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Overview

This document explains how to enable VoIP relaying on your Home Server with TURN.

The synapse Matrix Home Server supports integration with TURN server via the TURN server REST API. This allows the Home Server to generate credentials that are valid for use on the TURN server through the use of a secret shared between the Home Server and the TURN server.

The following sections describe how to install coturn (which implements the TURN REST API) and integrate it with synapse.

Requirements

For TURN relaying with coturn to work, it must be hosted on a server/endpoint with a public IP.

Hosting TURN behind a NAT (even with appropriate port forwarding) is known to cause issues and to often not work.

`coturn` Setup

Initial installation

The TURN daemon coturn is available from a variety of sources such as native package managers, or installation from source.

Debian installation

# apt install coturn

Source installation

Download the latest release from github. Unpack it and cd into the directory.
Configure it:
```
./configure
```
You may need to install libevent2: if so, you should do so in the way recommended by your operating system. You can ignore warnings about lack of database support: a database is unnecessary for this purpose.
Build and install it:
```
make
make install
```

Configuration

Create or edit the config file in /etc/turnserver.conf. The relevant lines, with example values, are:
```
use-auth-secret
static-auth-secret=[your secret key here]
realm=turn.myserver.org
```
See turnserver.conf for explanations of the options. One way to generate the static-auth-secret is with pwgen:
```
pwgen -s 64 1
```

Consider your security settings. TURN lets users request a relay which will connect to arbitrary IP addresses and ports. The following configuration is suggested as a minimum starting point:

# VoIP traffic is all UDP. There is no reason to let users connect to arbitrary TCP endpoints via the relay.
no-tcp-relay

# don't let the relay ever try to connect to private IP address ranges within your network (if any)
# given the turn server is likely behind your firewall, remember to include any privileged public IPs too.
denied-peer-ip=10.0.0.0-10.255.255.255
denied-peer-ip=192.168.0.0-192.168.255.255
denied-peer-ip=172.16.0.0-172.31.255.255

# special case the turn server itself so that client->TURN->TURN->client flows work
allowed-peer-ip=10.0.0.1

# consider whether you want to limit the quota of relayed streams per user (or total) to avoid risk of DoS.
user-quota=12 # 4 streams per video call, so 12 streams = 3 simultaneous relayed calls per user.
total-quota=1200

Ideally coturn should refuse to relay traffic which isn't SRTP; see https://github.com/matrix-org/synapse/issues/2009

Ensure your firewall allows traffic into the TURN server on the ports you've configured it to listen on (remember to allow both TCP and UDP TURN traffic)
If you've configured coturn to support TLS/DTLS, generate or import your private key and certificate.
Start the turn server:
```
 bin/turnserver -o
```

synapse Setup

Your home server configuration file needs the following extra keys:

"turn_uris": This needs to be a yaml list of public-facing URIs for your TURN server to be given out to your clients. Add separate entries for each transport your TURN server supports.
"turn_shared_secret": This is the secret shared between your Home server and your TURN server, so you should set it to the same string you used in turnserver.conf.
"turn_user_lifetime": This is the amount of time credentials generated by your Home Server are valid for (in milliseconds). Shorter times offer less potential for abuse at the expense of increased traffic between web clients and your home server to refresh credentials. The TURN REST API specification recommends one day (86400000).
"turn_allow_guests": Whether to allow guest users to use the TURN server. This is enabled by default, as otherwise VoIP will not work reliably for guests. However, it does introduce a security risk as it lets guests connect to arbitrary endpoints without having gone through a CAPTCHA or similar to register a real account.

As an example, here is the relevant section of the config file for matrix.org:

turn_uris: [ "turn:turn.matrix.org:3478?transport=udp", "turn:turn.matrix.org:3478?transport=tcp" ]
turn_shared_secret: "n0t4ctuAllymatr1Xd0TorgSshar3d5ecret4obvIousreAsons"
turn_user_lifetime: 86400000
turn_allow_guests: True

After updating the homeserver configuration, you must restart synapse:

cd /where/you/run/synapse
./synctl restart

..and your Home Server now supports VoIP relaying!

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