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.

CLAUDE.md

@@ -41,6 +41,7 @@ verb commands and scaffold, but a program stands on its own via its declared
 Stack guides (for writing *new* code, AI-facing):
 
 - @docs/registry.md — Registry architecture, castle.yaml structure, lifecycle
+- @docs/dns-and-tls.md — DNS + TLS approach: gateway routing, LAN name resolution, the three `gateway.tls` modes (off/internal/acme)
 - @docs/stacks/python-fastapi.md — FastAPI service patterns (config, routes, models, testing)
 - @docs/stacks/python-cli.md — CLI tool patterns (argparse, stdin/stdout, piping, testing)
 - @docs/stacks/react-vite.md — React/Vite/TypeScript frontend patterns

docs/dns-and-tls.md

@@ -0,0 +1,210 @@
+# DNS & TLS in Castle
+
+How services on a Castle node become reachable by name and trusted over HTTPS —
+while staying **internal-only** (no external exposure). This is the conceptual
+companion to the field-level gateway reference in
+[registry.md](registry.md#proxy--how-the-gateway-routes-to-it).
+
+Two independent questions decide whether `https://foo.example/` works from a
+browser on your LAN:
+
+1. **Resolve** — does the name `foo.example` point at the Castle node? *(DNS)*
+2. **Trust** — is the certificate the node serves one the browser accepts? *(TLS)*
+
+Castle answers #1 by leaning on your LAN's own DNS, and #2 with a per-node choice
+of three TLS modes. They're orthogonal: you pick a resolution strategy and a trust
+strategy, and any working combination is fine.
+
+## The gateway is the single ingress
+
+Every reachable service goes through the Caddy **gateway** (`:9000` by default). A
+gateway route maps a public **address** to a **target**. Two address shapes matter
+for DNS and TLS:
+
+- **path prefix** (`/foo`) — reached at `http://<node>:9000/foo/`. Shares the
+  node's own name/port; needs no per-service DNS. Caddy **strips** the prefix, so
+  this only suits apps that don't assume they live at the origin root.
+- **host route** (`foo.lan`, `foo.example.com`) — reached at `https://foo.…/`. A
+  whole hostname proxied to the backend root, nothing stripped. This is the shape
+  that gets its own DNS name and its own TLS cert.
+
+> **Rule of thumb:** a service that needs HTTPS, a real origin, WebSockets, or
+> root-relative asset URLs wants a **host route**. Path prefixes are for simple,
+> prefix-agnostic backends. See
+> [registry.md](registry.md#path-prefix-vs-host-route--pick-by-whether-the-app-is-prefix-aware)
+> for the failure modes of putting a root-based app under a stripped prefix.
+
+Only host routes are the subject of the rest of this document — they're what DNS
+and TLS act on.
+
+## DNS: making a name resolve to the node
+
+A host route does nothing until `foo.…` resolves **to this node** on the clients
+that will use it. Castle does **not** run DNS; it relies on whatever already serves
+your LAN. Two facts shape the approach:
+
+- **Resolve on the clients that matter.** A name only needs to resolve for the
+  devices that browse it. That's usually your LAN's DHCP/DNS authority — often the
+  router — not a central or mesh resolver.
+- **One wildcard beats many records.** A single wildcard entry routes every
+  subdomain of a zone to the node, so each new host-routed service works with no
+  further DNS edits.
+
+### Split-horizon: internal names, no public exposure
+
+The names Castle serves resolve **only inside the LAN**. For a private zone this is
+automatic; for a public domain you own, it's deliberate split-horizon — your LAN
+resolver answers with the node's private IP, and the **public** zone has no `A`
+records for the services, so nothing is reachable from the internet.
+
+### Two zone styles
+
+| Zone style | Example | Who's authoritative | Wildcard record |
+|------------|---------|---------------------|-----------------|
+| **private TLD** | `*.civil.lan` | the LAN router/DHCP server (owns `.lan`) | `address=/civil.lan/<node-ip>` |
+| **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>` |
+
+Both give the same result — every `*.<zone>` name resolves to the node's LAN IP.
+The difference is which TLS modes each can use (below): a private TLD like `.lan`
+**cannot** get a publicly-trusted cert (it isn't a real domain), while a real
+subdomain can.
+
+### Worked topology (this network)
+
+- The **router** (`192.168.8.1`, a GL.iNet box) owns the `.lan` zone: it
+  auto-registers DHCP hostnames (`civil.lan`) and answers `*.lan`. Unknown `.lan`
+  names are **not** forwarded upstream — the router keeps that zone to itself. A
+  `*.civil.lan` wildcard therefore lives on the **router**.
+- The router **forwards everything else** (including `*.payne.io`) to
+  **wild-central**'s dnsmasq. So a `*.civil.payne.io` wildcard lives on
+  **wild-central** (`/etc/dnsmasq.d/civil-payne.conf`,
+  `address=/civil.payne.io/192.168.8.222`), which the router already routes to.
+  `payne.io`'s **public** authority is Cloudflare, which holds no `A` records for
+  these names — so `civil.payne.io` services resolve on the LAN and nowhere else.
+
+Pin the node's IP with a **DHCP reservation** — the wildcard hardcodes it, so a
+drifting dynamic lease would break every host route at once.
+
+## TLS: three trust modes
+
+`gateway.tls` (in `castle.yaml`) picks how host routes are served. It's a per-node
+choice; the modes are mutually exclusive.
+
+| `gateway.tls` | What the browser gets | Client setup | Use when |
+|---------------|-----------------------|--------------|----------|
+| `off` *(default)* | plain HTTP on `:9000` | none | you don't need HTTPS; localhost-only tools |
+| `internal` | HTTPS from Caddy's **local CA** | **install & trust the CA** on every device | LAN with a private `.lan` zone, few devices you control |
+| `acme` | HTTPS from a **real Let's Encrypt wildcard** | **nothing** | you own a domain; multiple devices (phones, etc.) |
+
+### `off` — plain HTTP
+
+The gateway generates `auto_https off` and listens on a bare `:9000`. Reach it at
+`http://<node>:9000/`. Simple, but a non-`localhost` HTTP page is **not** a browser
+"secure context" (see below), and there's no encryption.
+
+### `internal` — Caddy's local CA
+
+Each host route becomes its own `tls internal` HTTPS site, signed by a CA Caddy
+generates on the node. Browsers get a real secure context — but only if they
+**trust that private CA**, which means distributing the root cert to every device's
+system/browser trust store. That's the catch: some platforms (notably Android
+browsers, and Firefox everywhere, which uses its own store) make installing a
+custom CA painful or impossible. Castle helps by exposing the public root at
+`GET /gateway/ca.crt` with a dashboard download button — but the per-device trust
+step is unavoidable, and it's why `internal` doesn't scale past a handful of
+machines you fully control.
+
+Good fit: a `.lan` zone (which can't get a public cert anyway) with a couple of
+trusted laptops.
+
+### `acme` — real Let's Encrypt wildcard via DNS-01
+
+The gateway obtains a genuine, publicly-trusted **wildcard** cert (`*.<domain>`)
+from Let's Encrypt using a **DNS-01** challenge. Every browser and phone trusts it
+with **zero setup** — while the services stay internal-only.
+
+How it stays internal:
+
+- **DNS-01 proves ownership without exposure.** Caddy writes a transient
+  `_acme-challenge.<domain>` TXT record to the **public** zone via your DNS
+  provider's API; Let's Encrypt reads it over public DNS and issues the cert. No
+  inbound connection, no open port, no public `A` record for any service is ever
+  needed. (HTTP-01 can't validate a wildcard, so DNS-01 — and thus a provider API
+  token — is mandatory here.)
+- **Only LAN DNS points at the node.** The public zone stays `A`-record-free; your
+  LAN resolver answers `*.<domain>` with the private IP. Public internet sees
+  nothing.
+
+One `*.<domain>` site means a **single cert** covers every host route, and Caddy
+**auto-renews** it — adding a service needs no new cert and no DNS-01 round trip.
+Host-route subdomains are derived from the **service name**: a service opts into a
+host route with `proxy.caddy.host`, and it's published at `<service>.<domain>`.
+
+This is the recommended mode when you own a domain and want to reach services from
+arbitrary devices.
+
+## Why HTTPS at all — the secure-context requirement
+
+Beyond eavesdropping protection, HTTPS unlocks browser capabilities gated to a
+**secure context**: `crypto.subtle` (WebCrypto), service workers, and anything
+built on them (device identity, end-to-end crypto). Browsers treat only `https://`
+and `http://localhost` as secure — a plain-HTTP page on a LAN hostname is **not**,
+so such apps break there. That's the concrete reason to move a host route to
+`internal` or `acme` rather than leaving it on `off`.
+
+Note: a host served over HTTPS has its own **origin** (`https://foo.example`, no
+port). An app that allowlists origins, or an OAuth/token flow, must include the new
+origin — moving a service between modes changes its origin.
+
+## Putting a service on trusted HTTPS — the recipe
+
+1. **Give it a host route.** In the service's `proxy.caddy`, set `host:` (drop any
+   `path_prefix`). The literal host value is used as-is in `internal` mode; in
+   `acme` mode the published name is derived as `<service>.<domain>`.
+2. **Make the name resolve.** Add (or rely on) the LAN wildcard for the zone
+   (§DNS). Verify: `dig +short <service>.<zone>` → the node's IP.
+3. **Pick a trust mode** on the gateway (`gateway.tls`), plus the operational
+   prerequisites for it (below).
+4. **Deploy & reload:** `castle deploy` regenerates the Caddyfile and reloads Caddy.
+5. **Update the app's origin allowlist** if it has one (§secure context).
+
+## Operational prerequisites
+
+Both HTTPS modes need the gateway to bind privileged ports; `acme` also needs a
+plugin-enabled Caddy and a DNS token.
+
+- **Bind `:443`/`:80`.** Caddy serves HTTPS on `:443` (and redirects `:80`). A
+  user-level gateway can't bind privileged ports under `NoNewPrivileges`, so lower
+  the floor once: `net.ipv4.ip_unprivileged_port_start=80`, persisted in
+  `/etc/sysctl.d/`. (This beats `setcap`, which `NoNewPrivileges=true` would void.)
+- **`acme` only — a DNS-plugin Caddy.** Stock Caddy has no DNS-provider modules.
+  Build one: `./install.sh --with-dns-plugin=<provider>` (uses `xcaddy`, installs
+  to `/usr/local/bin/caddy`, which precedes the apt binary on `PATH`, so the
+  gateway picks it up on the next deploy). Castle now owns updates to that binary.
+- **`acme` only — a provider API token.** Store it as a secret
+  (`~/.castle/secrets/<TOKEN_NAME>`, scope: the DNS provider's "edit DNS records"
+  permission for your zone) and map it into the gateway service env in
+  `services/castle-gateway.yaml` (`defaults.env`), so Caddy reads it as
+  `{env.<TOKEN_NAME>}`. `castle deploy` warns if the domain, env var, or secret is
+  missing.
+- **`acme` — stage first.** Set `CASTLE_ACME_STAGING=1` at deploy to use Let's
+  Encrypt's staging CA (generous rate limits) while verifying issuance, then unset
+  it and redeploy for a browser-trusted production cert.
+
+## Choosing a combination
+
+| You have… | Zone (DNS) | Trust (TLS) | Result |
+|-----------|-----------|-------------|--------|
+| a quick internal tool, HTTP is fine | path prefix or `.lan` host | `off` | `http://node:9000/tool/` |
+| a `.lan` LAN, a couple of trusted machines | `*.node.lan` on the router | `internal` | HTTPS, install the CA per device |
+| a domain you own + many devices (phones) | `*.sub.domain` on the LAN resolver | `acme` | HTTPS, **no client setup**, internal-only |
+
+The last row is the sweet spot for a multi-device personal LAN, and what this node
+runs today: `*.civil.payne.io` (wild-central DNS) + a Let's Encrypt wildcard via
+Cloudflare DNS-01, so e.g. `https://openclaw.civil.payne.io/` is trusted on any
+device with nothing to install.
+
+## See also
+
+- [registry.md — `proxy`, gateway routes, and the `gateway.tls` modes](registry.md#proxy--how-the-gateway-routes-to-it)
+  — the field-level reference (Caddyfile shapes, exact config keys, the CA-download endpoint).