T54S & T54W, suddenly STUN via UDP no longer works

[suthern101]

Over the weekend our ISP had some hardware failures. Since then, the phones were stuck in a very unstable loop where they would register, then unregister, over and over.

Setup:
- 3CX hosted on google cloud
- Phones set to use STUN.

I found that by setting the SIP Server: Transport to TCP on the phones made them register and stay registered. Prior to this they were all provisioned automatically by 3CX to use UDP.

I did a packet capture on a phone still set to UDP, and saw this interesting bit of info:

• Query for SRV _stun._udp.xxxxx.west.3cx.us
• Answer: no such name

(xxxxx.west.3cx.us = our FQDN)

So I used a few other DNS query tools to see if I can get anything out of xxxxx.west.3cx.us, and all I can get is an A record. No SRV records. I then tried doing queries to the subdomains mentioned:

• _udp.xxxxx.west.3cx.us
• _stun._udp.xxxxx.west.3cx.us
• udp.xxxxx.west.3cx.us
• stun.udp.xxxxx.west.3cx.us
• _tcp.xxxxx.west.3cx.us
• _stun._tcp.xxxxx.west.3cx.us
• ....etc

and have had no luck getting SRV records.

I'm glad that using TCP as a transport protocol works, but am concerned that if the phones provision again, they'll swap back to UDP and we'll be in for more manual configuration. Is there a setting somewhere in 3CX to say "set TCP" when provisioning?

Also, has there been a recent change to STUN servers or the DNS part of that setup? I don't know exactly how the STUN DNS resolution is supposed to work, but it seems odd that the SRV records would be missing.

Am I chasing the wrong horse here?

 

[N_G]

Yes you are chasing the wrong horse. The phones must not use STUN. You must deploy an SBC.

Now if you wait a little bit you can use one of those 54w as a router phone with integrated SBC if you have a small install with less then 10 phones. If you can not wait or have a larger network, deploy an SBC now.

 

[suthern101]

LOL, thanks @Nick Galea! We're only running 7 phones, so pretty lite. I just put an Debian SBC into a Hyper-V host and am connecting a few of the phones to it to see how it rolls.