Neilj/improved delegation doc 2 (#4832)

Improved federation configuration docs.  Specifically detailing  .well-known and SRV based delegation methods. 

Inspiration Valentin Lab <valentin.lab@kalysto.org> for https://github.com/matrix-org/synapse/pull/4781

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+Setting up Federation
+=====================
+
+Federation is the process by which users on different servers can participate
+in the same room. For this to work, those other servers must be able to contact
+yours to send messages.
+
+The ``server_name`` configured in the Synapse configuration file (often
+``homeserver.yaml``) defines how resources (users, rooms, etc.) will be
+identified (eg: ``@user:example.com``, ``#room:example.com``). By
+default, it is also the domain that other servers will use to
+try to reach your server (via port 8448). This is easy to set
+up and will work provided you set the ``server_name`` to match your
+machine's public DNS hostname, and provide Synapse with a TLS certificate
+which is valid for your ``server_name``.
+
+Once you have completed the steps necessary to federate, you should be able to 
+join a room via federation. (A good place to start is ``#synapse:matrix.org``
+- a room for Synapse admins.)
+
+
+## Delegation
+
+For a more flexible configuration, you can have ``server_name``
+resources (eg: ``@user:example.com``) served by a different host and
+port (eg: ``synapse.example.com:443``). There are two ways to do this:
+
+- adding a ``/.well-known/matrix/server`` URL served on ``https://example.com``.
+- adding a DNS ``SRV`` record in the DNS zone of domain
+  ``example.com``.
+
+Without configuring delegation, the matrix federation will
+expect to find your server via ``example.com:8448``. The following methods
+allow you retain a `server_name` of `example.com` so that your user IDs, room
+aliases, etc continue to look like `*:example.com`, whilst having your
+federation traffic routed to a different server.
+
+### .well-known delegation
+
+To use this method, you need to be able to alter the
+``server_name`` 's https server to serve the ``/.well-known/matrix/server``
+URL. Having an active server (with a valid TLS certificate) serving your
+``server_name`` domain is out of the scope of this documentation.
+
+The URL ``https://<server_name>/.well-known/matrix/server`` should
+return a JSON structure containing the key ``m.server`` like so:
+
+    {
+	    "m.server": "<synapse.server.name>[:<yourport>]"
+    }
+
+In our example, this would mean that URL ``https://example.com/.well-known/matrix/server``
+should return:
+
+    {
+	    "m.server": "synapse.example.com:443"
+    }
+
+Note, specifying a port is optional. If a port is not specified an SRV lookup
+is performed, as described below. If the target of the
+delegation does not have an SRV record, then the port defaults to 8448.
+
+Most installations will not need to configure .well-known. However, it can be
+useful in cases where the admin is hosting on behalf of someone else and
+therefore cannot gain access to the necessary certificate. With .well-known,
+federation servers will check for a valid TLS certificate for the delegated
+hostname (in our example: ``synapse.example.com``).
+
+.well-known support first appeared in Synapse v0.99.0. To federate with older
+servers you may need to additionally configure SRV delegation. Alternatively,
+encourage the server admin in question to upgrade :).
+
+### DNS SRV delegation
+
+To use this delegation method, you need to have write access to your
+``server_name`` 's domain zone DNS records (in our example it would be
+``example.com`` DNS zone).
+
+This method requires the target server to provide a
+valid TLS certificate for the original ``server_name``.
+domain zone. 
+
+You need to add a SRV record in your ``server_name`` 's DNS zone with
+this format:
+
+     _matrix._tcp.<yourdomain.com> <ttl> IN SRV <priority> <weight> <port> <synapse.server.name>
+
+In our example, we would need to add this SRV record in the
+``example.com`` DNS zone:
+
+     _matrix._tcp.example.com. 3600 IN SRV 10 5 443 synapse.example.com.
+
+
+Once done and set up, you can check the DNS record with ``dig -t srv
+_matrix._tcp.<server_name>``. In our example, we would expect this:
+
+    $ dig -t srv _matrix._tcp.example.com
+    _matrix._tcp.example.com. 3600    IN      SRV     10 0 443 synapse.example.com.
+
+Note that the target of a SRV record cannot be an alias (CNAME record): it has to point
+directly to the server hosting the synapse instance.
+
+## Troubleshooting
+
+You can use the [federation tester](
+<https://matrix.org/federationtester>) to check if your homeserver is
+configured correctly. Alternatively try the [JSON API used by the federation tester](https://matrix.org/federationtester/api/report?server_name=DOMAIN).
+Note that you'll have to modify this URL to replace ``DOMAIN`` with your
+``server_name``. Hitting the API directly provides extra detail.
+
+The typical failure mode for federation is that when the server tries to join
+a room, it is rejected with "401: Unauthorized". Generally this means that other
+servers in the room could not access yours. (Joining a room over federation is
+a complicated dance which requires connections in both directions).
+
+Another common problem is that people on other servers can't join rooms that
+you invite them to. This can be caused by an incorrectly-configured reverse
+proxy: see [reverse_proxy.rst](<reverse_proxy.rst>) for instructions on how to correctly
+configure a reverse proxy.
+
+
+## Running a Demo Federation of Synapses
+
+If you want to get up and running quickly with a trio of homeservers in a
+private federation, there is a script in the ``demo`` directory. This is mainly
+useful just for development purposes. See [demo/README](<../demo/README>).