diff --git a/synapse/http/proxy.py b/synapse/http/proxy.py
new file mode 100644
index 0000000000..58a5f7f0c9
--- /dev/null
+++ b/synapse/http/proxy.py
@@ -0,0 +1,150 @@
+# Copyright 2023 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import logging
+import urllib.parse
+from typing import TYPE_CHECKING, Any, Optional, Tuple, cast
+
+from twisted.internet import protocol
+from twisted.internet.interfaces import ITCPTransport
+from twisted.internet.protocol import connectionDone
+from twisted.python import failure
+from twisted.python.failure import Failure
+from twisted.web.client import ResponseDone
+from twisted.web.http import PotentialDataLoss
+from twisted.web.http_headers import Headers
+from twisted.web.iweb import IAgent, IResponse
+from twisted.web.resource import IResource
+from twisted.web.server import Site
+
+from synapse.http import QuieterFileBodyProducer
+from synapse.http.server import _AsyncResource
+from synapse.logging.context import make_deferred_yieldable, run_in_background
+from synapse.types import ISynapseReactor
+from synapse.util.async_helpers import timeout_deferred
+
+if TYPE_CHECKING:
+ from synapse.http.site import SynapseRequest
+
+logger = logging.getLogger(__name__)
+
+
+class ProxyResource(_AsyncResource):
+ isLeaf = True
+
+ def __init__(self, reactor: ISynapseReactor, federation_agent: IAgent):
+ super().__init__(True)
+
+ self.reactor = reactor
+ self.agent = federation_agent
+
+ async def _async_render(self, request: "SynapseRequest") -> Tuple[int, Any]:
+ assert request.uri.startswith(b"matrix://")
+
+ logger.info("Got proxy request %s", request.uri)
+
+ headers = Headers()
+ for header_name in (b"User-Agent", b"Authorization", b"Content-Type"):
+ header_value = request.getHeader(header_name)
+ if header_value:
+ headers.addRawHeader(header_name, header_value)
+
+ request_deferred = run_in_background(
+ self.agent.request,
+ request.method,
+ request.uri,
+ headers=headers,
+ bodyProducer=QuieterFileBodyProducer(request.content),
+ )
+ request_deferred = timeout_deferred(
+ request_deferred,
+ timeout=90,
+ reactor=self.reactor,
+ )
+
+ response = await make_deferred_yieldable(request_deferred)
+
+ logger.info("Got proxy response %s", response.code)
+
+ return response.code, response
+
+ def _send_response(
+ self,
+ request: "SynapseRequest",
+ code: int,
+ response_object: Any,
+ ) -> None:
+ response = cast(IResponse, response_object)
+
+ request.setResponseCode(code)
+
+ # Copy headers.
+ for k, v in response.headers.getAllRawHeaders():
+ request.responseHeaders.setRawHeaders(k, v)
+
+ response.deliverBody(_ProxyResponseBody(request))
+
+ def _send_error_response(
+ self,
+ f: failure.Failure,
+ request: "SynapseRequest",
+ ) -> None:
+ request.setResponseCode(502)
+ request.finish()
+
+
+class _ProxyResponseBody(protocol.Protocol):
+ transport: Optional[ITCPTransport] = None
+
+ def __init__(self, request: "SynapseRequest") -> None:
+ self._request = request
+
+ def dataReceived(self, data: bytes) -> None:
+ if self._request._disconnected and self.transport is not None:
+ self.transport.abortConnection()
+ return
+
+ self._request.write(data)
+
+ def connectionLost(self, reason: Failure = connectionDone) -> None:
+ if self._request.finished:
+ return
+
+ if reason.check(ResponseDone):
+ self._request.finish()
+ elif reason.check(PotentialDataLoss):
+ # TODO: ARGH
+ self._request.finish()
+ else:
+ self._request.transport.abortConnection()
+
+
+class ProxySite(Site):
+ def __init__(
+ self,
+ resource: IResource,
+ reactor: ISynapseReactor,
+ federation_agent: IAgent,
+ ):
+ super().__init__(resource, reactor=reactor)
+
+ self._proxy_resource = ProxyResource(reactor, federation_agent)
+
+ def getResourceFor(self, request: "SynapseRequest") -> IResource:
+ uri = urllib.parse.urlparse(request.uri)
+ if uri.scheme == b"matrix":
+ return self._proxy_resource
+
+ return super().getResourceFor(request)
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