watcha-synapse/docs/delegate.md

Delegation of incoming federation traffic

In the following documentation, we use the term server_name to refer to that setting in your homeserver configuration file. It appears at the ends of user ids, and tells other homeservers where they can find your server.

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 configure the server at https://<server_name> to serve a file at https://<server_name>/.well-known/matrix/server. There are two ways to do this, shown below.

Note that the .well-known file is hosted on the default port for https (port 443).

External server

For maximum flexibility, you need to configure an external server such as nginx, Apache or HAProxy to serve the https://<server_name>/.well-known/matrix/server file. Setting up such a server is out of the scope of this documentation, but note that it is often possible to configure your reverse proxy for this.

The URL https://<server_name>/.well-known/matrix/server should be configured return a JSON structure containing the key m.server like this:

{
    "m.server": "<synapse.server.name>[:<yourport>]"
}

In our example (where we want federation traffic to be routed to https://synapse.example.com, on port 443), this would mean that 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.

Serving a .well-known/matrix/server file with Synapse

If you are able to set up your domain so that https://<server_name> is routed to Synapse (i.e., the only change needed is to direct federation traffic to port 443 instead of port 8448), then it is possible to configure Synapse to serve a suitable .well-known/matrix/server file. To do so, add the following to your homeserver.yaml file:

serve_server_wellknown: true

Note: this only works if https://<server_name> is routed to Synapse, so is generally not suitable if Synapse is hosted at a subdomain such as https://synapse.example.com.

SRV DNS record delegation

It is also possible to do delegation using a SRV DNS record. However, that is generally not recommended, as it can be difficult to configure the TLS certificates correctly in this case, and it offers little advantage over .well-known delegation.

Please keep in mind that server delegation is a function of server-server communication, and as such using SRV DNS records will not cover use cases involving client-server comms. This means setting global client settings (such as a Jitsi endpoint, or disabling creating new rooms as encrypted by default, etc) will still require that you serve a file from the https://<server_name>/.well-known/ endpoints defined in the spec! If you are considering using SRV DNS delegation to avoid serving files from this endpoint, consider the impact that you will not be able to change those client-based default values globally, and will be relegated to the featureset of the configuration of each individual client.

However, if you really need it, you can find some documentation on what 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.

Should I use a reverse proxy for federation traffic?

Generally, using a reverse proxy for both the federation and client traffic is a good idea, since it saves handling TLS traffic in Synapse. See the reverse proxy documentation for information on setting up a reverse proxy.