[Serusers] proxy + nat in the same box

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 | | | 192.168.0.90 |______| IPpA IPpB |______| 10.0.0.70 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