Asterisk + Gafachi + T.38 = Finally Working

I thought I’d share what I learned about configuring Asterisk 1.8 to work with Gafachi inbound T.38 faxing. It took many days of trial and error but I finally have a configuration that works.

Here are the ingredients:

  • Asterisk 1.8 (using SVN-branch-1.8-r310834)
  • spandsp 0.0.5
  • res_fax and res_fax_spandsp

You must use spandsp-0.0.5, since spandsp-0.0.6 causes a segmentation fault in Asterisk. Also remember to either delete from your modules directory or put a noload statement in modules.conf, otherwise Asterisk will crash on startup.

It is not necessary to place t38pt_udptl=yes in the [general] section of sip.conf. It will suffice to place it in your [gafachi1?] peer. Also place insecure=invite and reinvite=yes in your [gafachi1?] peer. As of this version of Asterisk, the maxdatagram= setting is completely ignored, or at least I can find no way to make Asterisk honor the setting in the SIP INVITEs. sip show peer gafachi1? shows the specified maxdatagram but the actual SIP SDP message does not honor that setting.

I have my Gafachi fax DID going to ReceiveFax(filename) in my dialplan and it’s working correctly.

Also be aware that some advanced home firewalls randomly reassign outgoing ports. This was breaking my udptl port range implementation, and I needed to turn on static port mapping for outgoing NAT in my firewall to get T.38 faxing to work.

It took about 4 days of tweaking, recompiling, restarting, and reloading to find a configuration that works. Gafachi has no instructions specifically for Asterisk 1.8 on their website, but fortunately the insecure=invite seemed to take care of the authentication issue.

Aside from Asterisk module and peer configuration, my dialplan calls which needed tweaking for my system. Because Asterisk runs as -U asterisk I had to preface the mutt command with sudo -u asterisk, otherwise mutt failed to send the received fax in an email. Of course that makes it necessary to edit the sudoers command with visudo to allow asterisk to run sudo mutt passwordless.

Note: This uses Asterisk as the T.38 endpoint. Heaven help he who has a T.38-capable SIP ATA connected to a physical fax machine.


Author: PhilRW

software engineer, pianist, polymath

4 thoughts on “Asterisk + Gafachi + T.38 = Finally Working”

  1. Hi Phil
    Thanks for posting this information. Have you tested it for faxout yet?
    I am trying the same configuration but can’t get it working for faxout using a spool file. Any help will be much appreciated.


  2. I have not been able to get it working for outgoing faxes. I went so far as to capture a protocol trace for outgoing T.38, but I still cannot figure out what’s (not) going on.


  3. Hi Phil

    I got it working eventually with asterisk18- and the free fax digium module using sendfax with the z option. Very relieved!



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