Полный NAT

eth0 - wan
eth1 - lan

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT

/sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT