Federation outbound proxy (#15773)

Allow configuring the set of workers to proxy outbound federation traffic through (`outbound_federation_restricted_to`).

This is useful when you have a worker setup with `federation_sender` instances responsible for sending outbound federation requests and want to make sure *all* outbound federation traffic goes through those instances. Before this change, the generic workers would still contact federation themselves for things like profile lookups, backfill, etc. This PR allows you to set more strict access controls/firewall for all workers and only allow the `federation_sender`'s to contact the outside world.

The original code is from @erikjohnston's branches which I've gotten in-shape to merge.

1 files changed, 18 insertions, 8 deletions

diff --git a/synapse/http/site.py b/synapse/http/site.py
index 5b5a7c1e59..0ee2598345 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -21,25 +21,28 @@ from zope.interface import implementer
 
 from twisted.internet.address import UNIXAddress
 from twisted.internet.defer import Deferred
-from twisted.internet.interfaces import IAddress, IReactorTime
+from twisted.internet.interfaces import IAddress
 from twisted.python.failure import Failure
 from twisted.web.http import HTTPChannel
+from twisted.web.iweb import IAgent
 from twisted.web.resource import IResource, Resource
-from twisted.web.server import Request, Site
+from twisted.web.server import Request
 
 from synapse.config.server import ListenerConfig
 from synapse.http import get_request_user_agent, redact_uri
+from synapse.http.proxy import ProxySite
 from synapse.http.request_metrics import RequestMetrics, requests_counter
 from synapse.logging.context import (
     ContextRequest,
     LoggingContext,
     PreserveLoggingContext,
 )
-from synapse.types import Requester
+from synapse.types import ISynapseReactor, Requester
 
 if TYPE_CHECKING:
     import opentracing
 
+
 logger = logging.getLogger(__name__)
 
 _next_request_seq = 0
@@ -102,7 +105,7 @@ class SynapseRequest(Request):
         # A boolean indicating whether `render_deferred` should be cancelled if the
         # client disconnects early. Expected to be set by the coroutine started by
         # `Resource.render`, if rendering is asynchronous.
-        self.is_render_cancellable = False
+        self.is_render_cancellable: bool = False
 
         global _next_request_seq
         self.request_seq = _next_request_seq
@@ -601,7 +604,7 @@ class _XForwardedForAddress:
     host: str
 
 
-class SynapseSite(Site):
+class SynapseSite(ProxySite):
     """
     Synapse-specific twisted http Site
 
@@ -623,7 +626,8 @@ class SynapseSite(Site):
         resource: IResource,
         server_version_string: str,
         max_request_body_size: int,
-        reactor: IReactorTime,
+        reactor: ISynapseReactor,
+        federation_agent: IAgent,
     ):
         """
 
@@ -638,7 +642,11 @@ class SynapseSite(Site):
                 dropping the connection
             reactor: reactor to be used to manage connection timeouts
         """
-        Site.__init__(self, resource, reactor=reactor)
+        super().__init__(
+            resource=resource,
+            reactor=reactor,
+            federation_agent=federation_agent,
+        )
 
         self.site_tag = site_tag
         self.reactor = reactor
@@ -649,7 +657,9 @@ class SynapseSite(Site):
 
         request_id_header = config.http_options.request_id_header
 
-        self.experimental_cors_msc3886 = config.http_options.experimental_cors_msc3886
+        self.experimental_cors_msc3886: bool = (
+            config.http_options.experimental_cors_msc3886
+        )
 
         def request_factory(channel: HTTPChannel, queued: bool) -> Request:
             return request_class(