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?