Asterisk behind NAT, UDPTL SDP contains Internal IP

Asterisk Asterisk Fax
1

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:

v=0
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>
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxDatagram:1400
a=T38FaxUdpEC:t38UDPFEC

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.

2

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.