[SR-Users] rport parameter in VIA header in uac remote registration

Jetse Verschuren j.verschuren at vanderventions.nl
Wed Nov 11 08:38:04 CET 2020


I'm currently trying to connect Kamailio, running in docker, to my
FritzBox. The FritzBox is the modem provided by my ISP, with a builtin voip
server which I can use to dial out (basically a PSTN gateway). In my setup
I have a custom router/firewall/NAT between the FritzBox and the server.
When I connect LinPhone to the FritzBox, from behind the NAT, it works
fine. But when I use the Kamailio uac remote registration,  I don't get a
response. After some investigation with wireshark, I found that the the SIP
response from the FritzBox comes on the wrong port, and gets rejected by
the NAT. The request comes from port 1025, but the response is sent to port
5060. I compared the requests from LinPhone and Kamailio, and combined with
some research, found it probably was the rport option in the VIA header.
>From what I read, it basically signals the uas to use the source port to
reply to, instead of the port indicated in the request, which would be
necessary to effectively go through a NAT.
After reading a lot of documentation (and even parts of the source), I
wasn't able to find an option to enable this option. Is there some option I
completely missed, or is it not possible? Or am I completely wrong in my

Thanks in advance,
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