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Explain how SRV works in Matrix and discourage using it (#2577)

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* Explain how SRV works in Matrix and discourage using it

* Minor tweaks to formatting

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>
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docs/installation/2_domainname.md
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@ -14,15 +14,18 @@ that take the format `@user:example.com`.
For federation to work, the server name must be resolvable by other homeservers on the internet
— that is, the domain must be registered and properly configured with the relevant DNS records.
Matrix servers discover each other when federating using the following methods:
Matrix servers usually discover each other when federating using the following methods:
1. If a well-known delegation exists on `example.com`, use the path server from the
1. If a well-known delegation exists on `example.com`, use the domain and port from the
well-known file to connect to the remote homeserver;
2. If a DNS SRV delegation exists on `example.com`, use the hostname and port from the DNS SRV
2. If a DNS SRV delegation exists on `example.com`, use the IP address and port from the DNS SRV
record to connect to the remote homeserver;
3. If neither well-known or DNS SRV delegation are configured, attempt to connect to the remote
homeserver by connecting to `example.com` port TCP/8448 using HTTPS.
The exact details of how server name resolution works can be found in
[the spec](https://spec.matrix.org/v1.3/server-server-api/#resolving-server-names).
## TLS certificates
Matrix federation requires that valid TLS certificates are present on the domain. You must
@ -51,17 +54,12 @@ you will be able to delegate from `example.com` to `matrix.example.com` so that
Delegation can be performed in one of two ways:
* **Well-known delegation**: A well-known text file is served over HTTPS on the domain name
that you want to use, pointing to your server on `matrix.example.com` port 8448;
* **DNS SRV delegation**: A DNS SRV record is created on the domain name that you want to
use, pointing to your server on `matrix.example.com` port TCP/8448.
* **Well-known delegation (preferred)**: A well-known text file is served over HTTPS on the domain
name that you want to use, pointing to your server on `matrix.example.com` port 8448;
* **DNS SRV delegation (not recommended)**: See the SRV delegation section below for details.
If you are using a reverse proxy to forward `/_matrix` to Dendrite, your well-known or DNS SRV
delegation must refer to the hostname and port that the reverse proxy is listening on instead.
Well-known delegation is typically easier to set up and usually preferred. However, you can use
either or both methods to delegate. If you configure both methods of delegation, it is important
that they both agree and refer to the same hostname and port.
If you are using a reverse proxy to forward `/_matrix` to Dendrite, your well-known or delegation
must refer to the hostname and port that the reverse proxy is listening on instead.
## Well-known delegation
@ -74,20 +72,36 @@ and contain the following JSON document:
```json
{
"m.server": "https://matrix.example.com:8448"
"m.server": "matrix.example.com:8448"
}
```
You can also serve `.well-known` with Dendrite itself by setting the `well_known_server_name` config
option to the value you want for `m.server`. This is primarily useful if Dendrite is exposed on
`example.com:443` and you don't want to set up a separate webserver just for serving the `.well-known`
file.
```yaml
global:
...
well_known_server_name: "example.com:443"
```
## DNS SRV delegation
Using DNS SRV delegation requires creating DNS SRV records on the `example.com` zone which
refer to your Dendrite installation.
This method is not recommended, as the behavior of SRV records in Matrix is rather unintuitive:
SRV records will only change the IP address and port that other servers connect to, they won't
affect the domain name. In technical terms, the `Host` header and TLS SNI of federation requests
will still be `example.com` even if the SRV record points at `matrix.example.com`.
Assuming that your Dendrite installation is listening for HTTPS connections at `matrix.example.com`
port 8448, the DNS SRV record must have the following fields:
In practice, this means that the server must be configured with valid TLS certificates for
`example.com`, rather than `matrix.example.com` as one might intuitively expect. If there's a
reverse proxy in between, the proxy configuration must be written as if it's `example.com`, as the
proxy will never see the name `matrix.example.com` in incoming requests.
* Name: `@` (or whichever term your DNS provider uses to signal the root)
* Service: `_matrix`
* Protocol: `_tcp`
* Port: `8448`
* Target: `matrix.example.com`
This behavior also means that if `example.com` and `matrix.example.com` point at the same IP
address, there is no reason to have a SRV record pointing at `matrix.example.com`. It can still
be used to change the port number, but it won't do anything else.
If you understand how SRV records work and still want to use them, the service name is `_matrix` and
the protocol is `_tcp`.