Support for routing outbound HTTP requests via a proxy (#6239)

The `http_proxy` and `HTTPS_PROXY` env vars can be set to a `host[:port]` value which should point to a proxy.

The address of the proxy should be excluded from IP blacklists such as the `url_preview_ip_range_blacklist`.

The proxy will then be used for
 * push
 * url previews
 * phone-home stats
 * recaptcha validation
 * CAS auth validation

It will *not* be used for:
 * Application Services
 * Identity servers
 * Outbound federation
 * In worker configurations, connections from workers to masters

Fixes #4198.

2 files changed, 18 insertions, 0 deletions

diff --git a/synapse/server.py b/synapse/server.py

index 0b81af646c..f8aeebcff8 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -23,6 +23,7 @@
 # Imports required for the default HomeServer() implementation
 import abc
 import logging
+import os
 
 from twisted.enterprise import adbapi
 from twisted.mail.smtp import sendmail
@@ -168,6 +169,7 @@ class HomeServer(object):
         "filtering",
         "http_client_context_factory",
         "simple_http_client",
+        "proxied_http_client",
         "media_repository",
         "media_repository_resource",
         "federation_transport_client",
@@ -311,6 +313,13 @@ class HomeServer(object):
     def build_simple_http_client(self):
         return SimpleHttpClient(self)
 
+    def build_proxied_http_client(self):
+        return SimpleHttpClient(
+            self,
+            http_proxy=os.getenv("http_proxy"),
+            https_proxy=os.getenv("HTTPS_PROXY"),
+        )
+
     def build_room_creation_handler(self):
         return RoomCreationHandler(self)
 
diff --git a/synapse/server.pyi b/synapse/server.pyi

index 83d1f11283..b5e0b57095 100644
--- a/synapse/server.pyi
+++ b/synapse/server.pyi
@@ -12,6 +12,7 @@ import synapse.handlers.message
 import synapse.handlers.room
 import synapse.handlers.room_member
 import synapse.handlers.set_password
+import synapse.http.client
 import synapse.rest.media.v1.media_repository
 import synapse.server_notices.server_notices_manager
 import synapse.server_notices.server_notices_sender
@@ -38,6 +39,14 @@ class HomeServer(object):
         pass
     def get_state_resolution_handler(self) -> synapse.state.StateResolutionHandler:
         pass
+    def get_simple_http_client(self) -> synapse.http.client.SimpleHttpClient:
+        """Fetch an HTTP client implementation which doesn't do any blacklisting
+        or support any HTTP_PROXY settings"""
+        pass
+    def get_proxied_http_client(self) -> synapse.http.client.SimpleHttpClient:
+        """Fetch an HTTP client implementation which doesn't do any blacklisting
+        but does support HTTP_PROXY settings"""
+        pass
     def get_deactivate_account_handler(
         self,
     ) -> synapse.handlers.deactivate_account.DeactivateAccountHandler: