More NAT (05 Nov 2002)

I now have a working way of getting data back thru NATs: ICMP. Echo Requests open a tunnel back through the NAT so, with a server assisting, NATed hosts can setup bidirectional links. Unfortunately, the NAT mangles the ID number which the other host needs in order to send replies.

It so happens that the NAT at Imperial doesn't actually check the source address of the reply is correct, only the ID, so it would be easy to find the ID. But I cannot believe this is generally true so the only way to get the ID would be to use the fact that the NAT assigns IDs incrementally and try to hit the correct ID. Eww!