feature: Added an internally only exposed status web service.

ADR.md

@@ -26,3 +26,52 @@ because `vpn-router-docker` is already active. A published
 port is reachable from the external interface only when it is in `exposed_ports`. Other rules
 in `DOCKER-USER` are not touched, because the package owns only its own chain. Restarting Docker
 restores the rules. The rules cover IPv4 only.
+
+## ADR-2: A status web service of the standard library, bound to the internal address
+
+Status: Accepted
+
+Context: The router has no way to see its state without a shell. A check from the protected
+network, by a person or by a monitor, needs a page or an endpoint. The router is an
+internet-facing host, so what the service shows and where it listens matter.
+
+Decision: The `vpn-router-web` unit runs one Python script that uses only the standard library, in
+the way of a small test server. It binds to the address of the internal interface and to
+`web.port`, on by default at 8080. `/` is for people, in plain text for `curl` and `wget` and as
+a Tailwind CSS page for a browser, and `/status` is the same status as JSON. It shows the
+state of the services, the package version and the state of the site-to-site, road-warrior and
+WireGuard features, and nothing from the configuration. `vpn-router-setup` resolves the
+interface address, writes it with the port and the features to `/run/vpn-router/web.json`,
+opens the port on the internal interface in UFW and restarts the service, so the service does
+not need the package's configuration code, which needs Jinja2.
+
+Consequences: Nothing outside the internal network can reach it, since it listens on the internal
+address only and the firewall opens the port on the internal interface only. It has no
+authentication, which is acceptable because it is read only and shows no configuration. It runs
+as root, because `swanctl` and `wg` need it, with `NoNewPrivileges`, `ProtectSystem=strict`,
+`ProtectHome` and `PrivateTmp`. The Tailwind style comes from a CDN, so a browser on a network
+without internet shows the unstyled page, and `curl` is unaffected. A change of the internal
+address is picked up at the next run of `vpn-router-setup`.
+
+## ADR-3: /config is authenticated with a short-lived JWT signed with a shared secret
+
+Status: Accepted
+
+Context: A client such as manage-ui needs the configuration and the resolved state of the
+router, which `/` and `/status` must not show. The service has only the standard library, listens
+on plain HTTP on the internal network, and its configuration file is readable by root only.
+
+Decision: `/config` needs a JWT in an `Authorization: Bearer` header. The service verifies HS256
+tokens with `hmac` and `hashlib`, with the secret `web.jwt_secret`, and accepts no other
+algorithm, because verifying RS256 or ES256 needs a library outside the standard library. The
+token must carry `exp`, not more than ten minutes ahead, so that the client makes a fresh token for
+each use and a leaked one expires. Without a secret the page is not served. The secrets in the
+configuration shown are masked by name (`psk`, and options with `secret`, `password`, `token` or a
+`_key` ending in the name).
+
+Consequences: The client and the router share one secret, and whoever holds it can make tokens,
+so it is long and random; it is checked to have at least 32 characters. The secret is in the
+configuration file and in `/run/vpn-router/web.json`, both readable by root only. The token and
+the answer cross the network unencrypted, so the page is for a trusted internal network; HTTPS
+would need a certificate that the package does not have. The masking is by option name, so a new
+secret option needs a name that matches.