Add a new get_ip_address_from_request method

This method raises a `SynapseException` if Synapse is unable to extract the IP address of a client from an incoming request. This
typically indicates that there is an invalid configuration in one's reverse proxy.

Raise an exception rather than returning a dummy IP address, as it's typically better to fail loudly in this case.

synapse/api/auth/__init__.py

@@ -188,6 +188,22 @@ class Auth(Protocol):
                 request
         """
 
+    @staticmethod
+    def get_ip_address_from_request(request: Request) -> str:
+        """
+        Extract the IPv4 or IPv6 address from a client request.
+
+        Args:
+            request: The request to process.
+
+        Returns:
+            The IPv4 or IPv6 address of the client.
+
+        Raises:
+            SynapseError: If an IP address could not be extracted from the
+                request.
+        """
+
     async def check_user_in_room_or_world_readable(
         self, room_id: str, requester: Requester, allow_departed_users: bool = False
     ) -> Tuple[str, Optional[str]]:

synapse/api/auth/base.py

@@ -19,10 +19,12 @@
 #
 #
 import logging
+from http import HTTPStatus
 from typing import TYPE_CHECKING, Optional, Tuple
 
 from netaddr import IPAddress
 
+from twisted.internet.address import IPv4Address, IPv6Address
 from twisted.web.server import Request
 
 from synapse import event_auth
@@ -31,6 +33,7 @@ from synapse.api.errors import (
     AuthError,
     Codes,
     MissingClientTokenError,
+    SynapseError,
     UnstableSpecAuthError,
 )
 from synapse.appservice import ApplicationService
@@ -291,6 +294,36 @@ class BaseAuth:
 
             return query_params[0].decode("ascii")
 
+    @staticmethod
+    def get_ip_address_from_request(request: Request) -> str:
+        """
+        Extract the IPv4 or IPv6 address from a client request.
+
+        Args:
+            request: The request to process.
+
+        Returns:
+            The IPv4 or IPv6 address of the client.
+
+        Raises:
+            SynapseError: If an IP address could not be extracted from the
+                request.
+        """
+        client_address = request.getClientAddress()
+        if not isinstance(client_address, IPv4Address) and not isinstance(
+            client_address, IPv6Address
+        ):
+            logger.error(
+                "Unable to view IP address of the requester. Check that you are setting the X-Forwarded-For header correctly in your reverse proxy. Assuming '127.0.0.1'"
+            )
+            raise SynapseError(
+                HTTPStatus.INTERNAL_SERVER_ERROR,
+                "Unable to read client IP address",
+                Codes.UNKNOWN,
+            )
+
+        return client_address.host
+
     @cancellable
     async def get_appservice_user(
         self, request: Request, access_token: str