diff --git a/synapse/http/connectproxyclient.py b/synapse/http/connectproxyclient.py
new file mode 100644
index 0000000000..be7b2ceb8e
--- /dev/null
+++ b/synapse/http/connectproxyclient.py
@@ -0,0 +1,195 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 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
+
+from zope.interface import implementer
+
+from twisted.internet import defer, protocol
+from twisted.internet.error import ConnectError
+from twisted.internet.interfaces import IStreamClientEndpoint
+from twisted.internet.protocol import connectionDone
+from twisted.web import http
+
+logger = logging.getLogger(__name__)
+
+
+class ProxyConnectError(ConnectError):
+ pass
+
+
+@implementer(IStreamClientEndpoint)
+class HTTPConnectProxyEndpoint(object):
+ """An Endpoint implementation which will send a CONNECT request to an http proxy
+
+ Wraps an existing HostnameEndpoint for the proxy.
+
+ When we get the connect() request from the connection pool (via the TLS wrapper),
+ we'll first connect to the proxy endpoint with a ProtocolFactory which will make the
+ CONNECT request. Once that completes, we invoke the protocolFactory which was passed
+ in.
+
+ Args:
+ reactor: the Twisted reactor to use for the connection
+ proxy_endpoint (IStreamClientEndpoint): the endpoint to use to connect to the
+ proxy
+ host (bytes): hostname that we want to CONNECT to
+ port (int): port that we want to connect to
+ """
+
+ def __init__(self, reactor, proxy_endpoint, host, port):
+ self._reactor = reactor
+ self._proxy_endpoint = proxy_endpoint
+ self._host = host
+ self._port = port
+
+ def __repr__(self):
+ return "<HTTPConnectProxyEndpoint %s>" % (self._proxy_endpoint,)
+
+ def connect(self, protocolFactory):
+ f = HTTPProxiedClientFactory(self._host, self._port, protocolFactory)
+ d = self._proxy_endpoint.connect(f)
+ # once the tcp socket connects successfully, we need to wait for the
+ # CONNECT to complete.
+ d.addCallback(lambda conn: f.on_connection)
+ return d
+
+
+class HTTPProxiedClientFactory(protocol.ClientFactory):
+ """ClientFactory wrapper that triggers an HTTP proxy CONNECT on connect.
+
+ Once the CONNECT completes, invokes the original ClientFactory to build the
+ HTTP Protocol object and run the rest of the connection.
+
+ Args:
+ dst_host (bytes): hostname that we want to CONNECT to
+ dst_port (int): port that we want to connect to
+ wrapped_factory (protocol.ClientFactory): The original Factory
+ """
+
+ def __init__(self, dst_host, dst_port, wrapped_factory):
+ self.dst_host = dst_host
+ self.dst_port = dst_port
+ self.wrapped_factory = wrapped_factory
+ self.on_connection = defer.Deferred()
+
+ def startedConnecting(self, connector):
+ return self.wrapped_factory.startedConnecting(connector)
+
+ def buildProtocol(self, addr):
+ wrapped_protocol = self.wrapped_factory.buildProtocol(addr)
+
+ return HTTPConnectProtocol(
+ self.dst_host, self.dst_port, wrapped_protocol, self.on_connection
+ )
+
+ def clientConnectionFailed(self, connector, reason):
+ logger.debug("Connection to proxy failed: %s", reason)
+ if not self.on_connection.called:
+ self.on_connection.errback(reason)
+ return self.wrapped_factory.clientConnectionFailed(connector, reason)
+
+ def clientConnectionLost(self, connector, reason):
+ logger.debug("Connection to proxy lost: %s", reason)
+ if not self.on_connection.called:
+ self.on_connection.errback(reason)
+ return self.wrapped_factory.clientConnectionLost(connector, reason)
+
+
+class HTTPConnectProtocol(protocol.Protocol):
+ """Protocol that wraps an existing Protocol to do a CONNECT handshake at connect
+
+ Args:
+ host (bytes): The original HTTP(s) hostname or IPv4 or IPv6 address literal
+ to put in the CONNECT request
+
+ port (int): The original HTTP(s) port to put in the CONNECT request
+
+ wrapped_protocol (interfaces.IProtocol): the original protocol (probably
+ HTTPChannel or TLSMemoryBIOProtocol, but could be anything really)
+
+ connected_deferred (Deferred): a Deferred which will be callbacked with
+ wrapped_protocol when the CONNECT completes
+ """
+
+ def __init__(self, host, port, wrapped_protocol, connected_deferred):
+ self.host = host
+ self.port = port
+ self.wrapped_protocol = wrapped_protocol
+ self.connected_deferred = connected_deferred
+ self.http_setup_client = HTTPConnectSetupClient(self.host, self.port)
+ self.http_setup_client.on_connected.addCallback(self.proxyConnected)
+
+ def connectionMade(self):
+ self.http_setup_client.makeConnection(self.transport)
+
+ def connectionLost(self, reason=connectionDone):
+ if self.wrapped_protocol.connected:
+ self.wrapped_protocol.connectionLost(reason)
+
+ self.http_setup_client.connectionLost(reason)
+
+ if not self.connected_deferred.called:
+ self.connected_deferred.errback(reason)
+
+ def proxyConnected(self, _):
+ self.wrapped_protocol.makeConnection(self.transport)
+
+ self.connected_deferred.callback(self.wrapped_protocol)
+
+ # Get any pending data from the http buf and forward it to the original protocol
+ buf = self.http_setup_client.clearLineBuffer()
+ if buf:
+ self.wrapped_protocol.dataReceived(buf)
+
+ def dataReceived(self, data):
+ # if we've set up the HTTP protocol, we can send the data there
+ if self.wrapped_protocol.connected:
+ return self.wrapped_protocol.dataReceived(data)
+
+ # otherwise, we must still be setting up the connection: send the data to the
+ # setup client
+ return self.http_setup_client.dataReceived(data)
+
+
+class HTTPConnectSetupClient(http.HTTPClient):
+ """HTTPClient protocol to send a CONNECT message for proxies and read the response.
+
+ Args:
+ host (bytes): The hostname to send in the CONNECT message
+ port (int): The port to send in the CONNECT message
+ """
+
+ def __init__(self, host, port):
+ self.host = host
+ self.port = port
+ self.on_connected = defer.Deferred()
+
+ def connectionMade(self):
+ logger.debug("Connected to proxy, sending CONNECT")
+ self.sendCommand(b"CONNECT", b"%s:%d" % (self.host, self.port))
+ self.endHeaders()
+
+ def handleStatus(self, version, status, message):
+ logger.debug("Got Status: %s %s %s", status, message, version)
+ if status != b"200":
+ raise ProxyConnectError("Unexpected status on CONNECT: %s" % status)
+
+ def handleEndHeaders(self):
+ logger.debug("End Headers")
+ self.on_connected.callback(None)
+
+ def handleResponse(self, body):
+ pass
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