Support for serving server well-known files (#11211)

Fixes https://github.com/matrix-org/synapse/issues/8308

4 files changed, 69 insertions, 4 deletions

diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 51eadf122d..218826741e 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -100,6 +100,7 @@ from synapse.rest.client.register import (
 from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
 from synapse.rest.synapse.client import build_synapse_client_resource_tree
+from synapse.rest.well_known import well_known_resource
 from synapse.server import HomeServer
 from synapse.storage.databases.main.censor_events import CensorEventsStore
 from synapse.storage.databases.main.client_ips import ClientIpWorkerStore
@@ -318,6 +319,8 @@ class GenericWorkerServer(HomeServer):
                     resources.update({CLIENT_API_PREFIX: resource})
 
                     resources.update(build_synapse_client_resource_tree(self))
+                    resources.update({"/.well-known": well_known_resource(self)})
+
                 elif name == "federation":
                     resources.update({FEDERATION_PREFIX: TransportLayerServer(self)})
                 elif name == "media":
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 93e2299266..336c279a44 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -66,7 +66,7 @@ from synapse.rest.admin import AdminRestResource
 from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
 from synapse.rest.synapse.client import build_synapse_client_resource_tree
-from synapse.rest.well_known import WellKnownResource
+from synapse.rest.well_known import well_known_resource
 from synapse.server import HomeServer
 from synapse.storage import DataStore
 from synapse.util.httpresourcetree import create_resource_tree
@@ -189,7 +189,7 @@ class SynapseHomeServer(HomeServer):
                     "/_matrix/client/unstable": client_resource,
                     "/_matrix/client/v2_alpha": client_resource,
                     "/_matrix/client/versions": client_resource,
-                    "/.well-known/matrix/client": WellKnownResource(self),
+                    "/.well-known": well_known_resource(self),
                     "/_synapse/admin": AdminRestResource(self),
                     **build_synapse_client_resource_tree(self),
                 }
diff --git a/synapse/config/server.py b/synapse/config/server.py
index ed094bdc44..a387fd9310 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -262,6 +262,7 @@ class ServerConfig(Config):
         self.print_pidfile = config.get("print_pidfile")
         self.user_agent_suffix = config.get("user_agent_suffix")
         self.use_frozen_dicts = config.get("use_frozen_dicts", False)
+        self.serve_server_wellknown = config.get("serve_server_wellknown", False)
 
         self.public_baseurl = config.get("public_baseurl")
         if self.public_baseurl is not None:
@@ -774,6 +775,24 @@ class ServerConfig(Config):
         #
         #public_baseurl: https://example.com/
 
+        # Uncomment the following to tell other servers to send federation traffic on
+        # port 443.
+        #
+        # By default, other servers will try to reach our server on port 8448, which can
+        # be inconvenient in some environments.
+        #
+        # Provided 'https://<server_name>/' on port 443 is routed to Synapse, this
+        # option configures Synapse to serve a file at
+        # 'https://<server_name>/.well-known/matrix/server'. This will tell other
+        # servers to send traffic to port 443 instead.
+        #
+        # See https://matrix-org.github.io/synapse/latest/delegate.html for more
+        # information.
+        #
+        # Defaults to 'false'.
+        #
+        #serve_server_wellknown: true
+
         # Set the soft limit on the number of file descriptors synapse can use
         # Zero is used to indicate synapse should set the soft limit to the
         # hard limit.
diff --git a/synapse/rest/well_known.py b/synapse/rest/well_known.py
index 7ac01faab4..edbf5ce5d0 100644
--- a/synapse/rest/well_known.py
+++ b/synapse/rest/well_known.py
@@ -21,6 +21,7 @@ from twisted.web.server import Request
 from synapse.http.server import set_cors_headers
 from synapse.types import JsonDict
 from synapse.util import json_encoder
+from synapse.util.stringutils import parse_server_name
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -47,8 +48,8 @@ class WellKnownBuilder:
         return result
 
 
-class WellKnownResource(Resource):
-    """A Twisted web resource which renders the .well-known file"""
+class ClientWellKnownResource(Resource):
+    """A Twisted web resource which renders the .well-known/matrix/client file"""
 
     isLeaf = 1
 
@@ -67,3 +68,45 @@ class WellKnownResource(Resource):
         logger.debug("returning: %s", r)
         request.setHeader(b"Content-Type", b"application/json")
         return json_encoder.encode(r).encode("utf-8")
+
+
+class ServerWellKnownResource(Resource):
+    """Resource for .well-known/matrix/server, redirecting to port 443"""
+
+    isLeaf = 1
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self._serve_server_wellknown = hs.config.server.serve_server_wellknown
+
+        host, port = parse_server_name(hs.config.server.server_name)
+
+        # If we've got this far, then https://<server_name>/ must route to us, so
+        # we just redirect the traffic to port 443 instead of 8448.
+        if port is None:
+            port = 443
+
+        self._response = json_encoder.encode({"m.server": f"{host}:{port}"}).encode(
+            "utf-8"
+        )
+
+    def render_GET(self, request: Request) -> bytes:
+        if not self._serve_server_wellknown:
+            request.setResponseCode(404)
+            request.setHeader(b"Content-Type", b"text/plain")
+            return b"404. Is anything ever truly *well* known?\n"
+
+        request.setHeader(b"Content-Type", b"application/json")
+        return self._response
+
+
+def well_known_resource(hs: "HomeServer") -> Resource:
+    """Returns a Twisted web resource which handles '.well-known' requests"""
+    res = Resource()
+    matrix_resource = Resource()
+    res.putChild(b"matrix", matrix_resource)
+
+    matrix_resource.putChild(b"server", ServerWellKnownResource(hs))
+    matrix_resource.putChild(b"client", ClientWellKnownResource(hs))
+
+    return res