Net game, NAT-on-NAT action?

[scoorf]

Well, I added the ip of my external net on the internal machine .. this would allow me to change the source address to the appropriate one, but whenever the packets came back, my NAT box(FreeBSD) would naturally get confused and drop them.

Does anyone know of a packet filter that can edit individual packets on the fly? "tcpdump"ing the packets shows the ip in one of the udp packets, filtering that out and replacing it with the actual/real value would create the desired results..

Perhaps a VPN connection between affected parties could work out, but it's a bit troublesome to take it that far..