I have a homeserver, that is now connected to the internet with an own network device (ath0 - 192.168.1.x). It also has one more network interface (eth0 - 192.168.0.x).

Soon I will get a second internet line that will be connected the second network. The server then has both networks with different internet lines available, but i only want it to connect to the internet on the old ath0 interface - not the new eth0 (192.168.0.x).

Background of that constellation is that the new line has a volume-limit in traffic - the old hasn't and i need the new line for all mobile devices and laptops. The devices should be able to use the new network to connect to the internet and the server.

The homeserver is a debian 6 with iptables and some already written rules for it. I need now a rule to block all outgoing internet access on the eth0 interface - i guess it could be something with --target != 192.168.0.0 but i did not succeed in finding the proper solution.

Edit: found the solution:

iptables -A OUTPUT -o eth0 -d 192.168.0.0/24 -m state --state NEW,ESTABLISHED -j ACCEPT

With that setting, all traffic that uses the eth0 interface is only allowed if the destination is inside the network 192.168.0.x - all other traffic is denied .