diff --git a/changelog.d/9528.misc b/changelog.d/9528.misc
new file mode 100644
index 0000000000..14c7b78dd9
--- /dev/null
+++ b/changelog.d/9528.misc
@@ -0,0 +1 @@
+Fix incorrect type hints.
diff --git a/synapse/handlers/acme.py b/synapse/handlers/acme.py
index 5ecb2da1ac..132be238dd 100644
--- a/synapse/handlers/acme.py
+++ b/synapse/handlers/acme.py
@@ -73,7 +73,9 @@ class AcmeHandler:
                 "Listening for ACME requests on %s:%i", host, self.hs.config.acme_port
             )
             try:
-                self.reactor.listenTCP(self.hs.config.acme_port, srv, interface=host)
+                self.reactor.listenTCP(
+                    self.hs.config.acme_port, srv, backlog=50, interface=host
+                )
             except twisted.internet.error.CannotListenError as e:
                 check_bind_error(e, host, bind_addresses)
 
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 72901e3f95..af34d583ad 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -63,6 +63,7 @@ from synapse.http import QuieterFileBodyProducer, RequestTimedOutError, redact_u
 from synapse.http.proxyagent import ProxyAgent
 from synapse.logging.context import make_deferred_yieldable
 from synapse.logging.opentracing import set_tag, start_active_span, tags
+from synapse.types import ISynapseReactor
 from synapse.util import json_decoder
 from synapse.util.async_helpers import timeout_deferred
 
@@ -199,7 +200,7 @@ class _IPBlacklistingResolver:
         return r
 
 
-@implementer(IReactorPluggableNameResolver)
+@implementer(ISynapseReactor)
 class BlacklistingReactorWrapper:
     """
     A Reactor wrapper which will prevent DNS resolution to blacklisted IP
@@ -324,7 +325,7 @@ class SimpleHttpClient:
             # filters out blacklisted IP addresses, to prevent DNS rebinding.
             self.reactor = BlacklistingReactorWrapper(
                 hs.get_reactor(), self._ip_whitelist, self._ip_blacklist
-            )
+            )  # type: ISynapseReactor
         else:
             self.reactor = hs.get_reactor()
 
diff --git a/synapse/http/federation/matrix_federation_agent.py b/synapse/http/federation/matrix_federation_agent.py
index b07aa59c08..5935a125fd 100644
--- a/synapse/http/federation/matrix_federation_agent.py
+++ b/synapse/http/federation/matrix_federation_agent.py
@@ -35,6 +35,7 @@ from synapse.http.client import BlacklistingAgentWrapper
 from synapse.http.federation.srv_resolver import Server, SrvResolver
 from synapse.http.federation.well_known_resolver import WellKnownResolver
 from synapse.logging.context import make_deferred_yieldable, run_in_background
+from synapse.types import ISynapseReactor
 from synapse.util import Clock
 
 logger = logging.getLogger(__name__)
@@ -68,7 +69,7 @@ class MatrixFederationAgent:
 
     def __init__(
         self,
-        reactor: IReactorCore,
+        reactor: ISynapseReactor,
         tls_client_options_factory: Optional[FederationPolicyForHTTPS],
         user_agent: bytes,
         ip_blacklist: IPSet,
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 0f107714ea..da6866addf 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -59,7 +59,7 @@ from synapse.logging.opentracing import (
     start_active_span,
     tags,
 )
-from synapse.types import JsonDict
+from synapse.types import ISynapseReactor, JsonDict
 from synapse.util import json_decoder
 from synapse.util.async_helpers import timeout_deferred
 from synapse.util.metrics import Measure
@@ -237,14 +237,14 @@ class MatrixFederationHttpClient:
         # addresses, to prevent DNS rebinding.
         self.reactor = BlacklistingReactorWrapper(
             hs.get_reactor(), None, hs.config.federation_ip_range_blacklist
-        )
+        )  # type: ISynapseReactor
 
         user_agent = hs.version_string
         if hs.config.user_agent_suffix:
             user_agent = "%s %s" % (user_agent, hs.config.user_agent_suffix)
         user_agent = user_agent.encode("ascii")
 
-        self.agent = MatrixFederationAgent(
+        federation_agent = MatrixFederationAgent(
             self.reactor,
             tls_client_options_factory,
             user_agent,
@@ -254,7 +254,7 @@ class MatrixFederationHttpClient:
         # Use a BlacklistingAgentWrapper to prevent circumventing the IP
         # blacklist via IP literals in server names
         self.agent = BlacklistingAgentWrapper(
-            self.agent,
+            federation_agent,
             ip_blacklist=hs.config.federation_ip_range_blacklist,
         )
 
diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 0e6155cf53..7560706b4b 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -328,6 +328,6 @@ def lazyConnection(
     factory.continueTrying = reconnect
 
     reactor = hs.get_reactor()
-    reactor.connectTCP(host, port, factory, 30)
+    reactor.connectTCP(host, port, factory, timeout=30, bindAddress=None)
 
     return factory.handler
diff --git a/synapse/server.py b/synapse/server.py
index afd7cd72e7..369cc88026 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -36,7 +36,6 @@ from typing import (
     cast,
 )
 
-import twisted.internet.base
 import twisted.internet.tcp
 from twisted.internet import defer
 from twisted.mail.smtp import sendmail
@@ -130,7 +129,7 @@ from synapse.server_notices.worker_server_notices_sender import (
 from synapse.state import StateHandler, StateResolutionHandler
 from synapse.storage import Databases, DataStore, Storage
 from synapse.streams.events import EventSources
-from synapse.types import DomainSpecificString
+from synapse.types import DomainSpecificString, ISynapseReactor
 from synapse.util import Clock
 from synapse.util.distributor import Distributor
 from synapse.util.ratelimitutils import FederationRateLimiter
@@ -291,7 +290,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         for i in self.REQUIRED_ON_BACKGROUND_TASK_STARTUP:
             getattr(self, "get_" + i + "_handler")()
 
-    def get_reactor(self) -> twisted.internet.base.ReactorBase:
+    def get_reactor(self) -> ISynapseReactor:
         """
         Fetch the Twisted reactor in use by this HomeServer.
         """
diff --git a/synapse/types.py b/synapse/types.py
index 721343f0b5..0216d213c7 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -35,6 +35,14 @@ from typing import (
 import attr
 from signedjson.key import decode_verify_key_bytes
 from unpaddedbase64 import decode_base64
+from zope.interface import Interface
+
+from twisted.internet.interfaces import (
+    IReactorCore,
+    IReactorPluggableNameResolver,
+    IReactorTCP,
+    IReactorTime,
+)
 
 from synapse.api.errors import Codes, SynapseError
 from synapse.util.stringutils import parse_and_validate_server_name
@@ -67,6 +75,14 @@ MutableStateMap = MutableMapping[StateKey, T]
 JsonDict = Dict[str, Any]
 
 
+# Note that this seems to require inheriting *directly* from Interface in order
+# for mypy-zope to realize it is an interface.
+class ISynapseReactor(
+    IReactorTCP, IReactorPluggableNameResolver, IReactorTime, IReactorCore, Interface
+):
+    """The interfaces necessary for Synapse to function."""
+
+
 class Requester(
     namedtuple(
         "Requester",