Enable IP forwarding via sysctl - here the Linux-example - sysctl.conf:
# Controls IP packet forwarding
net.ipv4.ip_forward = 1
Disable source route verification
# Controls source route verification
net.ipv4.conf.default.rp_filter = 0
The enabling of IP forwarding is self explainitory. Source route verification
must be turned off in order for packets that reach the interface to
not dropped by the kernel. Route verification is the practice of discarding
packets received on an interface which does not expect to those handle
traffic from the given source address.
