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TLS documentation updates (#1733) 

* TLS documentation updates

Move "Bring your own certificates" to the top
  since the letsencrypt section is now much longer, it seems wrong to
  keep such a short section way down at the bottom.

Restructure "Challenge types" into separate sections

Add technical description of letsencrypt renewals
  this aims to answer:
  - what can be expected in terms of renewals
  - what logs can be expected (none)
  - how to validate that renewal happened successfully
  - the reason for some of the 'acme/autocert' logs, or at least
    some best-effort assumptions

* +prettier
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@ -1,8 +1,17 @@
# Running the service via TLS (optional)
## Bring your own certificate
Headscale can be configured to expose its web service via TLS. To configure the certificate and key file manually, set the `tls_cert_path` and `tls_cert_path` configuration parameters. If the path is relative, it will be interpreted as relative to the directory the configuration file was read from.
```yaml
tls_cert_path: ""
tls_key_path: ""
```
## Let's Encrypt / ACME
To get a certificate automatically via [Let's Encrypt](https://letsencrypt.org/), set `tls_letsencrypt_hostname` to the desired certificate hostname. This name must resolve to the IP address(es) headscale is reachable on (i.e., it must correspond to the `server_url` configuration parameter). The certificate and Let's Encrypt account credentials will be stored in the directory configured in `tls_letsencrypt_cache_dir`. If the path is relative, it will be interpreted as relative to the directory the configuration file was read from. The certificate will automatically be renewed as needed.
To get a certificate automatically via [Let's Encrypt](https://letsencrypt.org/), set `tls_letsencrypt_hostname` to the desired certificate hostname. This name must resolve to the IP address(es) headscale is reachable on (i.e., it must correspond to the `server_url` configuration parameter). The certificate and Let's Encrypt account credentials will be stored in the directory configured in `tls_letsencrypt_cache_dir`. If the path is relative, it will be interpreted as relative to the directory the configuration file was read from.
```yaml
tls_letsencrypt_hostname: ""
@ -11,21 +20,57 @@ tls_letsencrypt_cache_dir: ".cache"
tls_letsencrypt_challenge_type: HTTP-01
```
### Challenge type HTTP-01
### Challenge types
The default challenge type `HTTP-01` requires that headscale is reachable on port 80 for the Let's Encrypt automated validation, in addition to whatever port is configured in `listen_addr`. By default, headscale listens on port 80 on all local IPs for Let's Encrypt automated validation.
Headscale only supports two values for `tls_letsencrypt_challenge_type`: `HTTP-01` (default) and `TLS-ALPN-01`.
#### HTTP-01
For `HTTP-01`, headscale must be reachable on port 80 for the Let's Encrypt automated validation, in addition to whatever port is configured in `listen_addr`. By default, headscale listens on port 80 on all local IPs for Let's Encrypt automated validation.
If you need to change the ip and/or port used by headscale for the Let's Encrypt validation process, set `tls_letsencrypt_listen` to the appropriate value. This can be handy if you are running headscale as a non-root user (or can't run `setcap`). Keep in mind, however, that Let's Encrypt will _only_ connect to port 80 for the validation callback, so if you change `tls_letsencrypt_listen` you will also need to configure something else (e.g. a firewall rule) to forward the traffic from port 80 to the ip:port combination specified in `tls_letsencrypt_listen`.
### Challenge type TLS-ALPN-01
#### TLS-ALPN-01
Alternatively, `tls_letsencrypt_challenge_type` can be set to `TLS-ALPN-01`. In this configuration, headscale listens on the ip:port combination defined in `listen_addr`. Let's Encrypt will _only_ connect to port 443 for the validation callback, so if `listen_addr` is not set to port 443, something else (e.g. a firewall rule) will be required to forward the traffic from port 443 to the ip:port combination specified in `listen_addr`.
For `TLS-ALPN-01`, headscale listens on the ip:port combination defined in `listen_addr`. Let's Encrypt will _only_ connect to port 443 for the validation callback, so if `listen_addr` is not set to port 443, something else (e.g. a firewall rule) will be required to forward the traffic from port 443 to the ip:port combination specified in `listen_addr`.
## Bring your own certificate
### Technical description
headscale can also be configured to expose its web service via TLS. To configure the certificate and key file manually, set the `tls_cert_path` and `tls_cert_path` configuration parameters. If the path is relative, it will be interpreted as relative to the directory the configuration file was read from.
Headscale uses [autocert](https://pkg.go.dev/golang.org/x/crypto/acme/autocert), a Golang library providing [ACME protocol](https://en.wikipedia.org/wiki/Automatic_Certificate_Management_Environment) verification, to facilitate certificate renewals via [Let's Encrypt](https://letsencrypt.org/about/). Certificates will be renewed automatically, and the following can be expected:
```yaml
tls_cert_path: ""
tls_key_path: ""
- Certificates provided from Let's Encrypt have a validity of 3 months from date issued.
- Renewals are only attempted by headscale when 30 days or less remains until certificate expiry.
- Renewal attempts by autocert are triggered at a random interval of 30-60 minutes.
- No log output is generated when renewals are skipped, or successful.
#### Checking certificate expiry
If you want to validate that certificate renewal completed successfully, this can be done either manually, or through external monitoring software. Two examples of doing this manually:
1. Open the URL for your Headscale server in your browser of choice, and manually inspecting the expiry date of the certificate you receive.
2. Or, check remotely from CLI using `openssl`:
```bash
$ openssl s_client -servername [hostname] -connect [hostname]:443 | openssl x509 -noout -dates
(...)
notBefore=Feb 8 09:48:26 2024 GMT
notAfter=May 8 09:48:25 2024 GMT
```
#### Log output from the autocert library
As these log lines are from the autocert library, they are not strictly generated by headscale itself.
```plaintext
acme/autocert: missing server name
```
Likely caused by an incoming connection that does not specify a hostname, for example a `curl` request directly against the IP of the server, or an unexpected hostname.
```plaintext
acme/autocert: host "[foo]" not configured in HostWhitelist
```
Similarly to the above, this likely indicates an invalid incoming request for an incorrect hostname, commonly just the IP itself.
The source code for autocert can be found [here](https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.19.0:acme/autocert/autocert.go)