Add support for service reload and sync service file (#2102)

* Add support for service reload and sync service file

* Copy the systemd.service file to the manual linux docs and adjust the
  path to the headscale binary to match with the previous documentation
  blocks. Unfortunately, there seems to be no easy way to include a
  file in mkdocs.
* Remove a redundant "deprecation" block. The beginning of the
  documentation already states that.
* Add `ExecReload` to the systemd.service file.

Fixes: #2016

* Its called systemd

* Fix link to systemd homepage

docs/packaging/headscale.systemd.service

@@ -9,6 +9,7 @@ Type=simple
 User=headscale
 Group=headscale
 ExecStart=/usr/bin/headscale serve
+ExecReload=/usr/bin/kill -HUP $MAINPID
 Restart=always
 RestartSec=5
 

docs/running-headscale-linux-manual.md

@@ -8,7 +8,7 @@
 ## Goal
 
 This documentation has the goal of showing a user how-to set up and run `headscale` on Linux.
-In additional to the "get up and running section", there is an optional [SystemD section](#running-headscale-in-the-background-with-systemd)
+In additional to the "get up and running section", there is an optional [systemd section](#running-headscale-in-the-background-with-systemd)
 describing how to make `headscale` run properly in a server environment.
 
 ## Configure and run `headscale`
@@ -66,7 +66,7 @@ describing how to make `headscale` run properly in a server environment.
     To continue the tutorial, open a new terminal and let it run in the background.
     Alternatively use terminal emulators like [tmux](https://github.com/tmux/tmux) or [screen](https://www.gnu.org/software/screen/).
 
-    To run `headscale` in the background, please follow the steps in the [SystemD section](#running-headscale-in-the-background-with-systemd) before continuing.
+    To run `headscale` in the background, please follow the steps in the [systemd section](#running-headscale-in-the-background-with-systemd) before continuing.
 
 1. Verify `headscale` is running:
   Verify `headscale` is available:
@@ -109,42 +109,14 @@ This will return a pre-authenticated key that can be used to connect a node to `
 tailscale up --login-server <YOUR_HEADSCALE_URL> --authkey <YOUR_AUTH_KEY>
 ```
 
-## Running `headscale` in the background with SystemD
+## Running `headscale` in the background with systemd
 
-:warning: **Deprecated**: This part is very outdated and you should use the [pre-packaged Headscale for this](./running-headscale-linux.md)
-
-This section demonstrates how to run `headscale` as a service in the background with [SystemD](https://www.freedesktop.org/wiki/Software/systemd/).
+This section demonstrates how to run `headscale` as a service in the background with [systemd](https://systemd.io/).
 This should work on most modern Linux distributions.
 
-1. Create a SystemD service configuration at `/etc/systemd/system/headscale.service` containing:
-
-    ```systemd
-    [Unit]
-    Description=headscale controller
-    After=syslog.target
-    After=network.target
-
-    [Service]
-    Type=simple
-    User=headscale
-    Group=headscale
-    ExecStart=/usr/local/bin/headscale serve
-    Restart=always
-    RestartSec=5
-
-    # Optional security enhancements
-    NoNewPrivileges=yes
-    PrivateTmp=yes
-    ProtectSystem=strict
-    ProtectHome=yes
-    WorkingDirectory=/var/lib/headscale
-    ReadWritePaths=/var/lib/headscale /var/run/headscale
-    AmbientCapabilities=CAP_NET_BIND_SERVICE
-    RuntimeDirectory=headscale
-
-    [Install]
-    WantedBy=multi-user.target
-    ```
+1. Copy [headscale's systemd service file](./packaging/headscale.systemd.service) to
+   `/etc/systemd/system/headscale.service` and adjust it to suit your local setup. The following parameters likely need
+   to be modified: `ExecStart`, `WorkingDirectory`, `ReadWritePaths`.
 
     Note that when running as the headscale user ensure that, either you add your current user to the headscale group:
 
@@ -164,7 +136,7 @@ This should work on most modern Linux distributions.
     unix_socket: /var/run/headscale/headscale.sock
     ```
 
-1. Reload SystemD to load the new configuration file:
+1. Reload systemd to load the new configuration file:
 
     ```shell
     systemctl daemon-reload

docs/running-headscale-linux.md

@@ -8,7 +8,7 @@
 
 Get Headscale up and running.
 
-This includes running Headscale with SystemD.
+This includes running Headscale with systemd.
 
 ## Migrating from manual install
 

integration/dns_test.go

@@ -86,7 +86,7 @@ func TestResolveMagicDNS(t *testing.T) {
 // All the containers are based on Alpine, meaning Tailscale
 // will overwrite the resolv.conf file.
 // On other platform, Tailscale will integrate with a dns manager
-// if available (like Systemd-Resolved).
+// if available (like systemd-resolved).
 func TestValidateResolvConf(t *testing.T) {
 	IntegrationSkip(t)