How to enable VoIP relaying on your Home Server with TURN

Overview
--------
The synapse Matrix Home Server supports integration with TURN server via the
TURN server REST API
(http://tools.ietf.org/html/draft-uberti-behave-turn-rest-00). 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.

This document describes how to install coturn
(https://github.com/coturn/coturn) which also supports the TURN REST API,
and integrate it with synapse.

coturn Setup
============

You may be able to setup coturn via your package manager,  or set it up manually using the usual ``configure, make, make install`` process.  

 1. Check out coturn::
 
      git clone https://github.com/coturn/coturn.git coturn
      cd coturn

 2. 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.

 3. Build and install it::
 
      make
      make install

 4. Create or edit the config file in ``/etc/turnserver.conf``. The relevant
    lines, with example values, are::

      lt-cred-mech
      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

 5. Consider your security settings.  TURN lets users request a relay
    which will connect to arbitrary IP addresses and ports.  At the least
    we recommend:

       # 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

 6. 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)

 7. If you've configured coturn to support TLS/DTLS, generate or
    import your private key and certificate.

 8. Start the turn server::
 
       bin/turnserver -o


synapse Setup
=============

Your home server configuration file needs the following extra keys:

 1. "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.

 2. "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.

 3. "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).

  4. "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

Now, restart synapse::

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

...and your Home Server now supports VoIP relaying!
