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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 federation has been configured, you should be able to join a room over
federation. A good place to start is #synapse:matrix.org - a room for
Synapse admins.
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.md 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.