add howto for setting up your very own TURN server

docs/turn-howto.rst

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+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 described how to install coturn
+(https://code.google.com/p/coturn/) which also supports the TURN REST API,
+and integrate it with synapse.
+
+coturn Setup
+============
+
+ 1. Check out coturn::
+      svn checkout http://coturn.googlecode.com/svn/trunk/ 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. Make a config file in /etc/turnserver.conf. You can customise
+    a config file from turnserver.conf.default. The relevant
+    lines are::
+      lt-cred-mech
+      use-auth-secret
+      static-auth-secret=[your secret key here]
+    See turnserver.conf.default for explanations of the options.
+    One way to generate the static-auth-secret is with pwgen::
+
+       pwgen -s 64 1
+
+ 5. Ensure youe firewall allows traffic into the TURN server on
+    the ports you've configured it to listen on (remember to allow
+    both TCP and UDP if you've enabled both).
+
+ 6. If you've configured coturn to support TLS/DTLS, generate or
+    import your private key and certificate.
+
+ 7. 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 comma-separated
+    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).
+
+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
+
+Now, restart synapse::
+
+    cd /where/you/run/synapse
+    ./synctl restart
+
+...and your Home Server now supports VoIP relaying!