An federation whitelist query endpoint extension (#16848)

This is to allow clients to query the configured federation whitelist.
Disabled by default.

---------

Co-authored-by: Devon Hudson <devonhudson@librem.one>
Co-authored-by: devonh <devon.dmytro@gmail.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

2 files changed, 70 insertions, 0 deletions

diff --git a/synapse/rest/synapse/client/__init__.py b/synapse/rest/synapse/client/__init__.py
index ba6576d4db..7b5bfc0421 100644
--- a/synapse/rest/synapse/client/__init__.py
+++ b/synapse/rest/synapse/client/__init__.py
@@ -23,6 +23,7 @@ from typing import TYPE_CHECKING, Mapping
 
 from twisted.web.resource import Resource
 
+from synapse.rest.synapse.client.federation_whitelist import FederationWhitelistResource
 from synapse.rest.synapse.client.new_user_consent import NewUserConsentResource
 from synapse.rest.synapse.client.pick_idp import PickIdpResource
 from synapse.rest.synapse.client.pick_username import pick_username_resource
@@ -77,6 +78,9 @@ def build_synapse_client_resource_tree(hs: "HomeServer") -> Mapping[str, Resourc
         # To be removed in Synapse v1.32.0.
         resources["/_matrix/saml2"] = res
 
+    if hs.config.federation.federation_whitelist_endpoint_enabled:
+        resources[FederationWhitelistResource.PATH] = FederationWhitelistResource(hs)
+
     if hs.config.experimental.msc4108_enabled:
         resources["/_synapse/client/rendezvous"] = MSC4108RendezvousSessionResource(hs)
 
diff --git a/synapse/rest/synapse/client/federation_whitelist.py b/synapse/rest/synapse/client/federation_whitelist.py
new file mode 100644
index 0000000000..2b8f0320e0
--- /dev/null
+++ b/synapse/rest/synapse/client/federation_whitelist.py
@@ -0,0 +1,66 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2024 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+
+import logging
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.http.server import DirectServeJsonResource
+from synapse.http.site import SynapseRequest
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class FederationWhitelistResource(DirectServeJsonResource):
+    """Custom endpoint (disabled by default) to fetch the federation whitelist
+    config.
+
+    Only enabled if `federation_whitelist_endpoint_enabled` feature is enabled.
+
+    Response format:
+
+        {
+            "whitelist_enabled": true,  // Whether the federation whitelist is being enforced
+            "whitelist": [  // Which server names are allowed by the whitelist
+                "example.com"
+            ]
+        }
+    """
+
+    PATH = "/_synapse/client/v1/config/federation_whitelist"
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+
+        self._federation_whitelist = hs.config.federation.federation_domain_whitelist
+
+        self._auth = hs.get_auth()
+
+    async def _async_render_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        await self._auth.get_user_by_req(request)
+
+        whitelist = []
+        if self._federation_whitelist:
+            # federation_whitelist is actually a dict, not a list
+            whitelist = list(self._federation_whitelist)
+
+        return_dict: JsonDict = {
+            "whitelist_enabled": self._federation_whitelist is not None,
+            "whitelist": whitelist,
+        }
+
+        return 200, return_dict