asymmetric static routing linux

Question

I have 3 linux boxes in a triangle. All subnets are /24.

A has two interfaces: 1.1.1.1, 2.2.2.1

B has two interfaces: 1.1.1.2, 3.3.3.2

C has two interfaces: 3.3.3.1, 2.2.2.2

No routing protocol is running so I am adding a static route on each node.

When pinging A(1.1.1.1)-->C(3.3.3.1), all goes well when A has a static route to 3.3.3.0 via B and C has a static route to 1.1.1.0 via B. So symmetric routing works fine.

Problem happens when I set the return path on C via 2.2.2.1. The ICMP request reaches interface 3.3.3.1 (I see it in tcpdump) but C simply does nothing and no ICMP reply gets sent over any interface.

Any ideas about what is happening?

Answer 1

Your distribution may have enabled the "reverse path filter", which prevents asymmetric routing. On each machine, do

sysctl -a | grep '\.rp_filter'

and check that it is disabled for all interfaces.