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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:

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