How to route TCP/IP responses through a different interface?

Question

I have two machines each with two valid network interfaces, an Ethernet interface eth0 and a tun/tap interface gr0. The goal is to start a TCP connecti

Answer 1

You may be interested in enabling logging of martian packets net.ipv4.conf.all.log_martians, and disable reverse path filtering net.ipv4.conf..rp_filter on the affected interfaces.

This sysctl vars are accesible via the sysctl utility and/or the /proc filesystem.