A patched version of the Mac OSX built-in VPN client to allow it to work with Cisco certificate-based authentication.
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README

COMPATIBILITY AND FEATURES

Note: this hack doesn't seem to be needed anymore with recent OSX updates.
Can anyone else confirm this?

This is based on the source from the from OSX 1.6.5. (1.6.7+ patches not yet included)
I have tested this on OSX Snow Leopard 10.6.5, 1.6.6, 1.6.7, and 1.6.8.
It may work in closely-related Snow Leopard releases as well.
I have also heard reports that this works on Lion as well. (thanks, Alex!)

The main issue I discovered when attempting to use the build-in OSX
VPN client to connect to a Cisco VPN with certificate authentication
is that it refuses to accept the server's certificate if the FQDN is
not specified in a SubjectAltName (besides the usual place: CommonName).
This is non-standard for Cisco VPN certificates, so it has prevented the
use of the built-in client in those cases. (And the official Cisco VPN
client for Mac is out of date and causes kernel panics about once a
day on my machine)

The fix for this issue was to cause the certificate authentication
code to search the CommonName from the server's certificate before
attempting to search the SubjectAltNames.

DOWNLOADING AND INSTALLING

Downlaod the "racoon" binary and place it in "/usr/sbin/racoon"
(You probably want to make a backup copy of your original "racoon"
binary before doing this)
You may or may not need to reboot for this to take effect.
It may be sufficient to do "ps aux | grep racoon" and kill any running
/usr/sbin/racoon process.

SETTING UP YOUR VPN CONNECTION TO USE CISCO CERTIFICATES

In order to use the VPN with certificates, you will need to first
obtain a client certificate in an appropriate format.

If you already have your certificate in PKCS12 format, you are good to
go, otherwise, you will need to generate a new certificate request.
Unfortunately, if you are migrating from a Cisco VPN client, you may
not have the client certificate in a useful format. (the Cisco client
only exports in its own proprietary format)

To ease this process, I have included a set of scripts which use the
OpenSSL command-line utility to generate certificate requests and
reformat the response.

Use "generate_req.sh" to generate the certificate request and
"combine_cert.sh" to package your private key and request together for
import into your OSX Keychain.

You will also need to obtain a copy of the certificate for the
certificate authority who issued the certificate of your VPN server
(unless of course this was issued by an already-trusted CA like Verisign)

Next, open the Keychain Access application (found in
Applications/Utilities) and select the System keychain. (it is
important that you use the System keychain, not the default "login"
keychain)

Use the File->Import Items menu to import the CA certificate and your
PKCS12 cert+key file.

These should show up under the "Certificates" category.  Double-click
on the CA certificate, expand the "Trust" entry and set "When using
this certificate:" to "Always Trust"

You can also double-click on the other certificate to verify that it
imported correctly and contains the private key, but you shouldn't
need to set any particular trust settings there.

Finally, in your "Network Preferences" window, create a new VPN
interface with a type of "Cisco IPSec".

When configuring the Server Address, you should make sure to use a
FQDN or an IP address so as to match the CommonName that the server's
certificate will supply.

Clicking on the Authentication Settings button will bring up a window
where you can select the client certificate that you imported
previously.  If the correct certificate is not listed, go back to
Keychain Access and check that the certificate is in the "System"
keychain and that it has the private key attached to it.

Finally, you should be able to use the "Connect" button!

SOURCE CODE AND BUILDING

The file which I modified before recompiling the project is included
here as "oakley.c"

The full original source can be found in the ipsec package from Apple,
avialable here:

http://www.opensource.apple.com/source/ipsec/ipsec-93.10/

In order to build the "racoon" binary, you will need a Xcode and a
working build environment... which is really tricky for Darwin
components.  Generally, the "darwinbuild" tool from macosforge.com is
a good place to start:

http://darwinbuild.macosforge.org/

Unfortunately, the current version (as of 11/30/2010) of darwinbuild
does not like working with the latest sources and Xcode version, so
many manual hacks are necessary to get the build to work.

(That's why I'm also supplying the binary here!)