synapse-product/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. 

You will also need a valid TLS certificate for this `server_name` served
on port 8448 - the preferred way to do that is by using a reverse proxy,
see [reverse_proxy.md](<reverse_proxy.md>) for instructions on how to
correctly set one up.

In some cases you might not want Synapse to be running on the machine that
has the `server_name` as its public DNS hostname, or federation traffic
to use port than 8448 (e.g. you want to use `example.com` as your `server_name`
but want Synapse to be reachable on `synapse.example.com:443`). This can
be done using delegation, which allows an admin to dictate where federation
traffic should be sent, see [delegate.md](<delegate.md>) for instructions on
how to set this up. 

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](<https://matrix.org/federationtester>)
to check if your homeserver is configured correctly. Alternatively try the
[JSON API used by the federation tester](https://matrix.org/federationtester/api/report?server_name=DOMAIN).
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](<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](<../demo/README>).