Net game, NAT-on-NAT action?

[scoorf]

Hi folks,

recently I've begun expanding my SE experiences to networking games, and have run into some issues;

I've found that if you're not on a NAT'ed connection, connecting to a NAT'ed server works fine. However, behind NAT to not-NAT, you run into trouble. NAT-on-NAT doesn't work either.

Tracing this a bit more lowlevel, it seems the reciepient IP is included in the UDP packet(for some reason), and that doesn't change even when you change your 'source ip' in the connection settings.

What this leads to is when one machine says 'Hi' to the other, the other tries to reply to the source id in the udp packet, which would be the behind-the-NAT ip(and thus internet-invalid).

Any ideas for how to solve this? Does the 'Source IP' really count for anything?

[kshcsuf]

I noticed the exact same thing just a few days ago.

When the natted client attempts to connect, there is a source IP address list which allows you to choose your interface. Well, if your NATting is not done on your PC (I have ipfw running on my Sparc at home), you are SOL. Hand-typing in an IP address instead of selecting one from the list does not rewrite the address of the IP packet appropriately.

I hope that Shrapnel/Malfador addresses this, or uses DirectPlay 8 or something nice.

[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..