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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 :(.

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up vote 4 down vote accepted

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 to any ->

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.

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Is it any good guide how to configure simple test NAT on FreeBSD? I have installed FreeBSD 7.2-RELEASE on one of my computers, but adding your string to rc.conf will raise error "nat not found" :) – Eye of Hell Aug 27 '09 at 10:53
I have rebuilt kernel and sucesfully cnfigured NAT. Unfortunately, it's symmetric :(. – Eye of Hell Aug 27 '09 at 12:47
not rc.conf, but pf.conf, you also need kldload pf and pfctl -e for nat to start working. Also read FreeBSD hanbook about handling pf firewall – SaveTheRbtz Aug 27 '09 at 14:47
Thank you. It works! indeed a SYMMETRIC nat! – Eye of Hell Sep 17 '09 at 12:37

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.

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You can also use something like Dynamips or GUI utility like GNS3 ( ) to emulate cisco devices, using IOSes from, and connect virtual cisco network to the real one. – SaveTheRbtz Aug 25 '09 at 21:57

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