WAN by Pluto for public key calculations.
    GNU's Not Unix, the Free Software Foundation's project aimed at creating a free system with at least the capabilities of Unix. Linux uses GNU utilities extensively.
    see GNU Privacy Guard
    GNU General Public License(GPL, copyleft)
    The license developed by the Free Software Foundation under which Linux, Linux FreeS/WAN and many other pieces of software are distributed. The license allows anyone to redistribute and modify the code, but forbids anyone from distributing executables without providing access to source code. For more details see the file COPYING included with GPLed source distributions, including ours, or the GNU site's GPL page.
    GNU Privacy Guard
    An open source implementation of Open PGP as defined in RFC 2440. See their web site
    see GNU General Public License.
    see message digest
    Hashed Message Authentication Code (HMAC)
    using keyed message digest functions to authenticate a message. This differs from other uses of these functions:

    The exact techniques used in IPsec are defined in RFC 2104. They are referred to as HMAC-MD5-96 and HMAC-SHA-96 because they output only 96 bits of the hash. This makes some attacks on the hash functions harder.

    see Hashed Message Authentication Code
    see Hashed Message Authentication Code
    see Hashed Message Authentication Code
    Hybrid cryptosystem
    A system using both public key and symmetric cipher techniques. This works well. Public key methods provide key management and digital signature facilities which are not readily available using symmetric ciphers. The symmetric cipher, however, can do the bulk of the encryption work much more efficiently than public key methods.
    Internet Architecture Board.
    Internet Control M essage Protocol. This is used for various IP-connected devices to manage the network.
    International Data Encrypion Algorithm, developed in Europe as an alternative to exportable American ciphers such as DES which were too weak for serious use. IDEA is a block cipher using 64-bit blocks and 128-bit keys, and is used in products such as PGP.

    IDEA is not required by the IPsec RFCs and not currently used in Linux FreeS/WAN.

    IDEA is patented and, with strictly limited exceptions for personal use, using it requires a license from Ascom.

    Institute of Electrical and Electronic Engineers, a professional association which, among other things, sets some technical standards
    Internet Engineering Steering Group .
    Internet Engineering Task Force, the umbrella organisation whose various working groups make most of the technical decisions for the Internet. The IETF IPsec working group wrote the RFCs we are implementing.
    Internet Key Exchange, based on the Diffie-Hellman key exchange protocol. IKE is implemented in Linux FreeS/WAN by the Pluto daemon.
    Initialisation Vector (IV)
    Some cipher modes, including the CBC mode which IPsec uses, require some extra data at the beginning. This data is called the initialisation vector. It need not be secret, but should be different for each message. Its function is to prevent messages which begin with the same text from encrypting to the same ciphertext. That might give an analyst an opening, so it is best prevented.
    Internet Protocol.
    IP masquerade
    A mostly obsolete term for a method of allowing multiple machines to communicate over the Internet when only one IP address is available for their use. The more current term is Network Address Translation or NAT.
    "IP the Next Generation", see IPv6.
    The current version of the Internet protocol suite .
    IPv6 (IPng)
    Version six of the Internet protocol suite, currently being developed. It will replace the current version four. IPv6 has IPsec as a mandatory component.

    See this web site for more details, and our compatibility document for information on FreeS/WAN and the Linux implementation of IPv6.

    IPsec or IPSEC
    Internet Protocol SECurity, security functions (authentication and encryption) implemented at the IP level of the protocol stack. It is optional for IPv4 and mandatory for IPv6.

    This is the standard Linux FreeS/WAN is implementing. For more details, see our IPsec Overview. For the standards, see RFCs listed in our RFCs document.

    Novell's Netware protocol tunnelled over an IP link. Our firewalls document includes an example of using this through an IPsec tunnel.
    Internet Security Association and Key Management Protocol, defined in RFC 2408.
    International Traffic in Arms R egulations, US regulations administered by the State Department which until recently limited export of, among other things, cryptographic technology and software. ITAR still exists, but the limits on cryptography have now been transferred to the Export Administration Regulations under the Commerce Department's Bureau of Export Administration.
    see Initialisation vector
    The basic part of an operating system (e.g. Linux) which controls the hardware and provides services to all other programs.

    In the Linux release numbering system, an even second digit as in 2. 2.x indicates a stable or production kernel while an odd number as in 2.3.x indicates an experimental or development kernel. Most users should run a recent kernel version from the production series. The development kernels are primarily for people doing kernel development. Others should consider using development kernels only if they have an urgent need for some feature not yet available in production kernels.

    Keyed message digest
    See HMAC.
    Key length
    see brute force attack
    Kernel IP Security, the Linux FreeS/WAN project's changes to the Linux kernel to support the IPsec protocols.
    Lightweight Directory Access Protocol, defined in RFCs 1777 and 1778, a method of accessing information stored in directories. LDAP is used by several PKI implementations, often with X.501 directories and X.509 certificates. It may also be used by IPsec to obtain key certifications from those PKIs. This is not yet implemented in Linux FreeS/WAN.
    A publicly available library of DES code, written by Eric Young, which Linux FreeS/WAN uses in both KLIPS and Pluto .
    A freely available Unix-like operating system based on a kernel originally written for the Intel 386 architecture by (then) student Linus Torvalds. Once his 32-bit kernel was available, the GNU utilities made it a usable system and contributions from many others led to explosive growth.

    Today Linux is a complete Unix replacement available for several CPU architectures -- Intel, DEC/Compaq Alpha, Power PC, both 32-bit SPARC and the 64-bit UltraSPARC, SrongARM, . . . -- with support for multiple CPUs on some architectures.

    Linux FreeS/WAN is intended to run on all CPUs supported by Linux and is known to work on several. See our compatibility section for a list.

    Linux FreeS/WAN
    Our implementation of the IPsec protocols, intended to be freely redistributable source code with a GNU GPL license and no constraints under US or other export laws. Linux FreeS/WAN is intended to interoperate with other IPsec implementations. The name is partly taken, with permission, from the S/WAN multi-vendor IPsec compatability effort. Linux FreeS/WAN has two major components, KLIPS (KerneL IPsec Support) and the Pluto daemon which manages the whole thing.

    See our IPsec section for more detail. For the code see our primary site or one of the mirror sites on this list.

    Mailing list
    The Linux FreeS/WAN project has several public email lists for bug reports and software development discussions. See our document on mailing lists .
    Man-in-the-middle attack
    An active attack in which the attacker impersonates each of the legitimate players in a protocol to the other.

    For example, if Alice and Bob are negotiating a key via the Diffie-Hellman key agreement, and are not using authentication to be certain they are talking to each other, then an attacker able to insert himself in the communication path can deceive both players.

    Call the attacker Mallory. For Bob, he pretends to be Alice. For Alice, he pretends to be Bob. Two keys are then negotiated, Alice-to-Mallory and Bob-to-Mallory. Alice and Bob each think the key they have is Alice-to-Bob.

    A message from Alice to Bob then goes to Mallory who decrypts it, reads it and/or saves a copy, re-encrypts using the Bob-to-Mallory key and sends it along to Bob. Bob decrypts successfully and sends a reply which Mallory decrypts, reads, re-encrypts and forwards to Alice.

    To make this attack effective, Mallory must

    If he manages it, however, it is devastating. He not only gets to read all the messages; he can alter messages, inject his own, forge anything he likes, . . . In fact, he controls the communication completely.

    Manual keying
    An IPsec mode in which the keys are provided by the administrator. In FreeS/WAN, they are stored in /etc/ipsec.conf. The alternative, automatic keying, is preferred in most cases.
    Message Digest Algorithm Four from Ron Rivest of RSA. MD4 was widely used a few years ago, but is now considered obsolete. It has been replaced by its descendants MD5 and SHA.
    Message Digest Algorithm Five from Ron Rivest of RSA, an improved variant of his MD4. Like MD4, it produces a 128-bit hash. For details see RFC 1321.

    MD5 is one of two message digest algorithms available in IPsec. The other is SHA. SHA produces a longer hash and is therefore more resistant to birthday attacks, but this is not a concern for IPsec. The HMAC method used in IPsec is secure even if the underlying hash is not particularly strong against this attack.

    Meet-in-the-middle attack
    A divide-and-conquer attack which breaks a cipher into two parts, works against each separately, and compares results. Probably the best known example is an attack on double DES. This applies in principle to any pair of block ciphers, e.g. to an encryption system using, say, CAST-128 and Blowfish, but we will describe it for double DES.

    Double DES encryption and decryption can be written:

            C = E(k2,E(k1,P))
            P = D(k1,D(k2,C))

    Where C is ciphertext, P is plaintext, E is encryption, D is decryption, k1 is one key, and k2 is the other key. If we know a P, C pair, we can try and find the keys with a brute force attack, trying all possible k1, k2 pairs. Since each key is 56 bits, there are 2 112 such pairs and this attack is painfully inefficient.

    The meet-in-the middle attack re-writes the equations to calculate a middle value M:

            M = E(k1,P)
            M = D(k2,C)

    Now we can try some large number of D(k2,C) decryptions with various values of k2 and store the results in a table. Then start doing E(k1,P) encryptions, checking each result to see if it is in the table.

    With enough table space, this breaks double DES with 256 + 256 = 257 work. Against triple DES, you need 256 + 2112 ~= 2112.

    The memory requirements for such attacks can be prohibitive, but there is a whole body of research literature on methods of reducing them.

    Message Digest Algorithm
    An algorithm which takes a message as input and produces a hash or digest of it, a fixed-length set of bits which depend on the message contents in some highly complex manner. Design criteria include making it extremely difficult for anyone to counterfeit a digest or to change a message without altering its digest. One essential property is collision resistance. The main applications are in message authentication and digital signature schemes. Widely used algorithms include MD5 and SHA. In IPsec, message digests are used for HMAC authentication of packets.
    Maximum Transmission U nit, the largest size of packet that can be sent over a link. This is determined by the underlying network, but must be taken account of at the IP level.

    IP packets, which can be up to 64K bytes each, must be packaged into lower-level packets of the appropriate size for the underlying network(s) and re-assembled on the other end. When a packet must pass over multiple networks, each with its own MTU, and many of the MTUs are unknown to the sender, this becomes a fairly complex problem. See path MTU discovery for details.

    Often the MTU is a few hundred bytes on serial links and 1500 on Ethernet. There are, however, serial link protocols which use a larger MTU to avoid fragmentation at the ethernet/serial boundary, and newer (especially gigabit) Ethernet networks sometimes support much larger packets because these are more efficient in some applications.

    Network Associates, a conglomerate formed from PGP Inc., TIS (Trusted Information Systems, a firewall vendor) and Macaffee anti-virus products. Among other things, they offer an IPsec-based VPN products.
    Network Address Translation, a process by which firewall machines may change the addresses on packets as they go through. for discussion, see our background section.
    The US National Institute of Standards and Technology, responsible for FIPS standards including DES and its replacement, AES.
    A random value used in an authentication protocol.
    Non-routable IP address
    An IP address not normally allowed in the "to" or "from" IP address field header of IP packets.

    Almost invariably, the phrase "non-routable address" means one of the addresses reserved by RFC 1918 for private networks:

    These addresses are commonly used on private networks, e.g. behind a Linux machines doing IP masquerade. Machines within the private network can address each other with these addresses. All packets going outside that network, however, have these addresses replaced before they reach the Internet.

    If any packets using these addresses do leak out, they do not go far. Most routers automatically discard all such packets.

    Various other addresses -- the block reserved for local use,, various broadcast and network addresses -- cannot be routed over the Internet, but are not normally included in the meaning when the phrase "non-routable address" is used.

    The US National Security Agency, the American organisation for signals intelligence , the protection of US government messages and the interception and analysis of other messages. For details, see Bamford's "The Puzzle Palace".

    Some history of NSA documents were declassified in response to a FOIA (Freedom of Information Act) request.

    A key determination protocol, defined in RFC 2412.
    Oakley groups
    The groups used as the basis of Diffie-Hellman key exchange in the Oakley protocol, and in IKE. Four were defined in the original RFC, and a fifth has been added since.

    Linux FreeS/WAN currently supports the three groups based on finite fields modulo a prime (Groups 1, 2 and 5) and does not support the elliptic curve groups (3 and 4). For a description of the difference of the types, see discrete logarithms.

    One time pad
    A cipher in which the key is:

    Given those three conditions, it can easily be proved that the cipher is perfectly secure, in the sense that an attacker with intercepted message in hand has no better chance of guessing the message than an attacker who has not intercepted the message and only knows the message length. No such proof exists for any other cipher.

    There are, however, several problems with this "perfect" cipher.

    First, it is wildly impractical for most applications. Key management is at best difficult, often completely impossible.

    Second, it is extremely fragile. Small changes which violate the conditions listed above do not just weaken the cipher a bit; quite often they destroy its security completely.

    Marketing claims about the "unbreakable" security of various products which somewhat resemble one-time pads are common. Such claims are one of the surest signs of cryptographic snake oil. Systems marketed with such claims are usually completely worthless.

    Finally, even if the system is implemented and used correctly, it is highly vulnerable to certain types of attack. If an attacker knows the plaintext and has an intercepted message, he can discover the pad. This does not matter if the attacker is just a passive eavesdropper. It gives him no plaintext he didn't already know and we don't care that he learns a pad which we'll never re-use. However, knowing the pad lets an active attacker perform a man-in-the-middle attack, replacing your message with whatever he chooses.

    See also the one time pad FAQ.

    Opportunistic encryption
    A situation in which any two IPsec-aware machines can secure their communications, without a pre-shared secret and without a common PKI or previous exchange of public keys. This is one of the goals of the Linux FreeS/WAN project, discussed in our introduction section.

    Setting up for opportunistic encryption is described in our configuration document.

    P1363 standard
    An IEEE standard for public key cryptography. Web page.
    Passive attack
    An attack in which the attacker only eavesdrops and attempts to analyse intercepted messages, as opposed to an active attack in which he diverts messages or generates his own.
    Path MTU discovery
    The process of discovering the largest packet size which all links on a path can handle without fragmentation -- that is, without any router having to break the packet up into smaller pieces to match the MTU of its outgoing link.

    This is done as follows:

    Since this requires co-operation of many systems, and since the next packet may travel a different path, this is one of the trickier areas of IP programming. Bugs that have shown up over the years have included:

    Since IPsec adds a header, it increases packet size and may require fragmentation even where incoming and outgoing MTU are equal.

    Perfect forward secrecy (PFS)
    A property of systems such as Diffie-Hellman key exchange which use a long-term key (such as the shared secret in IKE) and generate short-term keys as required. If an attacker who acquires the long-term key provably can

    then the system has PFS. The attacker needs the short-term keys in order to read the trafiic and merely having the long-term key does not allow him to infer those. Of course, it may allow him to conduct another attack (such as man-in-the-middle) which gives him some short-term keys, but he does not automatically get them just by acquiring the long-term key.

    see Perfect Forward Secrecy
    Pretty Good Privacy, a personal encryption system for email based on public key technology, written by Phil Zimmerman.

    The 2.xx versions of PGP used the RSA public key algorithm and used IDEA as the symmetric cipher. These versions are described in RFC 1991 and in Garfinkel's book. Since version 5, the products from PGP Inc. have used Diffie-Hellman public key methods and CAST-128 symmetric encryption. These can verify signatures from the 2.xx versions, but cannot exchange encryted messages with them.

    An IETF working group has issued RFC 2440 for an "Open PGP" standard, similar to the 5.x versions. PGP Inc. staff were among the authors. A free Gnu Privacy Guard based on that standard is now available.

    For more information on PGP, including how to obtain it, see our cryptography links.

    PGP Inc.
    A company founded by Zimmerman, the author of PGP , now a division of NAI. See the corporate website. Zimmerman left in 2001.

    Versions 6.5 and later of the PGP product include PGPnet, an IPsec client for Macintosh or for Windows 95/98/NT.

    Another key negotiation protocol, an alternative to IKE, described in RFCs 2522 and 2523.
    Point-to-Point Tunneling Protocol. Papers discussing weaknesses in it are on
    Public Key Infrastructure, the things an organisation or community needs to set up in order to make public key cryptographic technology a standard part of their operating procedures.

    There are several PKI products on the market. Typically they use a hierarchy of Certification Authorities (CAs). Often they use LDAP access to X.509 directories to implement this.

    See Web of Trust for a different sort of infrastructure.

    PKI eXchange, an IETF standard that allows PKIs to talk to each other.

    This is required, for example, when users of a corporate PKI need to communicate with people at client, supplier or government organisations, any of which may have a different PKI in place. I should be able to talk to you securely whenever:

    At time of writing (March 1999), this is not yet widely implemented but is under quite active development by several groups.

    The unencrypted input to a cipher, as opposed to the encrypted ciphertext output.
    The Linux FreeS/WAN daemon which handles key exchange via the IKE protocol, connection negotiation, and other higher-level tasks. Pluto calls the KLIPS kernel code as required. For details, see the manual page ipsec_pluto(8).
    Public Key Cryptography
    In public key cryptography, keys are created in matched pairs. Encrypt with one half of a pair and only the matching other half can decrypt it. This contrasts with symmetric or secret key cryptography in which a single key known to both parties is used for both encryption and decryption.

    One half of each pair, called the public key, is made public. The other half, called the private key, is kept secret. Messages can then be sent by anyone who knows the public key to the holder of the private key. Encrypt with the public key and you know only someone with the matching private key can decrypt.

    Public key techniques can be used to create digital signatures and to deal with key management issues, perhaps the hardest part of effective deployment of symmetric ciphers. The resulting hybrid cryptosystems use public key methods to manage keys for symmetric ciphers.

    Many organisations are currently creating PKIs, public key infrastructures to make these benefits widely available.

    Public Key Infrastructure
    see PKI
    A remarkably tricky term, far too much so for me to attempt a definition here. Quite a few cryptosystems have been broken via attacks on weak random number generators, even when the rest of the system was sound.

    See RFC 1750 for the theory.

    See the manual pages for ipsec_ranbits(8) and random(4) for details of what we use.

    There has recently been discussion on several mailing lists of the limitations of Linux /dev/random and of whether we are using it correctly. Those discussions are archived on the /dev/random support page.

    A firewall product for Windows NT offerring IPsec-based VPN services. Linux FreeS/WAN interoperates with Raptor; see our interop document for details. Raptor have recently merged with Axent.
    Rivest Cipher four, designed by Ron Rivest of RSA and widely used. Believed highly secure with adequate key length, but often implemented with inadequate key length to comply with export restrictions.
    Rivest Cipher six, RSA's AES candidate cipher.
    Replay attack
    An attack in which the attacker records data and later replays it in an attempt to deceive the recipient.
    Request For Comments, an Internet document. Some RFCs are just informative. Others are standards.

    Our list of IPsec and other security-related RFCs is here, along with information on methods of obtaining them.

    A message digest algorithm. The current version is RIPEMD-160 which gives a 160-bit hash.
    Root CA
    The top level Certification Authority in a hierachy of such authorities.
    Routable IP address
    Most IP addresses can be used as "to" and "from" addresses in packet headers. These are the routable addresses; we expect routing to be possible for them. If we send a packet to one of them, we expect (in most cases; there are various complications) that it will be delivered if the address is in use and will cause an ICMP error packet to come back to us if not.

    There are also several classes of non-routable IP addresses.

    RSA algorithm
    Rivest Shamir Adleman public key encryption method, named for its three inventors. The algorithm is widely used and likely to become moreso since it became free of patent encumbrances in September 2000.

    For a full explanation of the algorithm, consult one of the standard references such as Applied Cryptography . A simple explanation is:

    The great 17th century French mathematician Fermat proved that, for any prime p and number x, 0 x^p == x modulo p x^(p-1) == 1 modulo p, non-zero x From this it follows that if we have a pair of primes p, q and two numbers e, d such that:

    	ed == 1			modulo lcm( p-1, q-1)
    where lcm() is least common multiple, then for all x, 0 x^(ed-1) == 1 modulo pq, non-zero x x^ed == x modulo pq So we construct such as set of numbers p, q, e, d and publish the product N=pq and e as the public key. Encryption is then:
    	c = x^e			modulo N
    An attacker cannot deduce x from the cyphertext c, short of either factoring N or solving the discrete logarithm problem for this field. If p, q are large primes (hundreds or thousands of bits) no efficient solution to either problem is known.

    The receiver, knowing the private key (N and d), can readily find x sixce:

    	c^d == (x^e)^d		modulo N
    	    == x^ed		modulo N
    	    == x		modulo N
    This gives an effective public key technique, with only a couple of problems. It uses a good deal of computer time, since calculations with large integers are not cheap, and there is no proof it is necessarily secure since no-one has proven either factoring or discrete log cannot be done efficiently.
    RSA Data Security
    A company founded by the inventors of the RSA public key algorithm.
    Security Association, the channel negotiated by the higher levels of an IPsec implementation and used by the lower. SAs are unidirectional; you need a pair of them for two-way communication.

    An SA is defined by three things -- the destination, the protocol (AH orESP) and the SPI , security parameters index. It is used to index other things such as session keys and intialisation vectors.

    For more detail, see our section on IPsec and/or RFC 2401.

    Secure DNS
    A version of the DNS or Domain Name Service enhanced with authentication services. This is being designed by the IETF DNS security working group. Check the Internet Software Consortium for information on implementation progress and for the latest version of BIND. Another site has more information .

    IPsec can use this plus Diffie-Hellman key exchange to bootstrap itself. This allows opportunistic encryption. Any pair of machines which can authenticate each other via DNS can communicate securely, without either a pre-existing shared secret or a shared PKI.

    Secret key cryptography
    See symmetric cryptography
    Security Association
    see SA
    Sequence number
    A number added to a packet or message which indicates its position in a sequence of packets or messages. This provides some security against replay attacks.

    For automatic keying mode, the IPsec RFCs require that the sender generate sequence numbers for each packet, but leave it optional whether the receiver does anything with them.

    Secure Hash Algorithm, a message digest algorithm developed by the NSA for use in the Digital Signature standard, FIPS number 186 from NIST. SHA is an improved variant of MD4 producing a 160-bit hash.

    SHA is one of two message digest algorithms available in IPsec. The other is MD5. Some people do not trust SHA because it was developed by the NSA. There is, as far as we know, no cryptographic evidence that SHA is untrustworthy, but this does not prevent that view from being strongly held.

    Signals intelligence (SIGINT)
    Activities of government agencies from various nations aimed at protecting their own communications and reading those of others. Cryptography, cryptanalysis, wiretapping, interception and monitoring of various sorts of signals. The players include the American NSA, British GCHQ and Canadian CSE.
    Simple Key management for Internet P rotocols, an alternative to IKE developed by Sun and being marketed by their Internet Commerce Group.
    Snake oil
    Bogus cryptography. See the Snake Oil FAQ or this paper by Schneier.
    Security Parameter Index, an index used within IPsec to keep connections distinct. A Security Association (SA) is defined by destination, protocol and SPI. Without the SPI, two connections to the same gateway using the same protocol could not be distinguished.

    For more detail, see our IPsec section and/or RFC 2401.

    Secure SHell, an encrypting replacement for the insecure Berkeley commands whose names begin with "r" for "remote": rsh, rlogin, etc.

    For more information on SSH, including how to obtain it, see our cryptography links.

    SSH Communications Security
    A company founded by the authors of SSH. Offices are in Finland and California. They have a toolkit for developers of IPsec applications.
    Secure Sockets Layer , a set of encryption and authentication services for web browsers, developed by Netscape. Widely used in Internet commerce. Also known as TLS.
    A free implementation of SSL by Eric Young (eay) and others. Developed in Australia; not subject to US export controls.
    Stream cipher
    A symmetric cipher which produces a stream of output which can be combined (often using XOR or bytewise addition) with the plaintext to produce ciphertext. Contrasts with block cipher.

    IPsec does not use stream ciphers. Their main application is link-level encryption, for example of voice, video or data streams on a wire or a radio signal.

    A group of IP addresses which are logically one network, typically (but not always) assigned to a group of physically connected machines. The range of addresses in a subnet is described using a subnet mask. See next entry.
    subnet mask
    A method of indicating the addresses included in a subnet. Here are two equivalent examples:

    The '24' is shorthand for a mask with the top 24 bits one and the rest zero. This is exactly the same as which has three all-ones bytes and one all-zeros byte.

    These indicate that, for this range of addresses, the top 24 bits are to be treated as naming a network (often referred to as "the subnet") while most combinations of the low 8 bits can be used to designate machines on that network. Two addresses are reserved; refers to the subnet rather than a specific machine while is a broadcast address. 1 to 254 are available for machines.

    It is common to find subnets arranged in a hierarchy. For example, a large company might have a /16 subnet and allocate /24 subnets within that to departments. An ISP might have a large subnet and allocate /26 subnets (64 addresses, 62 usable) to business customers and /29 subnets (8 addresses, 6 usable) to residential clients.

    Secure Wide Area Network, a project involving RSA Data Security and a number of other companies. The goal was to ensure that all their IPsec implementations would interoperate so that their customers can communicate with each other securely.
    Symmetric cryptography
    Symmetric cryptography, also referred to as conventional or secret key cryptography, relies on a shared secret key, identical for sender and receiver. Sender encrypts with that key, receiver decrypts with it. The idea is that an eavesdropper without the key be unable to read the messages. There are two main types of symmetric cipher, block ciphers and stream ciphers.

    Symmetric cryptography contrasts with public key or asymmetric systems where the two players use different keys.

    The great difficulty in symmetric cryptography is, of course, key management. Sender and receiver must have identical keys and those keys must be kept secret from everyone else. Not too much of a problem if only two people are involved and they can conveniently meet privately or employ a trusted courier. Quite a problem, though, in other circumstances.

    It gets much worse if there are many people. An application might be written to use only one key for communication among 100 people, for example, but there would be serious problems. Do you actually trust all of them that much? Do they trust each other that much? Should they? What is at risk if that key is compromised? How are you going to distribute that key to everyone without risking its secrecy? What do you do when one of them leaves the company? Will you even know?

    On the other hand, if you need unique keys for every possible connection between a group of 100, then each user must have 99 keys. You need either 99*100/2 = 4950 secure key exchanges between users or a central authority that securely distributes 100 key packets, each with a different set of 99 keys.

    Either of these is possible, though tricky, for 100 users. Either becomes an administrative nightmare for larger numbers. Moreover, keys must be changed regularly, so the problem of key distribution comes up again and again. If you use the same key for many messages then an attacker has more text to work with in an attempt to crack that key. Moreover, one successful crack will give him or her the text of all those messages.

    In short, the hardest part of conventional cryptography is key management. Today the standard solution is to build a hybrid system using public key techniques to manage keys.

    Trusted Information Systems, a firewall vendor now part of NAI. Their Gauntlet product offers IPsec VPN services. TIS implemented the first version of Secure DNS on a DARPA contract.
    Transport Layer Security, a newer name for SSL.
    Traffic analysis
    Deducing useful intelligence from patterns of message traffic, without breaking codes or reading the messages. In one case during World War II, the British guessed an attack was coming because all German radio traffic stopped. The "radio silence" order, intended to preserve security, actually gave the game away.

    In an industrial espionage situation, one might deduce something interesting just by knowing that company A and company B were talking, especially if one were able to tell which departments were involved, or if one already knew that A was looking for acquisitions and B was seeking funds for expansion.

    IPsec itself does not defend against this, but carefully thought out systems using IPsec can provide at least partial protection. In particular, one might want to encrypt more traffic than was strictly necessary, route things in odd ways, or even encrypt dummy packets, to confuse the analyst. We discuss this here.

    Transport mode
    An IPsec application in which the IPsec gateway is the destination of the protected packets, a machine acts as its own gateway. Contrast with tunnel mode.
    Triple DES
    see 3DES
    Tunnel mode
    An IPsec application in which an IPsec gateway provides protection for packets to and from other systems. Contrast with transport mode.
    Two-key Triple DES
    A variant of triple DES or 3DES in which only two keys are used. As in the three-key version, the order of operations is EDE or encrypt-decrypt-encrypt, but in the two-key variant the first and third keys are the same.

    3DES with three keys has 3*56 = 168 bits of key but has only 112-bit strength against a meet-in-the-middle attack, so it is possible that the two key version is just as strong. Last I looked, this was an open question in the research literature.

    RFC 2451 defines triple DES for IPsec as the three-key variant. The two-key variant should not be used and is not implemented directly in Linux FreeS/WAN. It cannot be used in automatically keyed mode without major fiddles in the source code. For manually keyed connections, you could make Linux FreeS/WAN talk to a two-key implementation by setting two keys the same in /etc/ipsec.conf.

    Virtual Interface
    A Linux feature which allows one physical network interface to have two or more IP addresses. See the Linux Network Administrator's Guide in book form or on the web for details.
    Virtual Private Network
    see VPN
    Virtual Private Network, a network which can safely be used as if it were private, even though some of its communication uses insecure connections. All traffic on those connections is encrypted.

    IPsec is not the only technique available for building VPNs, but it is the only method defined by RFCs and supported by many vendors. VPNs are by no means the only thing you can do with IPsec, but they may be the most important application for many users.

    Virtual Private Network Consortium , an association of vendors of VPN products.
    Wassenaar Arrangement
    An international agreement restricting export of munitions and other tools of war. Unfortunately, cryptographic software is also restricted under the current version of the agreement. Discussion.
    Web of Trust
    PGP's method of certifying keys. Any user can sign a key; you decide which signatures or combinations of signatures to accept as certification. This contrasts with the hierarchy of CAs (Certification Authorities) used in many PKIs (Public Key Infrastructures).

    See Global Trust Register for an interesting addition to the web of trust.

    WEP (Wired Equivalent Privacy)
    The cryptographic part of the IEEE standard for wireless LANs.

    As the name suggests, this is designed to be only as secure as a normal wired ethernet; anyone with a network conection can tap it. Its advocates would claim this is good design, refusing to build in complex features beyond the actual requirements. For example, see this Security Statement from a vendors' group.

    We consider "Wiretap Equivalent Privacy" a horribly flawed design. You do not control radio waves as you might control your wires, so the metaphor in the rationale is utterly inapplicable. A security policy that chooses not to invest resources in protecting against certain attacks which can only be conducted by people physically plugged into your LAN may or may not be reasonable. This depends at least on your needs, your building, your staffing policies, and the attack details. The same policy is likely completely unreasonable when someone can "plug in" from a laptop in a car or cafe half a block from your office.

    There has been considerable analysis indicating that WEP is seriously flawed. A FAQ on attacks against WEP is available. Part of it reads:

    ... attacks are practical to mount using only inexpensive off-the-shelf equipment. We recommend that anyone using an 802.11 wireless network not rely on WEP for security, and employ other security measures to protect their wireless network. Note that our attacks apply to both 40-bit and the so-called 128-bit versions of WEP equally well.

    WEP appears to be yet another instance of governments, and unfortunately some vendors and standards bodies, deliberately promoting hopelessly flawed "security" products, apprently mainly for the benefit of eavesdropping agencies. See this discussion.

    A standard from the ITU (International Telecommunication Union), for hierarchical directories with authentication services, used in many PKI implementations.

    Use of X.509 services, via the LDAP protocol, for certification of keys is allowed but not required by the IPsec RFCs. It is not yet implemented in Linux FreeS/WAN.

    A vendor of router and Internet access products, now part of Lucent. Their QVPN products interoperate with Linux FreeS/WAN; see our interop document.

    Bibliography for the Linux FreeS/WAN project

    For extensive bibliographic links, see the Collection of Computer Science Bibliographies

    See our web links for material available online.

    Carlisle Adams and Steve Lloyd Understanding Public Key Infrastructure
    Macmillan 1999 ISBN 1-57870-166-x
    An overview, mainly concentrating on policy and strategic issues rather than the technical details. Both authors work for PKI vendor Entrust.
    Albitz, Liu & Loukides DNS & BIND 3rd edition
    O'Reilly 1998 ISBN 1-56592-512-2
    The standard reference on the Domain Name Service and Berkeley Internet Name Daemon.
    Bamford The Puzzle Palace, A report on NSA, Americas's most Secret Agency
    Houghton Mifflin 1982 ISBN 0-395-31286-8

    David Bander, Linux Security Toolkit
    IDG Books, 2000, ISBN: 0764546902
    This book has a short section on FreeS/WAN and includes Caldera Linux on CD.
    Chapman, Zwicky & Russell Building Internet Firewalls
    O'Reilly 1995 ISBN 1-56592-124-0

    Cheswick and Bellovin Firewalls and Internet Security: Repelling the Wily Hacker
    Addison-Wesley 1994 ISBN 0201633574
    A fine book on firewalls in particular and security in general from two of AT&T's system adminstrators.

    Bellovin has also done a number of papers on IPsec and co-authored a paper on a large FreeS/WAN application.

    Comer Internetworking with TCP/IP
    Prentice Hall

    If you need to deal with the details of the network protocols, read either this series or the Stevens and Wright series before you start reading the RFCs.

    Diffie and Landau Privacy on the Line: The Politics of Wiretapping and Encryption
    MIT press 1998 ISBN 0-262-04167-7 (hardcover) or 0-262-54100-9

    Doraswamy and Harkins IP Sec: The New Security Standard for the Internet, Intranets and Virtual Private Networks
    Prentice Hall 1999 ISBN: 0130118982

    Electronic Frontier Foundation Cracking DES: Secrets of Encryption Research, Wiretap Politics and Chip Design
    O'Reilly 1998 ISBN 1-56592-520-3
    To conclusively demonstrate that DES is inadequate for continued use, the EFF built a machine for just over $200,000 that breaks DES encryption in under five days on average, under nine in the worst case.

    The book provides details of their design and, perhaps even more important, discusses why they felt the project was necessary. Recommended for anyone interested in any of the three topics mentioned in the subtitle.

    See also the EFF page on this project and our discussion of DES insecurity.

    Martin Freiss Protecting Networks with SATAN
    O'Reilly 1998 ISBN 1-56592-425-8
    translated from a 1996 work in German
    SATAN is a Security Administrator's Tool for Analysing Networks. This book is a tutorial in its use.
    Gaidosch and Kunzinger A Guide to Virtual Private Networks
    Prentice Hall 1999 ISBN: 0130839647
    Simson Garfinkel Database Nation: the death of privacy in the 21st century
    O'Reilly 2000 ISBN 1-56592-653-6
    A thoughtful and rather scary book.

    Simson Garfinkel PGP: Pretty Good Privacy
    O'Reilly 1995 ISBN 1-56592-098-8
    An excellent introduction and user manual for the
    PGP email-encryption package. PGP is a good package with a complex and poorly-designed user interface. This book or one like it is a must for anyone who has to use it at length.

    The book covers using PGP in Unix, PC and Macintosh environments, plus considerable background material on both the technical and political issues around cryptography. The only shortcoming is that it does not cover recent developments such as PGP 5 and Open PGP.

    Garfinkel and Spafford Practical Unix Security
    O'Reilly 1996 ISBN 1-56592-148-8
    A standard reference.
    Spafford's web page has an excellent collection of
    crypto and security links.
    David Kahn The Codebreakers: the Comprehensive History of Secret Communications from Ancient Times to the Internet
    second edition Scribner 1996 ISBN 0684831309
    A history of codes and code-breaking from ancient Egypt to the 20th century. Well-written and exhaustively researched. Highly recommended, even though it does not have much on computer cryptography.

    David Kahn Seizing the Enigma, The Race to Break the German U-Boat codes, 1939-1943
    Houghton Mifflin 1991 ISBN 0-395-42739-8
    Olaf Kirch Linux Network Administrator's Guide
    O'Reilly 1995 ISBN 1-56592-087-2
    Now becoming somewhat dated in places, but still a good introductory book and general reference.

    Pete Loshin Big Book of IPsec RFCs
    Morgan Kaufmann 2000 ISBN: 0-12-455839-9

    Steven Levy Crypto: How the Code Rebels Beat the Government -- Saving Privacy in the Digital Age
    Penguin 2001, ISBN 0-670--85950-8
    Highly recommended. A fine history of recent (about 1970-2000) developments in the field, and the related political controversies. FreeS/WAN project founder and leader John Gilmore appears several times.

    The book does not cover IPsec or FreeS/WAN, but this project is very much another battle in the same war. See our discussion of the politics.

    Matyas, Anderson et al. The Global Trust Register
    Northgate Consultants Ltd 1998 ISBN: 0953239705
    hard cover edition due April 1999 MIT Press ISBN 0262511053
    their web page:
    This book is a register of the fingerprints of the world's most important public keys; it implements a top-level certification authority (CA) using paper and ink rather than in an electronic system.

    Menezies, van Oorschot and Vanstone Handbook of Applied Cryptography
    CRC Press 1997
    ISBN 0-8493-8523-7
    An excellent reference. Read Schneier before tackling this.
    Michael Padlipsky Elements of Networking Style
    Prentice-Hall 1985 ISBN 0-13-268111-0 or 0-13-268129-3
    Probably the funniest technical book ever written , this is a vicious but well-reasoned attack on the OSI "seven layer model" and all that went with it. Several chapters of it are also available as RFCs 871 to 875.
    John S. Quarterman The Matrix: Computer Networks and Conferencing Systems Worldwide
    Digital Press 1990 ISBN 155558-033-5
    Prentice-Hall ISBN 0-13-565607-9
    The best general treatment of computer-mediated communication we have seen. It naturally has much to say about the Internet, but also covers UUCP, Fidonet and so on.

    David Ranch Securing Linux Step by Step
    SANS Institute, 1999
    SANS is a respected organisation, this guide is part of a well-known series, and Ranch has previously written the useful Trinity OS guide to securing Linux, so my guess would be this is a pretty good book. I haven't read it yet, so I'm not certain. It can be ordered online from SANS.
    Bruce Schneier Applied Cryptography, Second Edition
    John Wiley & Sons, 1996
    ISBN 0-471-12845-7 hardcover
    ISBN 0-471-11709-9 paperback
    A standard reference on computer cryptography. For more recent essays, see the
    author's company's web site.
    Bruce Schneier Secrets and Lies
    Wiley 2000, ISBN 0-471-25311-1
    An interesting discussion of security and privacy issues, written with more of an "executive overview" approach rather than a narrow focus on the technical issues. Highly recommended.
    Scott, Wolfe and Irwin Virtual Private Networks
    2nd edition, O'Reilly 1999 ISBN: 1-56592-529-7
    This is the only O'Reilly book, out of a dozen I own, that I'm disappointed with. It deals mainly with building VPNs with various proprietary tools -- PPTP, SSH, Cisco PIX, ... -- and touches only lightly on IPsec-based approaches.

    That said, it appears to deal competently with what it does cover and it has readable explanations of many basic VPN and security concepts. It may be exactly what some readers require, even if I find the emphasis unfortunate.

    Kurt Seifried Linux Administrator's Security Guide
    Available online from Security Portal. It has fairly extensive coverage of IPsec.
    Richard E Smith Internet Cryptography
    ISBN 0-201-92480-3, Addison Wesley, 1997
    See the book's home page
    Neal Stephenson Cryptonomicon
    Hardcover ISBN -380-97346-4, Avon, 1999.

    A novel in which cryptography and the net figure prominently. Highly recommended: I liked it enough I immediately went out and bought all the author's other books.

    There is also a paperback edition. Sequels are expected.

    Stevens and Wright TCP/IP Illustrated

    If you need to deal with the details of the network protocols, read either this series or the Comer series before you start reading the RFCs.

    Rubini Linux Device Drivers
    O'Reilly & Associates, Inc. 1998 ISBN 1-56592-292-1

    Robert Zeigler Linux Firewalls
    Newriders Publishing, 2000 ISBN 0-7537-0900-9

    IPsec RFCs and related documents

    The RFCs.tar.gz Distribution File

    The Linux FreeS/WAN distribution is available from:

    our primary distribution site and various mirror sites. To give people more control over their downloads, the RFCs that define IP security are bundled separately in the file RFCs.tar.gz.

    The file you are reading is included in the main distribution and is available on the web site. It describes the RFCs included in the RFCs.tar.gz bundle and gives some pointers to other ways to get them.

    Other sources for RFCs & Internet drafts


    RFCs are downloadble at many places around the net such as:

    browsable in HTML form at others such as:

    and some of them are available in translation:

    There is also a published Big Book of IPSEC RFCs .

    Internet Drafts

    Internet Drafts, working documents which sometimes evolve into RFCs, are also available.

    Note: some of these may be obsolete, replaced by later drafts or by RFCs.

    FIPS standards

    Some things used by IPsec, such as DES and SHA, are defined by US government standards called FIPS. The issuing organisation, NIST, have a FIPS home page.

    What's in the RFCs.tar.gz bundle?

    All filenames are of the form rfc*.txt, with the * replaced with the RFC number.

    RFC#        Title

    Overview RFCs

    2401        Security Architecture for the Internet Protocol
    2411        IP Security Document Roadmap

    Basic protocols

    2402        IP Authentication Header
    2406        IP Encapsulating Security Payload (ESP)

    Key management

    2367        PF_KEY Key Management API, Version 2
    2407        The Internet IP Security Domain of Interpretation for ISAKMP
    2408        Internet Security Association and Key Management Protocol (ISAKMP)
    2409        The Internet Key Exchange (IKE)
    2412        The OAKLEY Key Determination Protocol
    2528        Internet X.509 Public Key Infrastructure

    Details of various things used

    2085        HMAC-MD5 IP Authentication with Replay Prevention
    2104        HMAC: Keyed-Hashing for Message Authentication
    2202        Test Cases for HMAC-MD5 and HMAC-SHA-1
    2207        RSVP Extensions for IPSEC Data Flows
    2403        The Use of HMAC-MD5-96 within ESP and AH
    2404        The Use of HMAC-SHA-1-96 within ESP and AH
    2405        The ESP DES-CBC Cipher Algorithm With Explicit IV
    2410        The NULL Encryption Algorithm and Its Use With IPsec
    2451        The ESP CBC-Mode Cipher Algorithms
    2521        ICMP Security Failures Messages

    Older RFCs which may be referenced

    1321        The MD5 Message-Digest Algorithm
    1828        IP Authentication using Keyed MD5
    1829        The ESP DES-CBC Transform
    1851        The ESP Triple DES Transform
    1852        IP Authentication using Keyed SHA

    RFCs for secure DNS service, which IPsec may use

    2137        Secure Domain Name System Dynamic Update
    2230        Key Exchange Delegation Record for the DNS
    2535        Domain Name System Security Extensions
    2536        DSA KEYs and SIGs in the Domain Name System (DNS)
    2537        RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)
    2538        Storing Certificates in the Domain Name System (DNS)
    2539        Storage of Diffie-Hellman Keys in the Domain Name System (DNS)

    RFCs labelled "experimental"

    2521        ICMP Security Failures Messages
    2522        Photuris: Session-Key Management Protocol
    2523        Photuris: Extended Schemes and Attributes

    Related RFCs

    1750        Randomness Recommendations for Security
    1918        Address Allocation for Private Internets
    1984        IAB and IESG Statement on Cryptographic Technology and the Internet
    2144        The CAST-128 Encryption Algorithm

    Distribution Roadmap: What's Where in Linux FreeS/WAN

    This file is a guide to the locations of files within the FreeS/WAN distribution. Everything described here should be on your system once you download, gunzip, and untar the distribution.

    This distribution contains two major subsystems

    the kernel code
    the user-level key-management daemon

    plus assorted odds and ends.

    Top directory

    The top directory has essential information in text files:

    introduction to the software
    short experts-only installation procedures. More detalied procedures are in installation and configuration HTML documents.
    major known bugs in the current release.
    changes from previous releases
    acknowledgement of contributors
    licensing and distribution information


    The doc directory contains the bulk of the documentation, most of it in HTML format. See the index file for details.

    KLIPS: kernel IP security

    KLIPS is KerneL IP Security. It lives in the klips directory, of course.

    patches for existing kernel files
    test stuff
    low-level user utilities
    actual klips kernel files
    symbolic link to klips/net/ipsec

    The "make insert" step of installation installs the patches and makes a symbolic link from the kernel tree to klips/net/ipsec. The odd name of klips/net/ipsec is dictated by some annoying limitations of the scripts which build the Linux kernel. The symbolic-link business is a bit messy, but all the alternatives are worse.

    Utility programs:

    manipulate IPsec extended routing tables
    set Klips (kernel IPsec support) debug features and level
    manage IPsec Security Associations
    group/ungroup IPsec Security Associations
    associate IPsec virtual interface with real interface

    These are all normally invoked by ipsec(8) with commands such as

            ipsec tncfg arguments
    There are section 8 man pages for all of these; the names have "ipsec_" as a prefix, so your man command should be something like:
            man 8 ipsec_tncfg

    Pluto key and connection management daemon

    Pluto is our key management and negotiation daemon. It lives in the pluto directory, along with its low-level user utility, whack.

    There are no subdirectories. Documentation is a man page, pluto.8. This covers whack as well.


    The utils directory contains a growing collection of higher-level user utilities, the commands that administer and control the software. Most of the things that you will actually have to run yourself are in there.

    invoke IPsec utilities

    ipsec(8) is normally the only program installed in a standard directory, /usr/local/sbin. It is used to invoke the others, both those listed below and the ones in klips/utils mentioned above.

    control automatically-keyed IPsec connections
    take manually-keyed IPsec connections up and down
    generate copious debugging output
    generate moderate amounts of debugging output

    There are .8 manual pages for these. look is covered in barf.8. The man pages have an "ipsec_" prefix so your man command should be something like:

            man 8 ipsec_auto

    Examples are in various files with names utils/*.eg


    FreeS/WAN Library

    The lib directory is the FreeS/WAN library, also steadily growing, used by both user-level and kernel code.
    It includes section 3 man pages for the library routines.

    Imported Libraries


    The libdes library, originally from SSLeay, is used by both Klips and Pluto for Triple DES encryption. Single DES is not used because it is insecure.

    Note that this library has its own license, different from the GPL used for other code in FreeS/WAN.

    The library includes its own documentation.


    The GMP (GNU multi-precision) library is used for multi-precision arithmetic in Pluto's key-exchange code and public key code.

    Older versions (up to 1.7) of FreeS/WAN included a copy of this library in the FreeS/WAN distribution.

    Since 1.8, we have begun to rely on the system copy of GMP.