Routing traffic out over the interface the connection was received on in a setup where multiple NICs are present.

gistfile1.txt

# If you're using multiple NICs/IPs on AWS, traffic might be sent out over one of the 2 NICs only.
# And the other IP will not be usable.
#
# /etc/network/interfaces

iface eth0 inet dhcp
  post-up ip route add default via <ip-of-default-gateway-of-eth0> dev eth0 tab 1
  post-up ip rule add from <ip-of-eth0> tab 1
  pre-down ip rule del from <ip-of-eth0> tab 1
  pre-down ip route del default via <ip-of-default-gateway-of-eth0> dev eth0 tab 1

iface eth1 inet dhcp
  post-up ip route add default via <ip-of-default-gateway-of-eth1> dev eth1 tab 2
  post-up ip rule add from <ip-of-eth1> tab 2
  pre-down ip rule del from <ip-of-eth1> tab 2
  pre-down ip route del default via <ip-of-default-gateway-of-eth1> dev eth1 tab 2