226 lines
14 KiB
Markdown
226 lines
14 KiB
Markdown
# Architecture Decision Records
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## ADR-1: Docker's published ports are filtered in DOCKER-USER by the package
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Status: Accepted
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Context: Docker adds its own iptables rules for published ports. They are evaluated before
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UFW's, so a published port is reachable from the external interface whatever UFW says. Docker
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reserves the `DOCKER-USER` chain, evaluated first in `FORWARD`, for rules of the operator. The
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router is an internet-facing host, and Docker is installed on it for services run alongside the
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router.
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Decision: The package secures Docker itself. A `docker` configuration section has `mode`
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(`auto`, `on`, `off`) and `exposed_ports`. In `auto` mode the rules are applied when
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`docker.service` is installed, which is what the package looks up; `on` requires Docker and
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`off` removes the rules. The rules are in a chain of their own, `VPN-ROUTER-DOCKER`, entered by
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one jump at the top of `DOCKER-USER`. For traffic from the external interface they accept IPsec
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protected traffic, TCP to `exposed_ports` and established connections, and drop the rest. The
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`vpn-router-docker` unit applies them after `docker.service` starts and removes them when it
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stops; `vpn-router-setup` restarts it while Docker runs.
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Consequences: Installing Docker first is recommended; `auto` mode then finds it as soon as the
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package is configured. When Docker is installed after the package, the rules are applied by
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restarting `vpn-router-setup`, a reboot or a restart of Docker, not by Docker's first start,
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because `vpn-router-docker` is already active. A published
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port is reachable from the external interface only when it is in `exposed_ports`. Other rules
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in `DOCKER-USER` are not touched, because the package owns only its own chain. Restarting Docker
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restores the rules. The rules cover IPv4 only.
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## ADR-2: A status web service of the standard library, bound to the internal address
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Status: Accepted
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Context: The router has no way to see its state without a shell. A check from the protected
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network, by a person or by a monitor, needs a page or an endpoint. The router is an
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internet-facing host, so what the service shows and where it listens matter.
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Decision: The `vpn-router-web` unit runs one Python script that uses only the standard library, in
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the way of a small test server. It listens on the address of the internal interface and on
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`web.port`, 7081 by default, and is on unless `web.enabled` is `false` (ADR-7). `/` is for
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people, in plain text for `curl` and `wget` and as a Tailwind CSS page for a browser, and
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`/status` is the same status as JSON. It shows the state of the services, the package version
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and the state of the site-to-site, road-warrior, OpenVPN and WireGuard features, and nothing from
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the configuration. `vpn-router-setup` opens the port on the internal interface in UFW and writes
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the features and the facts it resolves to `/run/vpn-router/web.json`, so the service does not
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need the package's configuration code, which needs Jinja2.
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Consequences: Nothing outside the internal network can reach it, since it listens on the internal
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address only and the firewall opens the port on the internal interface only. `/` and `/status`
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have no authentication, which is acceptable because they are read only and show no
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configuration. It runs as root, because `swanctl` and `wg` need it, with `NoNewPrivileges`,
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`ProtectSystem=strict`, `ProtectHome` and `PrivateTmp`. The Tailwind style comes from a CDN, so a
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browser on a network without internet shows the unstyled page, and `curl` is unaffected.
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## ADR-3: /config is authenticated with a short-lived JWT signed with a shared secret
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Status: Accepted
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Context: A client such as the Router Manager needs the configuration and the resolved state of
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the router, which `/` and `/status` must not show. The service has only the standard library,
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listens on plain HTTP on the internal network, and its configuration file is readable by root
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only.
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Decision: `/config` needs a JWT in an `Authorization: Bearer` header. The service verifies HS256
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tokens with `hmac` and `hashlib`, with the secret `web.jwt_secret`, and accepts no other
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algorithm, because verifying RS256 or ES256 needs a library outside the standard library. The
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token must carry `exp`, not more than ten minutes ahead, so that the client makes a fresh token for
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each use and a leaked one expires. Without a secret the page is not served. The configuration
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shown has no secret options (ADR-8).
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Consequences: The client and the router share one secret, and whoever holds it can make tokens,
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so it is long and random; it is checked to have at least 32 characters. The secret is only in the
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configuration file, readable by root only. The token and the answer cross the network
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unencrypted, so the page is for a trusted internal network; HTTPS would need a certificate that
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the package does not have.
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## ADR-4: Users are kept in a local OpenLDAP directory
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Status: Accepted
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Context: OpenVPN needs user accounts with passwords, and the users are meant to cover every P2S
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system of the router, including IKEv2 EAP-TLS and WireGuard, which an OIDC provider cannot
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authenticate. Everything the router runs comes from the Ubuntu archive. slapd, its Python module
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`python3-ldap` and the OpenVPN plugin `openvpn-auth-ldap` are all in the archive; the plugin
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checks a password by binding as the user, so the hash scheme is slapd's alone. An OIDC provider
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can read the directory through its LDAP connector.
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Decision: The users are entries in slapd. The package keeps its own database,
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`dc=vpn-router,dc=local` in `/var/lib/ldap/vpn-router` (the path the slapd AppArmor profile
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allows), next to the database the slapd package creates from the host's domain, so the package
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does not depend on the debconf answers of slapd. Its root DN is root's peer-credential DN on the
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local socket, so `vpn-router-setup` manages it with SASL EXTERNAL and no administrator password
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exists. Users are changed by `cn=user-manager,ou=services`, whose ACLs allow writing `ou=users`
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and setting passwords (`=wx`) but not reading them. Passwords are hashed with ARGON2, and the
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ppolicy overlay locks a disabled user (`pwdAccountLockedTime`, effective with `pwdLockout`).
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slapd listens on the local socket only. Provisioning checks every step before changing it, so it
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runs at every start of `vpn-router-setup`.
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Consequences: The package depends on `slapd`, `python3-ldap`, `openvpn` and
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`openvpn-auth-ldap`, and `vpn-router-setup` and `vpn-router-user` import `python3-ldap`. The
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service account's password is a root-only file. The directory is user data: it is kept on purge,
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and it is backed up with `slapcat`, not by copying files.
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## ADR-5: OpenVPN clients need a certificate and the password of the user it names
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Status: Accepted
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Context: The router CA issues client certificates for IKEv2. OpenVPN can check a certificate, a
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password, or both. With both, a certificate alone and a password alone are not enough, but
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`openvpn-auth-ldap` checks only the password of the user name given at login, which need not be
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the user of the certificate.
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Decision: OpenVPN checks the certificate against the CA and the CRL, the password with
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`openvpn-auth-ldap`, and runs `openvpn-same-user` as `auth-user-pass-verify` with `via-env`,
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which accepts the login only when the user name equals the certificate's common name. OpenVPN
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calls both and needs both to pass. A user's certificate has the user name as its common name, is
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issued when the user is added and revoked when the user is removed. The CRL is created with the
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CA, so that `crl-verify` has a file from the start, and is valid for ten years, because the router
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renews it only when it revokes a certificate. The server uses the router's own certificate, whose
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common name is `wan.local_fqdn`; the client profile checks that name (`verify-x509-name`), since
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the CA's certificates carry both the server and the client purpose.
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Consequences: Disabling a user locks the password, which stops OpenVPN but not IKEv2, which
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checks certificates only; removing a user revokes the certificate for both. A user's
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certificate expires after the CA's default of a year and is renewed by removing and adding the
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user again.
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## ADR-6: The web service manages users by running vpn-router-user
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Status: Accepted
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Context: The web service has only the standard library (ADR-2), and managing users needs
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`python3-ldap` and `simple-ca`. Its unit is sandboxed with `ProtectSystem=strict`.
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Decision: `/users` runs `vpn-router-user` and returns its JSON answer, mapping its exit status
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to the HTTP status. Passwords go to the program on standard input, so they never appear in a
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command line. The unit allows writing the paths the program needs: `/run/vpn-router` for its
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lock, `/etc/vpn-router/pki` for the certificates and the CA's state, and `/etc/swanctl/x509crl`
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for the published CRL. A lock serializes the program's runs, because `simple-ca` keeps its state
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in one file.
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Consequences: The web service imports nothing outside the standard library, and the same program
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serves the command line and the API. Each request starts a process, which is fine for user
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administration. The API is protected by the token of `/config`, so whoever has the secret can
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manage users.
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## ADR-7: The web service starts on its own and reads its inputs at each request
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Status: Accepted
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Context: The status page is the tool to see a problem of `vpn-router-setup`, so it must run
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whatever state setup is in, also before setup has run and while it fails. The internal address,
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`web.port` and `web.enabled` can change at any time.
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Decision: The service depends on nothing to start and does not end on its own. A discovery
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thread works out, every 30 seconds, the internal address from `vpn-router.json` as
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`general.mode` directs, with the discovery code in `vpnrouter_net.py`, which uses only the
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standard library and which `vpnrouter.py` uses too, and `web.port` and `web.enabled` from the
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same file. It runs the HTTP server in a thread of its own at that address and port, stops it and
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starts a new one when they change, and runs none while the service is disabled, the address is
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not known or it cannot be bound. Each request reads the token secret from `vpn-router.json` and
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the features and resolved facts from `web.json`, which setup writes, because they need the
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configuration code; while that file is missing the router shows no features. The unit is not
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ordered after `vpn-router-setup`, and setup does not restart it.
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Consequences: The status page answers as soon as the internal address exists, whatever state
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setup is in, and follows a changed address, port or `web.enabled` within 30 seconds without a
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restart. A change of the secret applies to the next request. The interfaces are read every 30
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seconds, which costs a few `ip` commands.
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## ADR-8: /config leaves secret options out
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Status: Accepted
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Context: A secret has no use outside the router, so a client never needs it, and showing even
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that a secret is set tells a reader more than it needs.
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Decision: The configuration that `vpn-router-setup` writes for the web service has no secret
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options at all. They are recognised by name: `psk`, and options with `secret`, `password`,
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`token` or a `_key` ending in the name.
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Consequences: No secret reaches the web service's state file or any answer of the API. A client
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cannot tell whether a secret is set. A new secret option needs a name that matches.
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## ADR-9: The Router Manager is a separate package with a small Python server
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Status: Accepted
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Context: The router needs a web application to manage it, which may run on the router or on
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another Linux system. The router's web service needs a token signed with `web.jwt_secret` for
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`/config` and `/users` and sends no CORS headers, so a browser cannot call it with the secret
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kept away from the page.
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Decision: `vpn-router-manager` is a Debian package of its own, built in the repository
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`linux-cloud-router-manager`. A React and MUI application in the style of manage-ui is built
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with Vite when the package is built and shipped as static files. A Python server, using the
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Ubuntu packages `python3-jwt` and `python3-requests`, serves them and passes every `/api/<path>`
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call to the router's `/<path>`, signing a short-lived token with the router's secret; it holds
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no logic of the router's own. It reads its settings from `/etc/vpn-router-manager/manager.json`
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at each request; on the router the settings not given come from `vpn-router.json`, and it
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listens on the internal address like the status web service, with the same discovery thread.
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`vpn-router-setup` opens the manager's port on the internal interface while the manager is
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installed, so the firewall stays in one place. The manager asks for no authentication.
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Consequences: The router needs no Node.js; the package needs only `python3` and packages of the
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Ubuntu archive. The secret never reaches the browser. The application checks the shape of every
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answer of the router. Anyone who reaches the manager's port manages the router, so on another
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system it listens on `127.0.0.1` unless told otherwise. The router's firewall depends on whether
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the manager is installed, and a changed manager port needs `vpn-router-setup` to run again.
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## ADR-10: Users and their certificates do not depend on a P2S system being enabled
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Status: Accepted
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Context: Users are prepared before a P2S system is switched on, and the Router Manager manages
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them on any configured router, with or without one.
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Decision: Once a platform is chosen, `vpn-router-setup` always provisions the user directory in
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slapd and the CA with its CRL; the server certificate follows once `wan.local_fqdn` is set.
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`/users` and `vpn-router-user` rely only on the directory and the CA. Only the OpenVPN client
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profile needs OpenVPN to be enabled.
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Consequences: slapd always holds the directory and the CA always exists on a configured router,
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even when no P2S system uses them. An operator who wants a CA of their own must place it before
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choosing the platform.
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