matrix-docker-ansible-deploy/docs/howto-server-delegation.md

# Server Delegation

To have a server on a subdomain (e.g. `matrix.<your-domain>`) handle Matrix federation traffic for the base domain (`<your-domain>`), we need to instruct the Matrix network of such a delegation.

By default, this playbook guides you into setting up [Server Delegation via a well-known file](#server-delegation-via-a-well-known-file).
However, that method may have some downsides that are not to your liking. Hence this guide about alternative ways to set up Server Delegation.

It is a complicated matter, so unless you are affected by the [Downsides of well-known-based Server Delegation](#downsides-of-well-known-based-server-delegation), we suggest you stay on the simple/default path.


## Server Delegation via a well-known file

Serving a `/.well-known/matrix/server` file from the base domain is the most straightforward way to set up server delegation, but it suffers from some problems that we list in [Downsides of well-known-based Server Delegation](#downsides-of-well-known-based-server-delegation).

As we already mention in [Configuring DNS](configuring-dns.md) and [Configuring Service Discovery via .well-known](configuring-well-known.md),
this playbook already properly guides you into setting up such delegation by means of a `/.well-known/matrix/server` file served from the base domain (`<your-domain>`).

If this is okay with you, feel free to not read ahead.


### Downsides of well-known-based Server Delegation

Server Delegation by means of a `/.well-known/matrix/server` file is the most straightforward, but suffers from the following downsides:

- you need to have a working HTTPS server for the base domain (`<your-domain>`)

- any downtime on the base domain (`<your-domain>`) or network trouble between the matrix subdomain (`matrix.<your-domain>`) and the base `<domain>` may cause Matrix Federation outages. As the [Server-Server spec says](https://matrix.org/docs/spec/server_server/r0.1.0.html#server-discovery):

> Errors are recommended to be cached for up to an hour, and servers are encouraged to exponentially back off for repeated failures.

If this is not a concern for you, feel free to not read ahead.

Otherwise, you can decide to go against the default for this playbook, and instead set up [Server Delegation via a DNS SRV record (advanced)](#server-delegation-via-a-dns-srv-record-advanced).


## Server Delegation via a DNS SRV record (advanced)

**NOTE**: doing Server Delegation via a DNS SRV record is a more advanced way to do it and is not the default for this playbook.

As per the [Server-Server spec](https://matrix.org/docs/spec/server_server/r0.1.0.html#server-discovery), it's possible to do Server Delegation using only a SRV record (without a `/.well-known/matrix/server` file).

This prevents you from suffering the [Downsides of well-known-based Server Delegation](#downsides-of-well-known-based-server-delegation).

To use DNS SRV record validation, you need to:

- ensure that `/.well-known/matrix/server` is **not served** from the base domain, as that would interfere with DNS SRV record Server Delegation. To make the playbook **not** generate and serve the file, use the following configuration: `matrix_well_known_matrix_server_enabled: false`.

- ensure that you have a `_matrix._tcp` DNS SRV record for your base domain (`<your-domain>`) with a value of `10 0 8448 matrix.<your-domain>`

- ensure that you are serving the Matrix Federation API (tcp/8448) with a certificate for `<your-domain>` (not `matrix.<your-domain>`!). See below.


### Obtaining certificates

How you can obtain a valid certificate for `<your-domain>` on the `matrix.<your-domain>` server is up to you.

If `<your-domain>` and `matrix.<your-domain>` are hosted on the same machine, you can let the playbook obtain the certificate for you, by following our [Obtaining SSL certificates for additional domains](configuring-playbook-ssl-certificates.md#obtaining-ssl-certificates-for-additional-domains) guide.

If `<your-domain>` and `matrix.<your-domain>` are not hosted on the same machine, you can copy over the certificate files manually.
Don't forget that they may get renewed once in a while, so you may also have to transfer them periodically. How often you do that is up to you, as long as the certificate files don't expire.


### Serving the Federation API with your certificates

Regardless of which method for obtaining certificates you've used, once you've managed to get certificates for your base domain onto the `matrix.<your-domain>` machine you can put them to use.

Based on your setup, you have different ways to go about it:

- [Serving the Federation API with your certificates and matrix-nginx-proxy](#serving-the-federation-api-with-your-certificates-and-matrix-nginx-proxy)

- [Serving the Federation API with your certificates and another webserver](#serving-the-federation-api-with-your-certificates-and-another-webserver)

- [Serving the Federation API with your certificates and Synapse handling Federation](#serving-the-federation-api-with-your-certificates-and-synapse-handling-federation)


### Serving the Federation API with your certificates and matrix-nginx-proxy

**If you are using matrix-nginx-proxy**, a reverse-proxy webserver used by default in this playbook, you only need to override the certificates used for the Matrix Federation API. You can do that using:

```yaml
# Adjust paths below to point to your certificate.
#
# NOTE: these are in-container paths. `/matrix/ssl` on the host is mounted into the container
# at the same path (`/matrix/ssl`) by default, so if that's the path you need, it would be seamless.
matrix_nginx_proxy_proxy_matrix_federation_api_ssl_certificate: /matrix/ssl/config/live/matrix.<your-domain>/fullchain.pem
matrix_nginx_proxy_proxy_matrix_federation_api_ssl_certificate_key: /matrix/ssl/config/live/matrix.<your-domain>/privkey.pem
```

If your files are not in `/matrix/ssl` but in some other location, you would need to mount them into the container:

```yaml
matrix_nginx_proxy_container_extra_arguments:
  - "--mount type=bind,src=/some/path/on/the/host,dst=/some/path/inside/the/container,ro"
```

You then refer to them (for `matrix_nginx_proxy_proxy_matrix_federation_api_ssl_certificate` and `matrix_nginx_proxy_proxy_matrix_federation_api_ssl_certificate_key`) by using `/some/path/inside/the/container`.

Make sure to reload matrix-nginx-proxy once in a while (`systemctl reload matrix-nginx-proxy`), so that newer certificates can kick in.
Reloading doesn't cause any downtime.


### Serving the Federation API with your certificates and another webserver

**If you are NOT using matrix-nginx-proxy**, but rather some other webserver, you can set up reverse-proxying for the `tcp/8448` port by yourself.
Make sure to use the proper certificates for `<your-domain>` (not for `matrix.<your-domain>`) when serving the `tcp/8448` port.

Proxying needs to happen to `127.0.0.1:8048` (unencrypted Synapse federation listener).

Make sure to reload/restart your webserver once in a while, so that newer certificates can kick in.


### Serving the Federation API with your certificates and Synapse handling Federation

**Alternatively**, if you are **NOT using matrix-nginx-proxy** and **would rather not use your own webserver for Federation traffic**, you can let Synapse handle Federation by itself.

To do that, make sure the certificate files are mounted into the Synapse container:

```yaml
matrix_synapse_container_extra_arguments:
  - "--mount type=bind,src=/some/path/on/the/host,dst=/some/path/inside/the/container,ro"
```

You can then tell Synapse to serve Federation traffic over TLS on `tcp/8448`:

```yaml
matrix_synapse_tls_federation_listener_enabled: true
matrix_synapse_tls_certificate_path: /some/path/inside/the/container/certificate.crt
matrix_synapse_tls_private_key_path: /some/path/inside/the/container/private.key
```

Make sure to reload Synapse once in a while (`systemctl reload matrix-synapse`), so that newer certificates can kick in.
Reloading doesn't cause any downtime.