synapse/docs/federate.md

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 to check if your homeserver is configured correctly. Alternatively try the JSON API used by the federation tester. 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 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.