Solved SIP Trunk: Auto Discovery checkbox state doesn't persist -- and what does it do?

[Jon]

Hello,

Yesterday I upgraded from 3CX 15.5 to 16.0.5.619.

My SIP provider told me that starting in version 16, we need to tick the Auto Discovery checkbox in SIP Trunks > Edit > General:


Although the SIP provider uses the standard port 5060, and that's what was entered, apparently using Auto Discovery is some "black magic" that prevents trunk registration failures.

1. Does the Auto Discovery checkbox do anything other than what is documented in the manual, i.e., simply detect which port to use?

Enter a name for this VoIP provider account. Crosscheck the pre-filled “Registrar/Server/Gateway Hostname or IP” and optional “Outbound Proxy” with your VoIP provider info. You can use the “Auto Discovery” option to automatically detect the appropriate ports to use.

2. Bug Report: When I tried to tick the Auto Discovery checkbox, it would appear ticked and the Port field would be hidden, as expected. But then when I pressed OK, the change did not persist, i.e., when I went back into edit the trunk, the checkbox was back to the unticked state. I tried repeatedly without any luck. The next day, I tried again and it inexplicably started working as expected. (I am using Chrome.) I talked to my SIP provider's technical support and they told me that this UI bug is common.

Thank you.

 

[cobaltit]

Nope, no secret handshake or decoder ring here. It does what the manual says it does. If you want the technical nitty gritty it's documented here:

https://www.3cx.com/docs/sip-trunk-dns-record-resolution/

I suppose you could always ask your provider what it does as well if they are the ones telling you to check it.

As far as the 'bug report' I've not seen this issue nor do I recall seeing it reported. I'm guessing it was probably more of a browser cache issue than anything.

 

[Jon]

Thanks for the quick reply.

1. The link you sent indeed answers my question, and the answer is that Auto Discover does more than just automatically detect which port to use! It also enables or disables DNS lookup of SRV records, which perfectly explains why, when I initially had Auto Discover disabled, the trunk registration was failing!

From your link:

In the “General” tab next to the Registrar and Outbound Proxy fields there is the option “Auto Discovery”. When this option is disabled, the port must be manually specified. In this configuration 3CX will not attempt to look for NAPTR or SRV entries, it will only check for A or AAAA records.

Follow up action: I request that the manual at this link be updated to mention that important detail, because currently it only mentions that Auto Discovery controls automatic detection of ports. Even better would be if they rename the checkbox to better describe what it actually does and/or add a tooltip in the UI that explains it, for those of us who don't enjoy RTFM. ;-)

2.
As for the UI bug, next time I see this I will try closing the browser and/or clearing the cache. But I am using the recommended browser (Chrome), so I don't expect to see issues like this...

 

[cobaltit]

Well they are technically correct. I mean the only way for it to determine the protocol/port is via DNS lookups, it's basic SIP knowledge. Understand that the 3CX focus is for customers to stay with-in the supported environment, which includes using supported providers. And in doing so, you have templates where things like this are already configured for you.

 

[Jon]

Wanting automatic discovery of ports may require 3CX to lookup SRV records, but wanting 3CX to lookup SRV records does not mean you want automatic discovery of ports.

 

[cobaltit]

So my loose understanding of the RFC says 3CX is doing exactly what it should. If the SRV records don't have the ports listed in them, then the client (3CX) should use the default ports for that protocol.

https://tools.ietf.org/html/rfc3263