Asterisk behind NAT, UDPTL SDP contains Internal IP

Hi there,

I’m running Asterisk 13.6.0 (chan_sip), behind a NAT.

I’m trying to configure T.38 support, and it works, sort of, however the SDP Asterisk sends out when trying to RE-INVITE to UDPTL contains two c= lines, one with the external, and one with the internal IP.

I’ve set the following settings in sip.conf:

t38pt_udptl = yes
t38pt_usertpsource = yes

The SDP in the re-invite looks as follows:

o=root 1933471796 1933471797 IN IP4 <external ip>
s=Asterisk PBX 13.6.0
c=IN IP4 <external ip>
t=0 0
m=image 4404 udptl t38
c=IN IP4 <internal ip>

Is there some way to get rid of the c= line with the internal IP?

I suspect this is the reason why we’re seeing A LOT of errors receiving faxes when T.38 is enabled.

I’d suggest updating to the latest version of 13 first before trying to figure out what is going on, as the version you are using is quite old.