An federation whitelist query endpoint extension

docs/usage/configuration/config_documentation.md

@@ -4541,3 +4541,34 @@ background_updates:
     min_batch_size: 10
     default_batch_size: 50
 ```
+
+
+---
+## Extension features
+Configuration for extension features for Synapse
+
+---
+### `extension_federation_whitelist_endpoint`
+
+Enables an endpoint for fetching the federation whitelist config.
+
+The request path is `/_synapse/client/config/federation_whitelist`, and the
+response format is:
+
+```json
+{
+    "whitelist_enabled": true,  // Whether there is a federation whitelist
+    "whitelist": [  // Which hosts are allowed by the whitelist
+        "example.com"
+    ]
+}
+```
+
+If `whitelist_enabled` is `false` then the server can federate with all others.
+
+The endpoint requires authentication.
+
+Example configuration:
+```yaml
+extension_federation_whitelist_endpoint: true
+```

synapse/config/_base.pyi

@@ -31,6 +31,7 @@ from synapse.config import (  # noqa: F401
     database,
     emailconfig,
     experimental,
+    extensions,
     federation,
     jwt,
     key,
@@ -120,6 +121,7 @@ class RootConfig:
     federation: federation.FederationConfig
     retention: retention.RetentionConfig
     background_updates: background_updates.BackgroundUpdateConfig
+    extensions: extensions.ExtensionsConfig
 
     config_classes: List[Type["Config"]] = ...
     config_files: List[str]

synapse/config/extensions.py

@@ -0,0 +1,29 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2023 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>.
+#
+
+from typing import Any
+
+from synapse.config._base import Config
+from synapse.types import JsonDict
+
+
+class ExtensionsConfig(Config):
+    """Config section for enabling extension features"""
+
+    section = "extensions"
+
+    def read_config(self, config: JsonDict, **kwargs: Any) -> None:
+        self.federation_whitelist_endpoint: bool = config.get(
+            "extension_federation_whitelist_endpoint", False
+        )

synapse/config/homeserver.py

@@ -31,6 +31,7 @@
 from .database import DatabaseConfig
 from .emailconfig import EmailConfig
 from .experimental import ExperimentalConfig
+from .extensions import ExtensionsConfig
 from .federation import FederationConfig
 from .jwt import JWTConfig
 from .key import KeyConfig
@@ -105,4 +106,5 @@ class HomeServerConfig(RootConfig):
         RedisConfig,
         ExperimentalConfig,
         BackgroundUpdateConfig,
+        ExtensionsConfig,
     ]

synapse/rest/synapse/client/__init__.py

@@ -23,6 +23,7 @@
 
 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
@@ -76,6 +77,9 @@ 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.extensions.federation_whitelist_endpoint:
+        resources[FederationWhitelistResource.PATH] = FederationWhitelistResource(hs)
+
     return resources
 
 

synapse/rest/synapse/client/federation_whitelist.py

@@ -0,0 +1,67 @@
+#
+# 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` extension feature is
+    enabled.
+
+    Response format:
+
+        {
+            "whitelist_enabled": true,  // Whether there is a federation whitelist
+            "whitelist": [  // Which hosts are allowed by the whitelist
+                "example.com"
+            ]
+        }
+    """
+
+    PATH = "/_synapse/client/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