Routing outgoing packets over multiple interfaces?

Question

I have two network interfaces (eth0 and eth1) with separate IP addresses on the same subnet.  All outgoing traffic uses eth0 regardless of the interface the incoming traffic came in on.
I assume the outgoing packets still have the correct source IP address (not always eth0's), and I'd like the packets to go out on the interface with the corresponding IP address.
I think I have half the solution to my problem:
http://www.novell.com/support/viewConte … Id=7000318
The other half is that my IPs are dynamic, so ddclient could change my IPs and then the routing would be invalid.
Last edited by MindlessXD (2009-02-10 07:06:16)

Answer 1

Setup custom route tables to be used depending on the iptables conntrack marks below
ip route flush table 1
ip rule del fwmark 101 table 1
ip route add table 1 default via <ETH0 IP ADDRESS>
ip rule add fwmark 101 table 1
ip route flush table 2
ip rule del fwmark 102 table 2
ip route add table 2 default via <ETH1 IP ADDRESS>
ip rule add fwmark 102 table 2
I'm not 100% sure if you can add a route via the interfaces IP address. This code has been modified from a box using 2 different ISP's so they have different upstream routers. You might need to replace the 'via' parts with 'src'
# Ensure traffic in one interface goes back out the same interface
iptables -t mangle -F PREROUTING
iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
iptables -t mangle -A PREROUTING -m mark ! --mark 0 -j ACCEPT
iptables -t mangle -A PREROUTING -i eth0 -m state --state NEW -j MARK --set-mark 101
iptables -t mangle -A PREROUTING -i eth1 -m state --state NEW -j MARK --set-mark 102