Merge pull request #6940 from matrix-org/babolivier/federate.md

Clean up and update federation docs

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+# Delegation
+
+By default, other homeservers will expect to be able to reach yours via
+your `server_name`, on port 8448. For example, if you set your `server_name`
+to `example.com` (so that your user names look like `@user:example.com`),
+other servers will try to connect to yours at `https://example.com:8448/`.
+
+Delegation is a Matrix feature allowing a homeserver admin to retain a
+`server_name` of `example.com` so that user IDs, room aliases, etc continue
+to look like `*:example.com`, whilst having federation traffic routed
+to a different server and/or port (e.g. `synapse.example.com:443`).
+
+## .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:
+
+```json
+{
+    "m.server": "<synapse.server.name>[:<yourport>]"
+}
+```
+
+In our example, this would mean that URL `https://example.com/.well-known/matrix/server`
+should return:
+
+```json
+{
+    "m.server": "synapse.example.com:443"
+}
+```
+
+Note, specifying a port is optional. If no port is specified, then it defaults
+to 8448.
+
+With .well-known delegation, federating servers will check for a valid TLS
+certificate for the delegated hostname (in our example: `synapse.example.com`).
+
+## SRV DNS record delegation
+
+It is also possible to do delegation using a SRV DNS record. However, that is
+considered an advanced topic since it's a bit complex to set up, and `.well-known`
+delegation is already enough in most cases.
+
+However, if you really need it, you can find some documentation on how such a
+record should look like and how Synapse will use it in [the Matrix
+specification](https://matrix.org/docs/spec/server_server/latest#resolving-server-names).
+
+## Delegation FAQ
+
+### When do I need delegation?
+
+If your homeserver's APIs are accessible on the default federation port (8448)
+and the domain your `server_name` points to, you do not need any delegation.
+
+For instance, if you registered `example.com` and pointed its DNS A record at a
+fresh server, you could install Synapse on that host, giving it a `server_name`
+of `example.com`, and once a reverse proxy has been set up to proxy all requests
+sent to the port `8448` and serve TLS certificates for `example.com`, you
+wouldn't need any delegation set up.
+
+**However**, if your homeserver's APIs aren't accessible on port 8448 and on the
+domain `server_name` points to, you will need to let other servers know how to
+find it using delegation.
+
+### Do you still recommend against using a reverse proxy on the federation port?
+
+We no longer actively recommend against using a reverse proxy. Many admins will
+find it easier to direct federation traffic to a reverse proxy and manage their
+own TLS certificates, and this is a supported configuration.
+
+See [reverse_proxy.md](reverse_proxy.md) for information on setting up a
+reverse proxy.
+
+### Do I still need to give my TLS certificates to Synapse if I am using a reverse proxy?
+
+This is no longer necessary. If you are using a reverse proxy for all of your
+TLS traffic, then you can set `no_tls: True` in the Synapse config.
+
+In that case, the only reason Synapse needs the certificate is to populate a legacy
+`tls_fingerprints` field in the federation API. This is ignored by Synapse 0.99.0
+and later, and the only time pre-0.99 Synapses will check it is when attempting to
+fetch the server keys - and generally this is delegated via `matrix.org`, which
+is running a modern version of Synapse.
+
+### Do I need the same certificate for the client and federation port?
+
+No. There is nothing stopping you from using different certificates,
+particularly if you are using a reverse proxy.
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