Linux – How to Set Up Two Default Routes

TL;DR

How to set up two default routes in linux (corresponding to two interfaces), such that incoming traffic from both interfaces is accepted.

Scenario

I have a host having two NICs eth0 and eth1. Its connected to two networks, say:

• Dev: eth0, IP: 1.1.1.4, Netmask: 255.255.255.0, Gateway: 1.1.1.1

• Dev: eth1, IP: 2.2.2.4, Netmask: 255.255.255.0, Gateway: 2.2.2.1

(Both networks are connected to the internet.)

The routes are (ip route show):

default via 1.1.1.1 dev eth0 metric 1
default via 2.2.2.1 dev eth1 metric 2
1.1.1.0/24 dev eth0 src 1.1.1.4
2.2.2.0/24 dev eth1 src 2.2.2.4

(Lower metric means higher priority.)

Now when someone pings 2.2.2.4 from some external host (say 4.4.4.4), there is no reply.

The output of tcpdump -i eth1 on my host is:

22:41:27.431539 IP 4.4.4.4 > 2.2.2.4: ICMP echo request, id 8625, seq 4, length 64
22:41:28.439492 IP 4.4.4.4 > 2.2.2.4: ICMP echo request, id 8625, seq 5, length 64
22:41:29.447666 IP 4.4.4.4 > 2.2.2.4: ICMP echo request, id 8625, seq 6, length 64
22:41:30.455528 IP 4.4.4.4 > 2.2.2.4: ICMP echo request, id 8625, seq 7, length 64

After some research, it turns out that when linux network stack receives a packet from 4.4.4.4, it checks its routing table to see what interface should have been used for 4.4.4.4 (which is eth0 in the routing table). Now, since the packet came from eth1 interface, linux simply discards it (This policy is probably to prevent IP spoofing).

What I want is is:

• Outgoing traffic (locally originated) should use eth0 interface by default.

• Incoming traffic should be accepted from both interfaces. (So 4.4.4.4 should be able to ping both 1.1.1.4 and 2.2.2.4)