Hi,

We are building a service where we need to detect NAT when the clients register to our server. We are struggling in analyzing NAT status of some clients which modify their IP addresses/ports in the headers according to the value of "received" parameter sent during "401 Unauthorized" response.

Here's the flow:

Client->Server
REGISTER sip:...
Via: SIP/2.0/TLS 192.168.0.1:41157;rport;branch=z9hG4bKPj30093e5d-550d-4d4c-a9a2-22c3bd1cda7e;alias
Contact: <sip:user@192.168.0.1:42251;transport=TLS;ob>
...
Server->Client
SIP/2.0 401 Unauthorized
Via: SIP/2.0/TLS 192.168.0.1:41157;rport;branch=z9hG4bKPj30093e5d-550d-4d4c-a9a2-22c3bd1cda7e;alias;received=1.2.3.4
WWW-Authenticate: ...
...

Client->Server
REGISTER sip:...
Via: SIP/2.0/TLS 1.2.3.4:6201;rport;branch=z9hG4bKPj30093e5d-550d-4d4c-a9a2-22c3bd1cda7e;alias
Contact: <sip:user@ 1.2.3.4:6201;transport=TLS;ob>
Authorization: ...
...

By the time the client is authenticated, there is no way to detect whether the request was coming from a natted device or not by just analysing the Via or Contact headers.

Thanks in advance.