[Serusers] proxy + nat in the same box

Klaus Darilion klaus.mailinglists at pernau.at
Fri Feb 27 11:41:31 CET 2004

Eduard San Anselmo Mateu wrote:
> 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.

Why is ser listening on the private IP? Bind ser to the public IP (IPpA) 
and configure the Clients to use IPpA as proxy address.


> 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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