sni-router: break domain-fronting loop

contrib/sni-router/README.md

@@ -47,15 +47,52 @@ docker compose exec mtg mtg access /config/config.toml
 
 HAProxy forwards TCP connections to mtg and Caddy with a PROXY protocol
 v2 header so both backends see the real client IP instead of HAProxy's
-container address.  The three pieces must stay in sync:
+container address.  Caddy also receives PROXY v2 from mtg on the
+fronting path (see "Fronting loop" below), so all four pieces below
+must stay in sync:
 
 - `haproxy.cfg` — `send-proxy-v2` on the `mtg` and `web` backend `server` lines
-- `mtg-config.toml` — `proxy-protocol-listener = true`
+- `mtg-config.toml` — `proxy-protocol-listener = true` (HAProxy → mtg)
+- `mtg-config.toml` — `[domain-fronting].proxy-protocol = true` (mtg → Caddy on fronting)
 - `Caddyfile` — `listener_wrappers { proxy_protocol { ... } tls }` on `:8443`
 
-If you disable one, disable all three, otherwise the backend will fail
+If you disable one, disable all four, otherwise the backend will fail
 to parse the connection.
 
+## Fronting loop (why `[domain-fronting]` is set explicitly)
+
+When mtg sees TLS that isn't valid Telegram (a probe or a browser
+hitting the domain on `:443`), it forwards that connection to a real
+web server — "domain fronting".  By default mtg uses the secret's
+hostname as the fronting target and resolves it via DNS, which in
+this setup points back to this server: the fronting dial lands on
+HAProxy, SNI matches the secret, HAProxy routes the connection back
+to mtg → loop.
+
+The trigger is DNS, not name equality: any time the secret's hostname
+resolves to this host, the loop reproduces.  In an SNI-router
+deployment the secret's hostname has to point here for clients to
+reach mtg in the first place, so the loop is the default state unless
+mtg is steered away from HAProxy.
+
+`mtg-config.toml` therefore pins the fronting target to the Caddy
+container directly:
+
+```toml
+[domain-fronting]
+host = "web"
+port = 8443
+proxy-protocol = true
+```
+
+`host = "web"` resolves through compose-network DNS to the `web`
+service (Caddy), bypassing HAProxy.  `proxy-protocol = true` matches
+Caddy's `:8443` listener wrapper so the real client IP still
+propagates to Caddy's logs.
+
+Requires mtg ≥ 2.4 — hostname acceptance for the fronting target was
+added in #480.
+
 ## ACME (Let's Encrypt) notes
 
 HAProxy passes `/.well-known/acme-challenge/` requests on `:80` to

contrib/sni-router/mtg-config.toml

@@ -11,6 +11,17 @@ bind-to = "0.0.0.0:3128"
 # real client IP.  Keep this in sync with haproxy.cfg (`send-proxy-v2`).
 proxy-protocol-listener = true
 
+# Fronting target: point mtg at the Caddy container directly so its
+# fallback dial (for non-Telegram TLS) bypasses HAProxy and doesn't
+# loop back here.  Without this, mtg resolves the secret's hostname
+# via DNS, which in this setup resolves to this server -> HAProxy ->
+# mtg again.  See README's "Fronting loop" section for the long form.
+# Requires mtg >= 2.4 (#480 added hostname acceptance for the target).
+[domain-fronting]
+host = "web"
+port = 8443
+proxy-protocol = true
+
 [defense.anti-replay]
 enabled = true
 max-size = "1mib"