On a server with multiple network cards I tried to configure the eth3 interface by editing /etc/network/interfaces (this was an Ubuntu 12.04 machine).
This was the contents of /etc/networking/interfaces:
# The loopback network interface
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address xxx.yyy.zzz.mmm
netmask 255.255.255.0
gateway xxx.yyy.zzz.1
dns-nameservers xxx.yyy.zzz.aaa xxx.yyy.zzz.bbb
dns-search mydomain.nl
auto eth3
iface eth3 inet static
address 192.168.4.1
netmask 255.255.255.0
gateway 192.168.4.1
When I tried to bring the interface up I got an error message:
$ ifup eth3 RTNETLINK answers: File exists Failed to bring up eth3.
It took me a while to figure it out, but the problem was the gw line in the eth3 entry. Of course you can only have one default gateway in your setup. I missed this because I was also trying to add routes to networks behind the machine on the other end of eth3.
In the end, removing the gw line in the eth3 entry solved the problem.
My final /etc/networking/interfaces looks like this:
# The loopback network interface
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address xxx.yyy.zzz.mmm
netmask 255.255.255.0
gateway xxx.yyy.zzz.1
dns-nameservers xxx.yyy.zzz.aaa xxx.yyy.zzz.bbb
dns-search mydomain.nl
auto eth3
iface eth3 inet static
address 192.168.4.1
netmask 255.255.255.0
post-up /sbin/route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.4.250
post-up /sbin/route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.4.250
post-up /sbin/route add -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.4.250
post-down /sbin/route del -net 192.168.1.0 netmask 255.255.255.0
post-down /sbin/route del -net 192.168.2.0 netmask 255.255.255.0
post-down /sbin/route del -net 192.168.3.0 netmask 255.255.255.0
Update 2013-08-19: Removed network entries as per Ville’s suggestion.