improve documentation of federation

.env.sample

@@ -26,6 +26,10 @@ SECRET_REGISTRATION_VERSION=v1
 # Set "true" to enable federation endpoint on $DOMAIN/.well-known/matrix/server
 SERVE_SERVER_WELLKNOWN=false
 
+# Serve /.well-known/matrix/{server,client} on SERVER_NAME via Traefik.
+# Can be used when SERVER_NAME != DOMAIN and SERVER_NAME is served by Traefik.
+#COMPOSE_FILE="$COMPOSE_FILE:compose.wellknown.yml"
+
 ALLOW_PUBLIC_ROOMS_FEDERATION=false
 
 ## Registration

README.md

@@ -43,7 +43,9 @@ Federation is on by default (`DISABLE_FEDERATION=0`). Remote homeservers need a
 
 #### Option 1: built-in well-known (`SERVER_NAME` = `DOMAIN`)
 
-Set `SERVE_SERVER_WELLKNOWN=true` and leave `SERVER_NAME` unset (defaults to `DOMAIN`). The recipe's nginx serves `/.well-known/matrix/server` and `/.well-known/matrix/client` on `DOMAIN`. Suitable when users are e.g. `@alice:matrix.example.com`.
+Set `SERVE_SERVER_WELLKNOWN=true` and leave `SERVER_NAME` unset (defaults to `DOMAIN`). The recipe's nginx serves `/.well-known/matrix/server` and `/.well-known/matrix/client` on `DOMAIN`. 
+
+Suitable when users are e.g. `@alice:matrix.example.com`.
 
 #### Option 2: external well-known on `SERVER_NAME`
 
@@ -55,11 +57,25 @@ DOMAIN=matrix.example.com
 SERVE_SERVER_WELLKNOWN=false
 ```
 
-Then configure whatever web service hosts `example.com` to serve:
+The two paths that must be served on `SERVER_NAME` are:
 
 - `https://example.com/.well-known/matrix/server` → `{"m.server": "matrix.example.com:443"}`
 - `https://example.com/.well-known/matrix/client` → `{"m.homeserver": {"base_url": "https://matrix.example.com"}}`
 
+**Recommended — let this recipe serve them via Traefik** by enabling `compose.wellknown.yml`:
+
+```
+COMPOSE_FILE="$COMPOSE_FILE:compose.wellknown.yml"
+```
+
+This publishes a Traefik router `Host(${SERVER_NAME}) && PathPrefix(/.well-known/matrix)`
+pointing at the matrix nginx, which already serves both files. The path-scoped, high-priority
+rule coexists with any apex website that also serves `Host(${SERVER_NAME})` — that site keeps
+serving everything except `/.well-known/matrix`. `SERVER_NAME` must resolve to this Traefik so
+ACME can issue its certificate.
+
+**Alternative** — serve the two files yourself from whatever already hosts `example.com`.
+
 
 #### Option 3: Traefik `matrix-federation` entrypoint (port 8448)
 
@@ -72,6 +88,23 @@ Requirements:
 
 With these in place, the recipe publishes a Traefik router on `Host(${SERVER_NAME})` via the `matrix-federation` entrypoint, reusing the existing matrix nginx → synapse path.
 
+#### Option 4: DNS SRV records (usually not viable here)
+
+Federation can also be delegated with a DNS `SRV` record on `SERVER_NAME` instead of well-known:
+
+```
+_matrix-fed._tcp.example.com.  3600 IN SRV 10 0 8448 matrix.example.com.   # modern
+_matrix._tcp.example.com.      3600 IN SRV 10 0 8448 matrix.example.com.   # deprecated, for older peers
+```
+
+The catch is TLS: on the SRV path a remote validates the certificate against **`SERVER_NAME`**, *not* the SRV target. This recipe's Traefik only issues a cert for **`DOMAIN`**, so:
+
+- **SRV → `DOMAIN`:443 fails** — the presented cert is for `DOMAIN`, but the peer requires one for `SERVER_NAME`.
+- **SRV → `SERVER_NAME`:443 collides** — Traefik routes TLS by SNI, and `Host(SERVER_NAME)` on `:443` is already owned by whatever apex site serves `SERVER_NAME`.
+- **SRV → `SERVER_NAME`:8448 works** — the Option 3 `matrix-federation` router holds a cert for `SERVER_NAME` — but that's just Option 3 made explicit (the `:8448` fallback already works with no SRV record).
+
+So I think SRV does little here. Probably prefer Option 2 (well-known).
+
 #### Verifying
 
 The canonical test:
@@ -93,7 +126,8 @@ curl https://DOMAIN/_matrix/key/v2/server
 
 ### Getting client discovery on a custom domain
 
-You'll need to deploy something like [this](https://git.autonomic.zone/ruangrupa/well-known-uris). This could be implemented in this recipe but we haven't merged it in yet. Change sets are welcome.
+Enable `compose.wellknown.yml` (see Option 2 above) — it serves `/.well-known/matrix/client`
+on `SERVER_NAME` too, so clients signing in as `@alice:example.com` auto-discover the homeserver.
 
 ## Bridges
 For all Bridges:

compose.wellknown.yml

@@ -0,0 +1,24 @@
+---
+version: "3.8"
+
+# Serve /.well-known/matrix/{server,client} on SERVER_NAME via Traefik, routed to
+# the matrix nginx (`web`) — so server/client delegation works without hand-placing
+# files on whatever else hosts SERVER_NAME.
+#
+# Enable when SERVER_NAME != DOMAIN (users are @alice:example.com, Synapse runs at
+# matrix.example.com). The PathPrefix rule is more specific than a bare Host()
+# router, and the explicit high priority guarantees it wins over any apex website
+# that also serves Host(SERVER_NAME) — so the two coexist, the apex site keeps
+# serving everything except /.well-known/matrix.
+#
+# Requires SERVER_NAME to resolve to this Traefik so ACME can issue its cert.
+services:
+  web:
+    deploy:
+      labels:
+        - "traefik.http.routers.${STACK_NAME}-wellknown.rule=Host(`${SERVER_NAME}`) && PathPrefix(`/.well-known/matrix`)"
+        - "traefik.http.routers.${STACK_NAME}-wellknown.entrypoints=web-secure"
+        - "traefik.http.routers.${STACK_NAME}-wellknown.tls=true"
+        - "traefik.http.routers.${STACK_NAME}-wellknown.tls.certresolver=${LETS_ENCRYPT_ENV}"
+        - "traefik.http.routers.${STACK_NAME}-wellknown.service=${STACK_NAME}"
+        - "traefik.http.routers.${STACK_NAME}-wellknown.priority=1000"