Twilio SRV lookup while forwarding to SIP URI

Question

I am trying to forward calls to my sip device from my twilio number. However this is currently not working as expected because Twilio does not like SIP providers using a load balanced url. In my case the sip uri ends with @in.callcentric.com which in turn sends requests to three servers. If I directly use one of the servers, i.e. @alpha11.callcentric.com:5070 instead of @in.callcentric.com it works. However this is not ideal imo.

I have found a node module that does SRV lookups. Is this something I could use with twilio functions to solve the problem?

Is there a way to force twilio to lookup the SRV record and automatically use that to forward the calls?

Relevant link

Using the Node module to do the lookup and return the real server address to Twilio as TwiML sounds like a good idea. Have you tried it yourself? — philnash, Jan 09 '18 at 03:08
No, I haven't tried so far. Not sure how loading of modules would work in twilio functions. — charsi, Jan 09 '18 at 03:14
Check out the instructions here: https://www.twilio.com/blog/2017/09/npm-support-for-twilio-functions.html — philnash, Jan 09 '18 at 03:15
SRV lookups seems to be an odd feature to miss out on outbound calls as twilio actually uses it on its own SIP domains. Is there an issue related to this I can keep track of? — charsi, Jan 09 '18 at 03:16
There is a lot of internal discussion, but nothing I can publicly share right now. — philnash, Jan 09 '18 at 03:23

score 1 · Accepted Answer · edited Feb 01 '18 at 06:42

Using NPM's inbuilt dns module I was able to get things working. This is how my function looks now. Seems to be working okay.

const dns = require('dns');

let sipUri = '1777xxxxxxxxxx@in.callcentric.com';
let protocol = 'udp';
let region = 'us2' ;

exports.handler = function(context, event, callback) {    
  var user =   sipUri.split('@')[0];  
  var host =   sipUri.split('@')[1];  

  // generate the TwiML to tell Twilio how to forward this call
  let twiml = new Twilio.twiml.VoiceResponse();

  const dial = twiml.dial();

  dns.resolveSrv('_sip._'+protocol+'.'+host, (err, addresses) => {
    var resolvedhost = addresses[0].name+':'+addresses[0].port;
    dial.sip('sip:'+user+'@'+resolvedhost+';region='+region);
    console.log(twiml.toString());
    // return the TwiML
    callback(null, twiml);
  });
};

This manually queries the hostname for SRV records and then uses the first result returned. Does not take weight and priorities into account.

Gist

Twilio SRV lookup while forwarding to SIP URI

1 Answers1