I have an Ubuntu 9.10 machine with three ethernet interfaces, eth0, eth1 and eth2. eth2 is connected to a private network. eth0 and eth2 are connected to two different LANs. Either one will provide access to the internet. All three networks have DHCP servers.

Using Ubuntu's the default settings (And Gnome), when I boot up all the interfaces are active and my system gets three IP addresses. However any attempt to access the internet results in connection timeouts and other weirdness. I suspect that traffic is going out on one NIC (like eth0) and coming back in on another (like eth1). I'm not sure what's going on. The only way I can access the internet at the moment is to bring two of the devices down with ifdown.

How can I configure eth0 as my primary interface so all trafic goes out by default on that interface, while keeping the other two active? Also, I want to make sure Avahi broadcasts properly on all three IPs so that the computers on the LAN of eth1 can still connect to myHostname.local...