[Serusers] proxy + nat in the same box

Eduard San Anselmo Mateu esananselmo at albasoft.com
Fri Feb 27 10:50:01 CET 2004

Hello everyone.

I have read lots of posts to this list and whitepapers about NAT traversal with
SIP, but all of them refer to the same problem: the endpoint is behind a nat box
and SER is after that box. My question is regarding the situation in which SER
resides at the same box than NAT. Here's a diagram (sorry for that awful
ascii-art, never been a good artist):
    _____        ______                      ______         _____
   |     |      |      |                    |      |       |     |
   |  A  |------| FW   |-----  INTERNET ----|      |-------|  B  |
   |_____|      | NAT  |                    | NAT  |       |_____|
                | SER  |                    |      |    |______| IPpA          IPpB |______|

IPpA and IPpB are the public IP addresses of A and B, respectively. SER is
listening on its private IP. Of course, the SER box also has an RTP proxy.

I think there's no problem with RTP, because nathelper's exported functions can
deal with the problem of changing SDP's fields. My problem has to deal with SIP,
specially when B invites A into a conversation, and A has to give an answer. In
that case, when SER has to repeat that answer to B, what will it write in the
Record-Route header field? It should write IPpA so that B can get to SER in the
future, but if SER's listening on its private IP, I'm afraid it will write that
public IP in Record-Route.

Can anybody please point me in the right direction, or at least address me to a
place where all this may be explained (not the RFC, please!)?.

Thanks in advance.

Eduard San Anselmo

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