I have two NICs on a fresh install of Ubuntu 10.04 LTS. Both are config'd be dhcp. Both cards are on the same lan with same gateway. everything about the cards is the same except the IPs they get, which is what I want.
What I'm trying to achieve is having both NICs operational at the same time.
Currently when the server boots, it activates both NICs, but the server can't resolve any domains. If I ifdown one of them and bring it back up, the server is able to resolve domains, but the NIC I bring up won't respond to any traffic.
So basically both NICs should be on different subnets. The kernel will drop packets otherwise in the scenario I outlined.
A stopgap solution is to disable the dropping of packets with
# for i in /proc/sys/net/ipv4/conf/*/rp_filter ;
do echo 0 > $i done
More info here: http://lartc.org/howto/lartc.kernel.html