[OpenSIPS-Users] Change contact header and route OPTIONS back to client

[Jordan Kirby]

Hi,

I have OpenSIPS 2.1 installed with the purpose of handling client side
NAT for Internet facing phones for a PBX that is incapable of handling
NAT. I also have RTPEngine running.

The SIP proxying works fine when the client is not behind a firewall
(eg on 3G) but the PBX we're using, requires it get a reply to a SIP
OPTIONS request it initiates to the Contact header from the initial
REGISTER prior to allowing calls which presents a problem when the end
client is behind a firewall.

For example:

1. The softphone sends a REGISTER to the Internet facing IP of the
OpenSIPS server.
2. OpenSIPs handles the NAT and sends the REGISTER to the PBX internally.
3. PBX replies with 200 OK, via OpenSIPs to the client.
The above 3 steps work fine.

PBX then initiates a OPTIONS request to the IP address of the client
it saw in the Contact header, which is the public, Internet facing IP
of the client. This never gets to the client due to their firewall.

So, what I think I'm looking for is B2BUA type functionality for the
REGISTER request to put the OpenSIPs IP address in the Contact header
to allow the PBX to send its OPTIONS requests via OpenSIPs back to the
client via the established NAT'd path.

Hope that makes sense, any ideas on how to achieve this would be welcome.

Thanks

Jordan