I spent quite a long time googling for a way of re-creating symmetric cone nat in house without buying Cisco hardware. Hopefully someone else will find this useful. This was done on Ubuntu 8.04 beta.

eth1 = public ip
eth0 = lan ip

echo “1″ > /proc/sys/net/ipv4/ip_forward
/sbin/iptables –flush
/sbin/iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE [...]