I am trying to use latest Asterisk 15 (mainly configured with freepbx) for FAX T.38 passthrough. This is basically working.
But in a multihomed environment I cannot get the UDPTL packets on a pjsip trunk side to be routed with a correct source IP. I have 2 trunks to SBC1 and SBC2 (of the provider) which are connected over interface eth3 and eth4. A default route is configured to another interface eth0 (local LAN with ip phones). This works fine for calls, which are estabilshed over one of the 2 trunks.
But after the Re-INVITEs from T.38 to change to UDPTL transport, all outgoing packets over the trunks use the default route and IP address of eth0.
It seems, that asterisk/fax cannot set a source IP or transport for those outgoing UDPTL packets, as it would be done for RTP. The “trick” in my case is, that the source IP and interface shall be used as signalled in SIP/SDP and route (policy based routing) such that UDPTL is using the same asterisk IP address as the SIP signalling. As said, for the RTP traffic, this is properly working (using bind_rtp_to_media_address=yes or individual pjsip transport per interface). The media endpoints on the service provider SBCs are shared for all trunks and a normal route cannot be done, as it shall be chosen based on the source IPs.
Can this be solved by some udptl or pjsip configuration?