Using iproute2 you can do something like this:
echo "1 admin" >> /etc/iproute2/rt_tables echo "2 users" >> /etc/iproute2/rt_tables ip rule add from 192.168.122.40/32 dev eth0 table admin ip rule add from 192.168.123.41/32 dev eth1 table users ip route add default via 192.168.122.1 dev eth0 table admin ip route add default via 192.168.123.1 dev eth1 table users