synapse/docs/delegate.md

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:

{
    "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 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.

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 the reverse proxy documentation 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.