Add docs/dns-and-tls.md: castle's DNS + TLS approach
A conceptual companion to registry.md's field-level gateway reference. Covers the two questions (resolve / trust) an internal-only LAN service must answer: - DNS: gateway-as-ingress, path-prefix vs host routes, LAN name resolution, wildcard records, split-horizon (internal names, no public A records), and the private-TLD vs public-subdomain zone styles with the worked router/wild-central topology. - TLS: the three gateway.tls modes (off / internal / acme) with when-to-use, the internal-CA distribution pain vs the acme DNS-01 "no client setup" win, the secure-context (WebCrypto) rationale, the put-a-service-on-HTTPS recipe, operational prerequisites (443/80 bind, plugin caddy, DNS token, staging), and a decision table. Linked from CLAUDE.md's docs list.
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@@ -41,6 +41,7 @@ verb commands and scaffold, but a program stands on its own via its declared
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Stack guides (for writing *new* code, AI-facing):
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- @docs/registry.md — Registry architecture, castle.yaml structure, lifecycle
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- @docs/dns-and-tls.md — DNS + TLS approach: gateway routing, LAN name resolution, the three `gateway.tls` modes (off/internal/acme)
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- @docs/stacks/python-fastapi.md — FastAPI service patterns (config, routes, models, testing)
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- @docs/stacks/python-cli.md — CLI tool patterns (argparse, stdin/stdout, piping, testing)
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- @docs/stacks/react-vite.md — React/Vite/TypeScript frontend patterns
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# DNS & TLS in Castle
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How services on a Castle node become reachable by name and trusted over HTTPS —
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while staying **internal-only** (no external exposure). This is the conceptual
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companion to the field-level gateway reference in
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[registry.md](registry.md#proxy--how-the-gateway-routes-to-it).
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Two independent questions decide whether `https://foo.example/` works from a
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browser on your LAN:
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1. **Resolve** — does the name `foo.example` point at the Castle node? *(DNS)*
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2. **Trust** — is the certificate the node serves one the browser accepts? *(TLS)*
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Castle answers #1 by leaning on your LAN's own DNS, and #2 with a per-node choice
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of three TLS modes. They're orthogonal: you pick a resolution strategy and a trust
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strategy, and any working combination is fine.
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## The gateway is the single ingress
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Every reachable service goes through the Caddy **gateway** (`:9000` by default). A
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gateway route maps a public **address** to a **target**. Two address shapes matter
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for DNS and TLS:
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- **path prefix** (`/foo`) — reached at `http://<node>:9000/foo/`. Shares the
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node's own name/port; needs no per-service DNS. Caddy **strips** the prefix, so
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this only suits apps that don't assume they live at the origin root.
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- **host route** (`foo.lan`, `foo.example.com`) — reached at `https://foo.…/`. A
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whole hostname proxied to the backend root, nothing stripped. This is the shape
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that gets its own DNS name and its own TLS cert.
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> **Rule of thumb:** a service that needs HTTPS, a real origin, WebSockets, or
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> root-relative asset URLs wants a **host route**. Path prefixes are for simple,
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> prefix-agnostic backends. See
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> [registry.md](registry.md#path-prefix-vs-host-route--pick-by-whether-the-app-is-prefix-aware)
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> for the failure modes of putting a root-based app under a stripped prefix.
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Only host routes are the subject of the rest of this document — they're what DNS
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and TLS act on.
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## DNS: making a name resolve to the node
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A host route does nothing until `foo.…` resolves **to this node** on the clients
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that will use it. Castle does **not** run DNS; it relies on whatever already serves
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your LAN. Two facts shape the approach:
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- **Resolve on the clients that matter.** A name only needs to resolve for the
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devices that browse it. That's usually your LAN's DHCP/DNS authority — often the
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router — not a central or mesh resolver.
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- **One wildcard beats many records.** A single wildcard entry routes every
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subdomain of a zone to the node, so each new host-routed service works with no
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further DNS edits.
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### Split-horizon: internal names, no public exposure
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The names Castle serves resolve **only inside the LAN**. For a private zone this is
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automatic; for a public domain you own, it's deliberate split-horizon — your LAN
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resolver answers with the node's private IP, and the **public** zone has no `A`
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records for the services, so nothing is reachable from the internet.
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### Two zone styles
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| Zone style | Example | Who's authoritative | Wildcard record |
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|------------|---------|---------------------|-----------------|
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| **private TLD** | `*.civil.lan` | the LAN router/DHCP server (owns `.lan`) | `address=/civil.lan/<node-ip>` |
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| **subdomain of a public domain** | `*.civil.payne.io` | your public DNS host (e.g. Cloudflare), but **answered internally** by a LAN resolver | `address=/civil.payne.io/<node-ip>` |
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Both give the same result — every `*.<zone>` name resolves to the node's LAN IP.
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The difference is which TLS modes each can use (below): a private TLD like `.lan`
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**cannot** get a publicly-trusted cert (it isn't a real domain), while a real
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subdomain can.
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### Worked topology (this network)
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- The **router** (`192.168.8.1`, a GL.iNet box) owns the `.lan` zone: it
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auto-registers DHCP hostnames (`civil.lan`) and answers `*.lan`. Unknown `.lan`
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names are **not** forwarded upstream — the router keeps that zone to itself. A
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`*.civil.lan` wildcard therefore lives on the **router**.
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- The router **forwards everything else** (including `*.payne.io`) to
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**wild-central**'s dnsmasq. So a `*.civil.payne.io` wildcard lives on
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**wild-central** (`/etc/dnsmasq.d/civil-payne.conf`,
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`address=/civil.payne.io/192.168.8.222`), which the router already routes to.
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`payne.io`'s **public** authority is Cloudflare, which holds no `A` records for
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these names — so `civil.payne.io` services resolve on the LAN and nowhere else.
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Pin the node's IP with a **DHCP reservation** — the wildcard hardcodes it, so a
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drifting dynamic lease would break every host route at once.
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## TLS: three trust modes
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`gateway.tls` (in `castle.yaml`) picks how host routes are served. It's a per-node
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choice; the modes are mutually exclusive.
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| `gateway.tls` | What the browser gets | Client setup | Use when |
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|---------------|-----------------------|--------------|----------|
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| `off` *(default)* | plain HTTP on `:9000` | none | you don't need HTTPS; localhost-only tools |
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| `internal` | HTTPS from Caddy's **local CA** | **install & trust the CA** on every device | LAN with a private `.lan` zone, few devices you control |
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| `acme` | HTTPS from a **real Let's Encrypt wildcard** | **nothing** | you own a domain; multiple devices (phones, etc.) |
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### `off` — plain HTTP
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The gateway generates `auto_https off` and listens on a bare `:9000`. Reach it at
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`http://<node>:9000/`. Simple, but a non-`localhost` HTTP page is **not** a browser
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"secure context" (see below), and there's no encryption.
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### `internal` — Caddy's local CA
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Each host route becomes its own `tls internal` HTTPS site, signed by a CA Caddy
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generates on the node. Browsers get a real secure context — but only if they
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**trust that private CA**, which means distributing the root cert to every device's
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system/browser trust store. That's the catch: some platforms (notably Android
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browsers, and Firefox everywhere, which uses its own store) make installing a
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custom CA painful or impossible. Castle helps by exposing the public root at
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`GET /gateway/ca.crt` with a dashboard download button — but the per-device trust
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step is unavoidable, and it's why `internal` doesn't scale past a handful of
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machines you fully control.
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Good fit: a `.lan` zone (which can't get a public cert anyway) with a couple of
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trusted laptops.
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### `acme` — real Let's Encrypt wildcard via DNS-01
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The gateway obtains a genuine, publicly-trusted **wildcard** cert (`*.<domain>`)
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from Let's Encrypt using a **DNS-01** challenge. Every browser and phone trusts it
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with **zero setup** — while the services stay internal-only.
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How it stays internal:
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- **DNS-01 proves ownership without exposure.** Caddy writes a transient
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`_acme-challenge.<domain>` TXT record to the **public** zone via your DNS
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provider's API; Let's Encrypt reads it over public DNS and issues the cert. No
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inbound connection, no open port, no public `A` record for any service is ever
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needed. (HTTP-01 can't validate a wildcard, so DNS-01 — and thus a provider API
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token — is mandatory here.)
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- **Only LAN DNS points at the node.** The public zone stays `A`-record-free; your
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LAN resolver answers `*.<domain>` with the private IP. Public internet sees
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nothing.
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One `*.<domain>` site means a **single cert** covers every host route, and Caddy
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**auto-renews** it — adding a service needs no new cert and no DNS-01 round trip.
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Host-route subdomains are derived from the **service name**: a service opts into a
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host route with `proxy.caddy.host`, and it's published at `<service>.<domain>`.
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This is the recommended mode when you own a domain and want to reach services from
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arbitrary devices.
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## Why HTTPS at all — the secure-context requirement
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Beyond eavesdropping protection, HTTPS unlocks browser capabilities gated to a
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**secure context**: `crypto.subtle` (WebCrypto), service workers, and anything
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built on them (device identity, end-to-end crypto). Browsers treat only `https://`
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and `http://localhost` as secure — a plain-HTTP page on a LAN hostname is **not**,
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so such apps break there. That's the concrete reason to move a host route to
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`internal` or `acme` rather than leaving it on `off`.
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Note: a host served over HTTPS has its own **origin** (`https://foo.example`, no
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port). An app that allowlists origins, or an OAuth/token flow, must include the new
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origin — moving a service between modes changes its origin.
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## Putting a service on trusted HTTPS — the recipe
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1. **Give it a host route.** In the service's `proxy.caddy`, set `host:` (drop any
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`path_prefix`). The literal host value is used as-is in `internal` mode; in
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`acme` mode the published name is derived as `<service>.<domain>`.
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2. **Make the name resolve.** Add (or rely on) the LAN wildcard for the zone
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(§DNS). Verify: `dig +short <service>.<zone>` → the node's IP.
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3. **Pick a trust mode** on the gateway (`gateway.tls`), plus the operational
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prerequisites for it (below).
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4. **Deploy & reload:** `castle deploy` regenerates the Caddyfile and reloads Caddy.
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5. **Update the app's origin allowlist** if it has one (§secure context).
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## Operational prerequisites
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Both HTTPS modes need the gateway to bind privileged ports; `acme` also needs a
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plugin-enabled Caddy and a DNS token.
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- **Bind `:443`/`:80`.** Caddy serves HTTPS on `:443` (and redirects `:80`). A
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user-level gateway can't bind privileged ports under `NoNewPrivileges`, so lower
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the floor once: `net.ipv4.ip_unprivileged_port_start=80`, persisted in
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`/etc/sysctl.d/`. (This beats `setcap`, which `NoNewPrivileges=true` would void.)
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- **`acme` only — a DNS-plugin Caddy.** Stock Caddy has no DNS-provider modules.
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Build one: `./install.sh --with-dns-plugin=<provider>` (uses `xcaddy`, installs
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to `/usr/local/bin/caddy`, which precedes the apt binary on `PATH`, so the
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gateway picks it up on the next deploy). Castle now owns updates to that binary.
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- **`acme` only — a provider API token.** Store it as a secret
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(`~/.castle/secrets/<TOKEN_NAME>`, scope: the DNS provider's "edit DNS records"
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permission for your zone) and map it into the gateway service env in
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`services/castle-gateway.yaml` (`defaults.env`), so Caddy reads it as
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`{env.<TOKEN_NAME>}`. `castle deploy` warns if the domain, env var, or secret is
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missing.
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- **`acme` — stage first.** Set `CASTLE_ACME_STAGING=1` at deploy to use Let's
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Encrypt's staging CA (generous rate limits) while verifying issuance, then unset
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it and redeploy for a browser-trusted production cert.
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## Choosing a combination
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| You have… | Zone (DNS) | Trust (TLS) | Result |
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|-----------|-----------|-------------|--------|
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| a quick internal tool, HTTP is fine | path prefix or `.lan` host | `off` | `http://node:9000/tool/` |
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| a `.lan` LAN, a couple of trusted machines | `*.node.lan` on the router | `internal` | HTTPS, install the CA per device |
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| a domain you own + many devices (phones) | `*.sub.domain` on the LAN resolver | `acme` | HTTPS, **no client setup**, internal-only |
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The last row is the sweet spot for a multi-device personal LAN, and what this node
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runs today: `*.civil.payne.io` (wild-central DNS) + a Let's Encrypt wildcard via
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Cloudflare DNS-01, so e.g. `https://openclaw.civil.payne.io/` is trusted on any
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device with nothing to install.
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## See also
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- [registry.md — `proxy`, gateway routes, and the `gateway.tls` modes](registry.md#proxy--how-the-gateway-routes-to-it)
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— the field-level reference (Caddyfile shapes, exact config keys, the CA-download endpoint).
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