STUN steps

Client queries a STUN server for a shared secret username and password
Server responds with a unique username/password combination for this client
Client sends a binding request using this username/password to the server via UDP
Server copies the source IP and port number into a binding response, and sends this response back to the client
Client compares the IP address and port number received from the server with its local IP address and port number. If they do not match, then the client is behind some type of NAT.
- A full flowchart to find each of the potential situations is shown as Figure 14 “Flow Chart: Determining NAT type” in [Keyes 2004].

Lawrence Keyes, “A Low Density Voice Over IP Gateway”, Master of Science in Information Technology thesis, Rochester Institute of Technology, B. Thomas Golisano College of Computing and Information Sciences, May 17, 2004 http://mxdesign.net/VoIP/onfolio-files/Low%20Density%20Voice%20Over%20IP%20Gateway.pdf Links to an external site.

[slide374] And here are the steps that basically STUN does.