feature: Added an internally only exposed status web service.

README.md

@@ -115,6 +115,9 @@ must have the type shown below; an unknown section or setting is an error.
 | `wireguard` | `listen_port` | integer | `51820` | UDP port |
 | `docker` | `mode` | string | `auto` | `auto` secures Docker when its service is installed, `on` requires it, `off` removes the rules |
 | `docker` | `exposed_ports` | list | | TCP ports and port ranges (`8000:8100`) reachable from the external interface; at most 15, a range counting as two |
+| `web` | `enabled` | boolean | `true` | The status web service on the internal address |
+| `web` | `port` | integer | `8080` | TCP port of the status web service |
+| `web` | `jwt_secret` | string | | Secret, at least 32 characters, that signs the tokens for `/config`; without it `/config` is not served |
 
 A minimal site-to-site configuration in `auto` mode:
 
@@ -321,6 +324,74 @@ EXTRA_LOCAL_TS = '192.0.2.53/32'
 Set `general.platform` to `example` and restart the service. The debconf question offers only the
 modules shipped with the package; the configuration file accepts any module installed.
 
+## Status web service
+
+The `vpn-router-web` service shows the state of the router on its internal address and
+`web.port` (8080 by default). It uses only the Python standard library, serves HTTP without
+authentication, and shows no configuration: no key, identity, address or subnet.
+
+| Page | Content |
+| --- | --- |
+| `/` | The status for people: plain text for `curl` and `wget`, a page styled with Tailwind CSS for a browser (the style sheet is loaded from a CDN, so the browser needs internet access; the text is readable without it) |
+| `/status` | The same status as JSON |
+| `/config` | The configuration with its secrets masked, and what the router resolved and runs, as JSON; needs a token |
+
+The status holds the state of `vpn-router-setup`, of `strongswan` when site-to-site or road-warrior
+access is configured, and of `wg-quick@wg0` when WireGuard is; the package version; whether
+Docker is installed, and when it is, the state of `docker.service` and of `vpn-router-docker`
+(the Docker rules), which do not change `status`; whether the
+site-to-site tunnel is established; the number of connected road-warrior (IPsec P2S) clients; and
+the WireGuard peers with the age of their last handshake and their traffic. `status` is `ok` when
+all these services are active, and `degraded` otherwise.
+
+```bash
+curl http://10.0.0.4:8080/
+curl http://10.0.0.4:8080/status
+```
+
+### The /config page
+
+`/config` shows what `/` and `/status` leave out, for a client such as manage-ui:
+
+| Key | Content |
+| --- | --- |
+| `configuration` | `vpn-router.json` with the defaults applied and the secrets masked (`psk`, `jwt_secret`, and any option named like a password, token or key) |
+| `network` | The external and internal interface, their addresses, the internal gateway, the local and remote IKE identities and subnets, and the IPv4 routes |
+| `ipsec` | The output of `swanctl --list-conns` and `swanctl --list-sas`, as lists of lines |
+| `wireguard` | The public key and port of `wg0`, and each peer's public key, endpoint, allowed IPs, handshake age and traffic; no private or preshared key |
+| `docker` | `docker.exposed_ports` and the rules of the `VPN-ROUTER-DOCKER` chain |
+
+The page needs a JWT in an `Authorization: Bearer` header, signed with HS256 by `web.jwt_secret`.
+The token must have an `exp` claim, not more than ten minutes from now (a minute of clock
+difference is allowed); `nbf` is checked when present. Other algorithms are refused, with 401 and
+`WWW-Authenticate: Bearer`. While `web.jwt_secret` is not set the page is not served (404). The
+token and the page travel over plain HTTP on the internal network, and the token can be used
+until it expires, so keep the lifetime short. A client makes a token from the secret with only
+HMAC-SHA256; with Python:
+
+```python
+import base64, hashlib, hmac, json, time
+
+def b64(data):
+    return base64.urlsafe_b64encode(data).rstrip(b'=').decode()
+
+secret = 'the web.jwt_secret value'
+head = b64(json.dumps({'alg': 'HS256', 'typ': 'JWT'}).encode())
+body = b64(json.dumps({'exp': int(time.time()) + 300}).encode())
+sign = b64(hmac.new(secret.encode(), f'{head}.{body}'.encode(), hashlib.sha256).digest())
+print(f'{head}.{body}.{sign}')
+```
+
+```bash
+curl -H "Authorization: Bearer $TOKEN" http://10.0.0.4:8080/config
+```
+
+`vpn-router-setup` opens `web.port` on the internal interface in the firewall, writes the address,
+the port, the secret and the facts for `/config` to `/run/vpn-router/web.json` (readable by root
+only) and restarts the service, so a changed `web` section
+applies with `systemctl restart vpn-router-setup`. The service ends without an error when
+`web.enabled` is `false`, and keeps trying to start until the router has an internal address.
+
 ## Removal
 
 `apt-get remove` stops `vpn-router-docker`, which removes the Docker rules. It terminates the