docs(wireguard): update setup instruction

docs/getting-started/vpn-setup.md

@@ -1,6 +1,8 @@
 # VPN setup
 
-You can choose between [Tailscale](https://tailscale.com), [Wireguard](https://www.wireguard.com), or use both like me.
+You can choose between [Tailscale](https://tailscale.com),
+[Wireguard](https://www.wireguard.com), or use both like me. I primarily use
+WireGuard but keep Tailscale as a backup for when the WireGuard server is down.
 
 ## Tailscale (requires third-party account)
 
@@ -17,6 +19,8 @@ extra_secrets = {
 }
 ```
 
+You may want to back up the `external/terraform.tfvars` file to a secure location.
+
 Apply the secret:
 
 ```sh
@@ -30,17 +34,21 @@ You can now connect to your homelab via Tailscale and [invite user to your Tails
 
 ## Wireguard (requires port-forwarding)
 
-Update the peer list in `apps/wireguard/values.yaml`:
+### Prerequisites
 
-```yaml
-PEERS: |
-  UserDevice
-  FooPhone
-  FooLaptop
-  BarDesktop
+Find your public IP address using:
+
+```sh
+curl -4 ifconfig.me
 ```
 
-Go to your router settings and forward the Wireguard service.
+If you don’t have a static IP address, use dynamic DNS and replace the IP with
+your domain name.
+
+Next, configure port forwarding in your router for the WireGuard service.
+
+!!! example
+
     Each router is different, here's mine for reference:
 
     - Protocol: `UDP`
@@ -50,12 +58,124 @@ Each router is different, here's mine for reference:
     - Start Port Local: `51820`
     - End Port Local: `51820`
 
-To get the QR code (for mobile) and config (for desktop), run:
-
-!!! warning
-
-    This command will print sensitive secrets to the terminal.
+Generate a key pair for the server:
 
 ```sh
-./scripts/get-wireguard-config FooPhone
+wg genkey | tee /dev/tty | wg pubkey
+```
+
+This will generate a private key and a public key, in that order. Add the
+private key to `external/terraform.tfvars` as an extra secret:
+
+```hcl
+extra_secrets = {
+  wireguard-private-key = "privatekeyhere"
+}
+```
+
+You may want to back up the `external/terraform.tfvars` file to a secure location.
+
+Apply the secret:
+
+```sh
+make external
+```
+
+I use `172.16.0.0/12` as the private IP range for WireGuard, but you can choose
+any private IP address range you prefer in `./apps/wireguard/values.yaml`. I
+also recommend removing my peers and adding your own.
+
+### Add a new device to the server
+
+!!! info
+
+    Each device requires its own configuration.
+
+Generate a new key pair for the device. You can generate it for the user, or
+they can generate it themselves if they prefer to keep the private key
+confidential:
+
+```sh
+wg genkey | tee /dev/tty | wg pubkey
+```
+
+This will generate a private key and a public key, in that order. The private
+key must be saved in a secure password manager, and save the public key for the
+next step.
+
+Update the list of peers in `./apps/wireguard/values.yaml`, make sure you
+replace all of my peers with yours.
+
+!!! example
+
+    Example configuration for my phone:
+
+    ```ini
+    [Peer]
+    PublicKey = nITHFdgTkNZOTWeSWqnGXjgwlCJMKRCnnUsjMx2yp2U=
+    AllowedIPs = 172.16.0.12/32
+    ```
+
+    - The public key is the one generated in the previous step.
+    - `172.16.0.12/32` is the device's private IP address, manually selected from
+      the `172.16.0.0/12` range mentioned above.
+
+### Add the Wireguard config to the device
+
+Create a new configuration file for the device:
+
+```ini
+[Interface]
+Address = <CLIENT PRIVATE IP>/32
+PrivateKey = <CLIENT PRIVATE KEY>
+
+[Peer]
+PublicKey = <SERVER PUBLIC KEY>
+Endpoint = <SERVER PUBLIC IP>:51820
+AllowedIPs = <SERVER PRIVATE IP>/32, <LOAD BALANCER IP RANGE>
+```
+
+Replace placeholders with actual values and save as `wg0.conf`.
+
+!!! example
+
+    Example configuration for my phone:
+
+    ```ini
+    [Interface]
+    Address = 172.16.0.12/32
+    PrivateKey = <REDACTED>
+
+    [Peer]
+    PublicKey = sSAZS1Z3vB7Wx8e2yVqXfeHjgWTa80wnSYoma3mZkiU
+    Endpoint = <HOME IP>:51820
+    AllowedIPs = 172.16.0.1/32, 192.168.1.224/27
+    ```
+
+The client can now import this configuration and connect to your WireGuard
+mesh. Make sure you clean up the `wg0.conf` file after importing it to the
+client.
+
+=== "Mobile"
+
+    Generate a QR code from the configuration file:
+
+    ```sh
+    qrencode -t ansiutf8 -r wg0.conf
+    ```
+
+    Then scan the QR code using the official WireGuard app.
+
+=== "Linux"
+
+    Import the WireGuard configuration using NetworkManager:
+
+    ```sh
+    nmcli connection import type wireguard file wg0.conf
+    ```
+
+    Activate the connection:
+
+    ```sh
+    nmcli connection up wg0
     ```