Dropped IP traffic with a multihomed server

Assuming you don't have any iptables rules that would prevent this, you need to disable the return path filtering. You can do this using:

# echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter

You can also use a particular interface name instead of all and there is also a default, which would affect newly created interfaces.

From:

reverse path filter; it is a check to see if, for a packet arriving on an interface, a packet sent to the original packet's source address would be sent out on that interface; if not, the arriving packet is dropped. it can be considered an attempt at detecting packets with spoofed source addresses.