diff --git a/synapse/http/proxyagent.py b/synapse/http/proxyagent.py
new file mode 100644
index 0000000000..332da02a8d
--- /dev/null
+++ b/synapse/http/proxyagent.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
+import re
+
+from zope.interface import implementer
+
+from twisted.internet import defer
+from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
+from twisted.python.failure import Failure
+from twisted.web.client import URI, BrowserLikePolicyForHTTPS, _AgentBase
+from twisted.web.error import SchemeNotSupported
+from twisted.web.iweb import IAgent
+
+from synapse.http.connectproxyclient import HTTPConnectProxyEndpoint
+
+logger = logging.getLogger(__name__)
+
+_VALID_URI = re.compile(br"\A[\x21-\x7e]+\Z")
+
+
+@implementer(IAgent)
+class ProxyAgent(_AgentBase):
+ """An Agent implementation which will use an HTTP proxy if one was requested
+
+ Args:
+ reactor: twisted reactor to place outgoing
+ connections.
+
+ contextFactory (IPolicyForHTTPS): A factory for TLS contexts, to control the
+ verification parameters of OpenSSL. The default is to use a
+ `BrowserLikePolicyForHTTPS`, so unless you have special
+ requirements you can leave this as-is.
+
+ connectTimeout (float): The amount of time that this Agent will wait
+ for the peer to accept a connection.
+
+ bindAddress (bytes): The local address for client sockets to bind to.
+
+ pool (HTTPConnectionPool|None): connection pool to be used. If None, a
+ non-persistent pool instance will be created.
+ """
+
+ def __init__(
+ self,
+ reactor,
+ contextFactory=BrowserLikePolicyForHTTPS(),
+ connectTimeout=None,
+ bindAddress=None,
+ pool=None,
+ http_proxy=None,
+ https_proxy=None,
+ ):
+ _AgentBase.__init__(self, reactor, pool)
+
+ self._endpoint_kwargs = {}
+ if connectTimeout is not None:
+ self._endpoint_kwargs["timeout"] = connectTimeout
+ if bindAddress is not None:
+ self._endpoint_kwargs["bindAddress"] = bindAddress
+
+ self.http_proxy_endpoint = _http_proxy_endpoint(
+ http_proxy, reactor, **self._endpoint_kwargs
+ )
+
+ self.https_proxy_endpoint = _http_proxy_endpoint(
+ https_proxy, reactor, **self._endpoint_kwargs
+ )
+
+ self._policy_for_https = contextFactory
+ self._reactor = reactor
+
+ def request(self, method, uri, headers=None, bodyProducer=None):
+ """
+ Issue a request to the server indicated by the given uri.
+
+ Supports `http` and `https` schemes.
+
+ An existing connection from the connection pool may be used or a new one may be
+ created.
+
+ See also: twisted.web.iweb.IAgent.request
+
+ Args:
+ method (bytes): The request method to use, such as `GET`, `POST`, etc
+
+ uri (bytes): The location of the resource to request.
+
+ headers (Headers|None): Extra headers to send with the request
+
+ bodyProducer (IBodyProducer|None): An object which can generate bytes to
+ make up the body of this request (for example, the properly encoded
+ contents of a file for a file upload). Or, None if the request is to
+ have no body.
+
+ Returns:
+ Deferred[IResponse]: completes when the header of the response has
+ been received (regardless of the response status code).
+ """
+ uri = uri.strip()
+ if not _VALID_URI.match(uri):
+ raise ValueError("Invalid URI {!r}".format(uri))
+
+ parsed_uri = URI.fromBytes(uri)
+ pool_key = (parsed_uri.scheme, parsed_uri.host, parsed_uri.port)
+ request_path = parsed_uri.originForm
+
+ if parsed_uri.scheme == b"http" and self.http_proxy_endpoint:
+ # Cache *all* connections under the same key, since we are only
+ # connecting to a single destination, the proxy:
+ pool_key = ("http-proxy", self.http_proxy_endpoint)
+ endpoint = self.http_proxy_endpoint
+ request_path = uri
+ elif parsed_uri.scheme == b"https" and self.https_proxy_endpoint:
+ endpoint = HTTPConnectProxyEndpoint(
+ self._reactor,
+ self.https_proxy_endpoint,
+ parsed_uri.host,
+ parsed_uri.port,
+ )
+ else:
+ # not using a proxy
+ endpoint = HostnameEndpoint(
+ self._reactor, parsed_uri.host, parsed_uri.port, **self._endpoint_kwargs
+ )
+
+ logger.debug("Requesting %s via %s", uri, endpoint)
+
+ if parsed_uri.scheme == b"https":
+ tls_connection_creator = self._policy_for_https.creatorForNetloc(
+ parsed_uri.host, parsed_uri.port
+ )
+ endpoint = wrapClientTLS(tls_connection_creator, endpoint)
+ elif parsed_uri.scheme == b"http":
+ pass
+ else:
+ return defer.fail(
+ Failure(
+ SchemeNotSupported("Unsupported scheme: %r" % (parsed_uri.scheme,))
+ )
+ )
+
+ return self._requestWithEndpoint(
+ pool_key, endpoint, method, parsed_uri, headers, bodyProducer, request_path
+ )
+
+
+def _http_proxy_endpoint(proxy, reactor, **kwargs):
+ """Parses an http proxy setting and returns an endpoint for the proxy
+
+ Args:
+ proxy (bytes|None): the proxy setting
+ reactor: reactor to be used to connect to the proxy
+ kwargs: other args to be passed to HostnameEndpoint
+
+ Returns:
+ interfaces.IStreamClientEndpoint|None: endpoint to use to connect to the proxy,
+ or None
+ """
+ if proxy is None:
+ return None
+
+ # currently we only support hostname:port. Some apps also support
+ # protocol://<host>[:port], which allows a way of requiring a TLS connection to the
+ # proxy.
+
+ host, port = parse_host_port(proxy, default_port=1080)
+ return HostnameEndpoint(reactor, host, port, **kwargs)
+
+
+def parse_host_port(hostport, default_port=None):
+ # could have sworn we had one of these somewhere else...
+ if b":" in hostport:
+ host, port = hostport.rsplit(b":", 1)
+ try:
+ port = int(port)
+ return host, port
+ except ValueError:
+ # the thing after the : wasn't a valid port; presumably this is an
+ # IPv6 address.
+ pass
+
+ return hostport, default_port
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