I found some informations but all confusing. I have an ubuntu box 12.04 connected to internet via cable with a fixed ip inside a wide internal network. The cable is plugged into motherboard network interface, that is configured with his ip, dns and so on. I bought an extra network interface card and plugged a apple airport extreme router to the second interface. I want the router to share connection with other devices using the connection taken by the cable. How i can do this?
If I understood correctly, the network layout is as follows,
clients <-wireless-> airport <-eth1-> ubuntu <-eth0-> ISP
With this setup you will need to have ip forwarding enabled and have iptables setup correctly to masquerade all the traffic from eth1 to eth0.
sysctl -w net.ipv4.ip_forward=1 iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE iptables -A FORWARD -i eth1 -j ACCEPT
All you need to do is then to persist these changes.