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TURN server
You need an TURN server to connect participants that are behind a NAT firewall. Because IPv6 doesn't really need TURN, and Chrome can get confused if it has to use TURN over IPv6, we'll stick to a strict IPv4-only configuration.
Also, because VoIP traffic is only UDP, we won't do TCP.
Installation
Installation is short:
apt install coturn
For sake of maintainability we'll move the only configuration file into its own directoy:
mkdir /etc/coturn
mv /etc/turnserver.conf /etc/coturn
We need to tell systemd to start it with the configuration file on the new place. Edit the service file with:
systemctl edit coturn
Contrary to what the comment suggests, only the parts you add will override
the content that's already there. We have to "clean" the ExecStart first,
before we assign a new line to it, so this is the bit we add:
[Service]
ExecStart=
ExecStart=/usr/bin/turnserver -c /etc/coturn/turnserver.conf --pidfile=/etc/coturn/run/turnserver.pid
Create the directory /etc/coturn/run and chgrp it to turnserver, so that
coturn can write its pid there: /run/turnserver.pid can't be written because
coturn doesn't run as root.
This prepares us for the next step: configuring the whole thing.
DNS and certificate
As stated before, we only use IPv4, so a CNAME to our machine that also does
IPv6 is a bad idea. Fix a new entry in DNS for TURN only, we'll use
turn.matrix.example.com here.
Make sure this entry only has an A record, no AAAA.
Get a certificate for this name:
certbot certonly --nginx -d turn.matrix.example.com
This assumes you've already setup and started nginx (see nginx).
Of course, when this certificate is renewed, coturn must be restarted!
Configuration
Synapse's documentation gives a reasonable default config.
We'll need a shared secret that Synapse can use to control coturn, so let's create that first:
pwgen -s 64 1
Now that we have this, we can configure our configuration file under
/etc/coturn/turnserver.conf.
# We don't need more than 10000 connections:
min-port=50000
max-port=60000
use-auth-secret
static-auth-secret=<previously created secret>
realm=turn.matrixdev.example.com
user-quota=12
total-quota=1200
# Of course: substitute correct IPv4 address:
listening-ip=185.206.232.60
# VoIP traffic is only UDP
no-tcp-relay
# coturn doesn't run as root, so the certificate has
# to be copied/chowned here.
cert=/etc/coturn/ssl/fullchain.pem
pkey=/etc/coturn/ssl/privkey.pem
denied-peer-ip=0.0.0.0-255.255.255.255
denied-peer-ip=127.0.0.0-0.255.255.255
denied-peer-ip=10.0.0.0-10.255.255.255
denied-peer-ip=172.16.0.0-172.31.255.255
denied-peer-ip=192.168.0.0-192.168.255.255
denied-peer-ip=100.64.0.0-100.127.255.255
denied-peer-ip=192.0.0.0-192.0.0.255
denied-peer-ip=169.254.0.0-169.254.255.255
denied-peer-ip=192.88.99.0-192.88.99.255
denied-peer-ip=198.18.0.0-198.19.255.255
denied-peer-ip=192.0.2.0-192.0.2.255
denied-peer-ip=198.51.100.0-198.51.100.255
denied-peer-ip=203.0.113.0-203.0.113.255
# We do only IPv4
allocation-default-address-family="ipv4"
# No weak TLS
no-tlsv1
no-tlsv1_1
All other options in the configuration file are either commented out, or defaults.
The certificate files reside under /etc/letsencrypt/live, but coturn doesn't
run as root, and can't read them. Therefore we create the directory
/etc/coturn/ssl where we copy the files to. This script should be run after
each certificate renewal:
#!/bin/bash
# This script is hooked after a renewal of the certificate, so
# that it's copied and chowned and made readable by coturn:
cd /etc/coturn/ssl
cp /etc/letsencrypt/live/turn.matrixdev.example.com/{fullchain,privkey}.pem .
chown turnserver:turnserver *.pem
systemctl restart coturn
Run this automatically after every renewal by adding this line to
/etc/letsencrypt/renewal/turn.matrixdev.example.com.conf:
renew_hook = /etc/coturn/fixssl
Yes, it's a bit primitive and could (should?) be polished. But for now: it works.
Be sure to open the correct ports in the firewall.