Where to find symmetric NAT?

Currently i'm working on some NAT traversal code (that allows to computers each behind it's own NAT to communicate with each other). But, surprisingly, i can't find any symmetric NAT's! (it's the most hard case). Anyone knows where can i find one, wither in software or hardware? It seems that windows 'internet connection sharing' is not symmetric :(.

Solution 1:

As i know OpenBSD/FreeBSD pf nat is symmetric, it can be done using one line in /etc/pf.conf i.e.

nat on tl0 from 192.168.1.0/24 to any -> 24.5.0.5 

UPD: Of cause you can download OpenBSD/FreeBSD or pfSense and run it under some VM, i.e. VirtualBox as popular free one, just like Jimsmithkka suggested.

Solution 2:

i would suggest seeing if you can get your hands on a cisco virtual device (example/sample), i know their routers can support Symmetric Nat, so you could build one real quick and see if it works for you in VMs.

Also check VMwares site for other possible appliances that might do this for you.