NAT Router Traversal

[stevenutt]

Guys, I have a NAT traversal issue that I need to solve using VB.Net and I can't do it with the .Net socket class. Here is an overview.... Remote client sends out a UDP packet from its internal IP on port 1234. The NAT router maps it to the external IP plus a mapped port (let's say 9999) My UDP listening software picks up the packet and I am able to find out the IP address and mapped port number (source port). The problem with NAT is that if I do not send a packet back from the same source port - it will be rejected (symmetric NAT). So, in my VB/net app, I connect and send back a response as follows.... udpClient.Connect(RemoteIP, SourcePort) 'code to send the packet goes here I use Ethereal to view the packet going out and the destination port is 9999 as expected, but the source port of the response gets mapped to a different port number. So, even though the packet hits the correct remote router port, it gets rejected because it came from a different source port. My question is, what hardware device is mapping the source port of my packet and how do I stop this from happening - or work around it ?