The host is expected to be bytes.

synapse/replication/tcp/handler.py

@@ -302,7 +302,7 @@ def start_replication(self, hs):
                 hs, outbound_redis_connection
             )
             hs.get_reactor().connectTCP(
-                hs.config.redis.redis_host,
+                hs.config.redis.redis_host.encode(),
                 hs.config.redis.redis_port,
                 self._factory,
             )
@@ -311,7 +311,7 @@ def start_replication(self, hs):
             self._factory = DirectTcpReplicationClientFactory(hs, client_name, self)
             host = hs.config.worker_replication_host
             port = hs.config.worker_replication_port
-            hs.get_reactor().connectTCP(host, port, self._factory)
+            hs.get_reactor().connectTCP(host.encode(), port, self._factory)
 
     def get_streams(self) -> Dict[str, Stream]:
         """Get a map from stream name to all streams."""

synapse/replication/tcp/redis.py

@@ -365,6 +365,6 @@ def lazyConnection(
     factory.continueTrying = reconnect
 
     reactor = hs.get_reactor()
-    reactor.connectTCP(host, port, factory, timeout=30, bindAddress=None)
+    reactor.connectTCP(host.encode(), port, factory, timeout=30, bindAddress=None)
 
     return factory.handler

tests/replication/_base.py

@@ -233,7 +233,7 @@ def setUp(self):
         if self.hs.config.redis.redis_enabled:
             # Handle attempts to connect to fake redis server.
             self.reactor.add_tcp_client_callback(
-                "localhost",
+                b"localhost",
                 6379,
                 self.connect_any_redis_attempts,
             )
@@ -415,7 +415,7 @@ def connect_any_redis_attempts(self):
         clients = self.reactor.tcpClients
         while clients:
             (host, port, client_factory, _timeout, _bindAddress) = clients.pop(0)
-            self.assertEqual(host, "localhost")
+            self.assertEqual(host, b"localhost")
             self.assertEqual(port, 6379)
 
             client_protocol = client_factory.buildProtocol(None)