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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.

IMPORTANT! TURN can also be offered by [LiveKit](../element-call#livekit), in
which case you should not run coturn (unless you don't use LiveKit's built-in
TURN server).


# 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.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.example.com
```

This assumes you've already setup and started nginx (see [nginx](../nginx)).

Of course, when this certificate is renewed, coturn must be restarted!


# Configuration {#configuration}

Synapse's documentation gives a reasonable [default
config](https://element-hq.github.io/synapse/latest/setup/turn/coturn.html).

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.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.example.com/{fullchain,privkey}.pem .
chown turnserver:turnserver *.pem

# We should restart either coturn or LiveKit, they cannot run both!
systemctl restart coturn
#systemctl restart livekit-server

```

Run this automatically after every renewal by adding this line to
`/etc/letsencrypt/renewal/turn.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](../firewall).
