ping source is firewall ? set sourceIP to vpn IP address
The problem can be seen easily in Puzzle 2, VPN Demo.
From firewall2, ping firewall0. The ping request tunnels
to firewall0 with a source address of 0.0.0.0, and so
the reply returns untunneled and drops at the default gateway.
This patch probably needs to be tweaked a bit in case it
covers too many situations, but in general something like
this is needed.