Running a VOIP/SIP homelab with 3CX (free) PBX

2020-12-05 15:25:00

Just yesterday, a lucrative dumpster dive netted me two brandnew IP desk phones, very spiffy Grandstream GPX2130 models. Because studying for my upcoming Ansible exam isn't much fun (OMG two weeks!!), procrastination struck!

Let's add VOIP to my simulated company Broehaha in my homelab!

Until this weekend I had zero experience with VOIP, SIP and the likes beyond using Cisco phones as an end-user. I'd heard plenty of colleagues talk about Asterisk and I remember hacking an Asterisk server in the PWK labs at Offensive Security, but that's about as far as my exposure went. 

Wanting to save time and to simulate an actual company, I quickly gave up on both Asterisk and FreeSwitch. As the meme goes: "Ain't nobody got time fo' that!"

A little search further led me to 3CX, a commercial PBX solution that provides a free edition for (very limited) small environments. They offer a Debian-based soft-appliance that you can deploy from ISO anywhere you like.

So:

• I added a new vSwitch and port group to VMware ESXi, dedicated to VOIP.
• In it I added a 1CPU, 512MB RAM VM for 3CX.
• I added one of the free NICs of my Dell R710 to the vSwitch.
• One of the GXP2130s was hooked up directly to said NIC. Later I may add a switch and the other phone. 
• Later on I deployed another VM in the VOIP subnet, using Vagrant, to make a quick firmware upgrade (web) server that doubles as sniffing station for debugging. 

Last night I spent from 2200-0100 mucking around with 3CX because no matter what I tried, the GXP2130 would not show up on the admin UI. The phone's in the network just fine and could also talk to 3CX, but there were a few steps missing.

Continuing this morning, I used tcpdump and other tools to ascertain that:

• Yes the phone was getting all needed info through DHCP.
• The phone can talk to the 3CX host just fine. 
• Running a Python web server on my sniffer-host, shows the phone succeeded in pulling a firmware image from there. But it wasn't upgrading, or doing anything with it. 
• The HTTP response codes from the 3CX web server kept saying "403 forbidden" on any file that the Grandstream phone tried to get. 
• The GS phone was doing SIP SUBSCRIBE calls on multicast, but 3CX refused to pick'm up. 

After lunch, things fell into place :)

1. The phone's firmware was too old to PNP with 3CX. 
2. Upgrading from 1.0.7.25 to 1.0.11.16 failed because the gap was too large. 

So... I upgrade the phone's firmware in four steps, using an on-prem update server. Then, after resetting the phone to factory defaults it showed up just fine and I could add it to one of my extensions!

The cool part is that 3CX comes with a web UI for end-users, that also works with their browser extension for Chrome or Edge. Now I can simulate a working-from-home situation, with one user on a Windows 10 VM calling the "reception" on the Grandstream phone. Or vice versa. 

kilala.nl tags: work, sysadmin, homelab,

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