synapse/docs/delegate.md

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# 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.
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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/`.
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Delegation is a Matrix feature allowing a homeserver admin to retain a
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`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](reverse_proxy.md) 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:
```json
{
"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:
```json
{
"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:
```yaml
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.
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](https://matrix.org/docs/spec/server_server/latest#resolving-server-names).
## 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](reverse_proxy.md) for information on setting up a
reverse proxy.