Describe which rate limiter was hit in logs (#16135)

18 files changed, 235 insertions, 121 deletions

diff --git a/changelog.d/16135.misc b/changelog.d/16135.misc
new file mode 100644
index 0000000000..cba8733d02
--- /dev/null
+++ b/changelog.d/16135.misc
@@ -0,0 +1 @@
+Describe which rate limiter was hit in logs.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 578e798773..fdb2955be8 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -211,6 +211,11 @@ class SynapseError(CodeMessageException):
     def error_dict(self, config: Optional["HomeServerConfig"]) -> "JsonDict":
         return cs_error(self.msg, self.errcode, **self._additional_fields)
 
+    @property
+    def debug_context(self) -> Optional[str]:
+        """Override this to add debugging context that shouldn't be sent to clients."""
+        return None
+
 
 class InvalidAPICallError(SynapseError):
     """You called an existing API endpoint, but fed that endpoint
@@ -508,8 +513,8 @@ class LimitExceededError(SynapseError):
 
     def __init__(
         self,
+        limiter_name: str,
         code: int = 429,
-        msg: str = "Too Many Requests",
         retry_after_ms: Optional[int] = None,
         errcode: str = Codes.LIMIT_EXCEEDED,
     ):
@@ -518,12 +523,17 @@ class LimitExceededError(SynapseError):
             if self.include_retry_after_header and retry_after_ms is not None
             else None
         )
-        super().__init__(code, msg, errcode, headers=headers)
+        super().__init__(code, "Too Many Requests", errcode, headers=headers)
         self.retry_after_ms = retry_after_ms
+        self.limiter_name = limiter_name
 
     def error_dict(self, config: Optional["HomeServerConfig"]) -> "JsonDict":
         return cs_error(self.msg, self.errcode, retry_after_ms=self.retry_after_ms)
 
+    @property
+    def debug_context(self) -> Optional[str]:
+        return self.limiter_name
+
 
 class RoomKeysVersionError(SynapseError):
     """A client has tried to upload to a non-current version of the room_keys store"""
diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 511790c7c5..887b214d64 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -61,12 +61,16 @@ class Ratelimiter:
     """
 
     def __init__(
-        self, store: DataStore, clock: Clock, rate_hz: float, burst_count: int
+        self,
+        store: DataStore,
+        clock: Clock,
+        cfg: RatelimitSettings,
     ):
         self.clock = clock
-        self.rate_hz = rate_hz
-        self.burst_count = burst_count
+        self.rate_hz = cfg.per_second
+        self.burst_count = cfg.burst_count
         self.store = store
+        self._limiter_name = cfg.key
 
         # An ordered dictionary representing the token buckets tracked by this rate
         # limiter. Each entry maps a key of arbitrary type to a tuple representing:
@@ -305,7 +309,8 @@ class Ratelimiter:
 
         if not allowed:
             raise LimitExceededError(
-                retry_after_ms=int(1000 * (time_allowed - time_now_s))
+                limiter_name=self._limiter_name,
+                retry_after_ms=int(1000 * (time_allowed - time_now_s)),
             )
 
 
@@ -322,7 +327,9 @@ class RequestRatelimiter:
 
         # The rate_hz and burst_count are overridden on a per-user basis
         self.request_ratelimiter = Ratelimiter(
-            store=self.store, clock=self.clock, rate_hz=0, burst_count=0
+            store=self.store,
+            clock=self.clock,
+            cfg=RatelimitSettings(key=rc_message.key, per_second=0, burst_count=0),
         )
         self._rc_message = rc_message
 
@@ -332,8 +339,7 @@ class RequestRatelimiter:
             self.admin_redaction_ratelimiter: Optional[Ratelimiter] = Ratelimiter(
                 store=self.store,
                 clock=self.clock,
-                rate_hz=rc_admin_redaction.per_second,
-                burst_count=rc_admin_redaction.burst_count,
+                cfg=rc_admin_redaction,
             )
         else:
             self.admin_redaction_ratelimiter = None
diff --git a/synapse/config/ratelimiting.py b/synapse/config/ratelimiting.py
index a5514e70a2..4efbaeac0d 100644
--- a/synapse/config/ratelimiting.py
+++ b/synapse/config/ratelimiting.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Any, Dict, Optional
+from typing import Any, Dict, Optional, cast
 
 import attr
 
@@ -21,16 +21,47 @@ from synapse.types import JsonDict
 from ._base import Config
 
 
+@attr.s(slots=True, frozen=True, auto_attribs=True)
 class RatelimitSettings:
-    def __init__(
-        self,
-        config: Dict[str, float],
+    key: str
+    per_second: float
+    burst_count: int
+
+    @classmethod
+    def parse(
+        cls,
+        config: Dict[str, Any],
+        key: str,
         defaults: Optional[Dict[str, float]] = None,
-    ):
+    ) -> "RatelimitSettings":
+        """Parse config[key] as a new-style rate limiter config.
+
+        The key may refer to a nested dictionary using a full stop (.) to separate
+        each nested key. For example, use the key "a.b.c" to parse the following:
+
+        a:
+          b:
+            c:
+              per_second: 10
+              burst_count: 200
+
+        If this lookup fails, we'll fallback to the defaults.
+        """
         defaults = defaults or {"per_second": 0.17, "burst_count": 3.0}
 
-        self.per_second = config.get("per_second", defaults["per_second"])
-        self.burst_count = int(config.get("burst_count", defaults["burst_count"]))
+        rl_config = config
+        for part in key.split("."):
+            rl_config = rl_config.get(part, {})
+
+        # By this point we should have hit the rate limiter parameters.
+        # We don't actually check this though!
+        rl_config = cast(Dict[str, float], rl_config)
+
+        return cls(
+            key=key,
+            per_second=rl_config.get("per_second", defaults["per_second"]),
+            burst_count=int(rl_config.get("burst_count", defaults["burst_count"])),
+        )
 
 
 @attr.s(auto_attribs=True)
@@ -49,15 +80,14 @@ class RatelimitConfig(Config):
         # Load the new-style messages config if it exists. Otherwise fall back
         # to the old method.
         if "rc_message" in config:
-            self.rc_message = RatelimitSettings(
-                config["rc_message"], defaults={"per_second": 0.2, "burst_count": 10.0}
+            self.rc_message = RatelimitSettings.parse(
+                config, "rc_message", defaults={"per_second": 0.2, "burst_count": 10.0}
             )
         else:
             self.rc_message = RatelimitSettings(
-                {
-                    "per_second": config.get("rc_messages_per_second", 0.2),
-                    "burst_count": config.get("rc_message_burst_count", 10.0),
-                }
+                key="rc_messages",
+                per_second=config.get("rc_messages_per_second", 0.2),
+                burst_count=config.get("rc_message_burst_count", 10.0),
             )
 
         # Load the new-style federation config, if it exists. Otherwise, fall
@@ -79,51 +109,59 @@ class RatelimitConfig(Config):
                 }
             )
 
-        self.rc_registration = RatelimitSettings(config.get("rc_registration", {}))
+        self.rc_registration = RatelimitSettings.parse(config, "rc_registration", {})
 
-        self.rc_registration_token_validity = RatelimitSettings(
-            config.get("rc_registration_token_validity", {}),
+        self.rc_registration_token_validity = RatelimitSettings.parse(
+            config,
+            "rc_registration_token_validity",
             defaults={"per_second": 0.1, "burst_count": 5},
         )
 
         # It is reasonable to login with a bunch of devices at once (i.e. when
         # setting up an account), but it is *not* valid to continually be
         # logging into new devices.
-        rc_login_config = config.get("rc_login", {})
-        self.rc_login_address = RatelimitSettings(
-            rc_login_config.get("address", {}),
+        self.rc_login_address = RatelimitSettings.parse(
+            config,
+            "rc_login.address",
             defaults={"per_second": 0.003, "burst_count": 5},
         )
-        self.rc_login_account = RatelimitSettings(
-            rc_login_config.get("account", {}),
+        self.rc_login_account = RatelimitSettings.parse(
+            config,
+            "rc_login.account",
             defaults={"per_second": 0.003, "burst_count": 5},
         )
-        self.rc_login_failed_attempts = RatelimitSettings(
-            rc_login_config.get("failed_attempts", {})
+        self.rc_login_failed_attempts = RatelimitSettings.parse(
+            config,
+            "rc_login.failed_attempts",
+            {},
         )
 
         self.federation_rr_transactions_per_room_per_second = config.get(
             "federation_rr_transactions_per_room_per_second", 50
         )
 
-        rc_admin_redaction = config.get("rc_admin_redaction")
         self.rc_admin_redaction = None
-        if rc_admin_redaction:
-            self.rc_admin_redaction = RatelimitSettings(rc_admin_redaction)
+        if "rc_admin_redaction" in config:
+            self.rc_admin_redaction = RatelimitSettings.parse(
+                config, "rc_admin_redaction", {}
+            )
 
-        self.rc_joins_local = RatelimitSettings(
-            config.get("rc_joins", {}).get("local", {}),
+        self.rc_joins_local = RatelimitSettings.parse(
+            config,
+            "rc_joins.local",
             defaults={"per_second": 0.1, "burst_count": 10},
         )
-        self.rc_joins_remote = RatelimitSettings(
-            config.get("rc_joins", {}).get("remote", {}),
+        self.rc_joins_remote = RatelimitSettings.parse(
+            config,
+            "rc_joins.remote",
             defaults={"per_second": 0.01, "burst_count": 10},
         )
 
         # Track the rate of joins to a given room. If there are too many, temporarily
         # prevent local joins and remote joins via this server.
-        self.rc_joins_per_room = RatelimitSettings(
-            config.get("rc_joins_per_room", {}),
+        self.rc_joins_per_room = RatelimitSettings.parse(
+            config,
+            "rc_joins_per_room",
             defaults={"per_second": 1, "burst_count": 10},
         )
 
@@ -132,31 +170,37 @@ class RatelimitConfig(Config):
         # * For requests received over federation this is keyed by the origin.
         #
         # Note that this isn't exposed in the configuration as it is obscure.
-        self.rc_key_requests = RatelimitSettings(
-            config.get("rc_key_requests", {}),
+        self.rc_key_requests = RatelimitSettings.parse(
+            config,
+            "rc_key_requests",
             defaults={"per_second": 20, "burst_count": 100},
         )
 
-        self.rc_3pid_validation = RatelimitSettings(
-            config.get("rc_3pid_validation") or {},
+        self.rc_3pid_validation = RatelimitSettings.parse(
+            config,
+            "rc_3pid_validation",
             defaults={"per_second": 0.003, "burst_count": 5},
         )
 
-        self.rc_invites_per_room = RatelimitSettings(
-            config.get("rc_invites", {}).get("per_room", {}),
+        self.rc_invites_per_room = RatelimitSettings.parse(
+            config,
+            "rc_invites.per_room",
             defaults={"per_second": 0.3, "burst_count": 10},
         )
-        self.rc_invites_per_user = RatelimitSettings(
-            config.get("rc_invites", {}).get("per_user", {}),
+        self.rc_invites_per_user = RatelimitSettings.parse(
+            config,
+            "rc_invites.per_user",
             defaults={"per_second": 0.003, "burst_count": 5},
         )
 
-        self.rc_invites_per_issuer = RatelimitSettings(
-            config.get("rc_invites", {}).get("per_issuer", {}),
+        self.rc_invites_per_issuer = RatelimitSettings.parse(
+            config,
+            "rc_invites.per_issuer",
             defaults={"per_second": 0.3, "burst_count": 10},
         )
 
-        self.rc_third_party_invite = RatelimitSettings(
-            config.get("rc_third_party_invite", {}),
+        self.rc_third_party_invite = RatelimitSettings.parse(
+            config,
+            "rc_third_party_invite",
             defaults={"per_second": 0.0025, "burst_count": 5},
         )
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 59ecafa6a0..2b0c505130 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -218,19 +218,17 @@ class AuthHandler:
         self._failed_uia_attempts_ratelimiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=self.hs.config.ratelimiting.rc_login_failed_attempts.per_second,
-            burst_count=self.hs.config.ratelimiting.rc_login_failed_attempts.burst_count,
+            cfg=self.hs.config.ratelimiting.rc_login_failed_attempts,
         )
 
         # The number of seconds to keep a UI auth session active.
         self._ui_auth_session_timeout = hs.config.auth.ui_auth_session_timeout
 
-        # Ratelimitier for failed /login attempts
+        # Ratelimiter for failed /login attempts
         self._failed_login_attempts_ratelimiter = Ratelimiter(
             store=self.store,
             clock=hs.get_clock(),
-            rate_hz=self.hs.config.ratelimiting.rc_login_failed_attempts.per_second,
-            burst_count=self.hs.config.ratelimiting.rc_login_failed_attempts.burst_count,
+            cfg=self.hs.config.ratelimiting.rc_login_failed_attempts,
         )
 
         self._clock = self.hs.get_clock()
diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 17ff8821d9..798c7039f9 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -90,8 +90,7 @@ class DeviceMessageHandler:
         self._ratelimiter = Ratelimiter(
             store=self.store,
             clock=hs.get_clock(),
-            rate_hz=hs.config.ratelimiting.rc_key_requests.per_second,
-            burst_count=hs.config.ratelimiting.rc_key_requests.burst_count,
+            cfg=hs.config.ratelimiting.rc_key_requests,
         )
 
     async def on_direct_to_device_edu(self, origin: str, content: JsonDict) -> None:
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 3031384d25..472879c964 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -66,14 +66,12 @@ class IdentityHandler:
         self._3pid_validation_ratelimiter_ip = Ratelimiter(
             store=self.store,
             clock=hs.get_clock(),
-            rate_hz=hs.config.ratelimiting.rc_3pid_validation.per_second,
-            burst_count=hs.config.ratelimiting.rc_3pid_validation.burst_count,
+            cfg=hs.config.ratelimiting.rc_3pid_validation,
         )
         self._3pid_validation_ratelimiter_address = Ratelimiter(
             store=self.store,
             clock=hs.get_clock(),
-            rate_hz=hs.config.ratelimiting.rc_3pid_validation.per_second,
-            burst_count=hs.config.ratelimiting.rc_3pid_validation.burst_count,
+            cfg=hs.config.ratelimiting.rc_3pid_validation,
         )
 
     async def ratelimit_request_token_requests(
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 1d8d4a72e7..de0f04e3fe 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -112,8 +112,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._join_rate_limiter_local = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_joins_local.per_second,
-            burst_count=hs.config.ratelimiting.rc_joins_local.burst_count,
+            cfg=hs.config.ratelimiting.rc_joins_local,
         )
         # Tracks joins from local users to rooms this server isn't a member of.
         # I.e. joins this server makes by requesting /make_join /send_join from
@@ -121,8 +120,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._join_rate_limiter_remote = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_joins_remote.per_second,
-            burst_count=hs.config.ratelimiting.rc_joins_remote.burst_count,
+            cfg=hs.config.ratelimiting.rc_joins_remote,
         )
         # TODO: find a better place to keep this Ratelimiter.
         #   It needs to be
@@ -135,8 +133,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._join_rate_per_room_limiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_joins_per_room.per_second,
-            burst_count=hs.config.ratelimiting.rc_joins_per_room.burst_count,
+            cfg=hs.config.ratelimiting.rc_joins_per_room,
         )
 
         # Ratelimiter for invites, keyed by room (across all issuers, all
@@ -144,8 +141,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._invites_per_room_limiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_invites_per_room.per_second,
-            burst_count=hs.config.ratelimiting.rc_invites_per_room.burst_count,
+            cfg=hs.config.ratelimiting.rc_invites_per_room,
         )
 
         # Ratelimiter for invites, keyed by recipient (across all rooms, all
@@ -153,8 +149,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._invites_per_recipient_limiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_invites_per_user.per_second,
-            burst_count=hs.config.ratelimiting.rc_invites_per_user.burst_count,
+            cfg=hs.config.ratelimiting.rc_invites_per_user,
         )
 
         # Ratelimiter for invites, keyed by issuer (across all rooms, all
@@ -162,15 +157,13 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self._invites_per_issuer_limiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_invites_per_issuer.per_second,
-            burst_count=hs.config.ratelimiting.rc_invites_per_issuer.burst_count,
+            cfg=hs.config.ratelimiting.rc_invites_per_issuer,
         )
 
         self._third_party_invite_limiter = Ratelimiter(
             store=self.store,
             clock=self.clock,
-            rate_hz=hs.config.ratelimiting.rc_third_party_invite.per_second,
-            burst_count=hs.config.ratelimiting.rc_third_party_invite.burst_count,
+            cfg=hs.config.ratelimiting.rc_third_party_invite,
         )
 
         self.request_ratelimiter = hs.get_request_ratelimiter()
diff --git a/synapse/handlers/room_summary.py b/synapse/handlers/room_summary.py
index dad3e23470..dd559b4c45 100644
--- a/synapse/handlers/room_summary.py
+++ b/synapse/handlers/room_summary.py
@@ -35,6 +35,7 @@ from synapse.api.errors import (
     UnsupportedRoomVersionError,
 )
 from synapse.api.ratelimiting import Ratelimiter
+from synapse.config.ratelimiting import RatelimitSettings
 from synapse.events import EventBase
 from synapse.types import JsonDict, Requester, StrCollection
 from synapse.util.caches.response_cache import ResponseCache
@@ -94,7 +95,9 @@ class RoomSummaryHandler:
         self._server_name = hs.hostname
         self._federation_client = hs.get_federation_client()
         self._ratelimiter = Ratelimiter(
-            store=self._store, clock=hs.get_clock(), rate_hz=5, burst_count=10
+            store=self._store,
+            clock=hs.get_clock(),
+            cfg=RatelimitSettings("<room summary>", per_second=5, burst_count=10),
         )
 
         # If a user tries to fetch the same page multiple times in quick succession,
diff --git a/synapse/http/server.py b/synapse/http/server.py
index 5109cec983..3bbf91298e 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -115,7 +115,13 @@ def return_json_error(
         if exc.headers is not None:
             for header, value in exc.headers.items():
                 request.setHeader(header, value)
-        logger.info("%s SynapseError: %s - %s", request, error_code, exc.msg)
+        error_ctx = exc.debug_context
+        if error_ctx:
+            logger.info(
+                "%s SynapseError: %s - %s (%s)", request, error_code, exc.msg, error_ctx
+            )
+        else:
+            logger.info("%s SynapseError: %s - %s", request, error_code, exc.msg)
     elif f.check(CancelledError):
         error_code = HTTP_STATUS_REQUEST_CANCELLED
         error_dict = {"error": "Request cancelled", "errcode": Codes.UNKNOWN}
diff --git a/synapse/rest/client/login.py b/synapse/rest/client/login.py
index d724c68920..7be327e26f 100644
--- a/synapse/rest/client/login.py
+++ b/synapse/rest/client/login.py
@@ -120,14 +120,12 @@ class LoginRestServlet(RestServlet):
         self._address_ratelimiter = Ratelimiter(
             store=self._main_store,
             clock=hs.get_clock(),
-            rate_hz=self.hs.config.ratelimiting.rc_login_address.per_second,
-            burst_count=self.hs.config.ratelimiting.rc_login_address.burst_count,
+            cfg=self.hs.config.ratelimiting.rc_login_address,
         )
         self._account_ratelimiter = Ratelimiter(
             store=self._main_store,
             clock=hs.get_clock(),
-            rate_hz=self.hs.config.ratelimiting.rc_login_account.per_second,
-            burst_count=self.hs.config.ratelimiting.rc_login_account.burst_count,
+            cfg=self.hs.config.ratelimiting.rc_login_account,
         )
 
         # ensure the CAS/SAML/OIDC handlers are loaded on this worker instance.
diff --git a/synapse/rest/client/login_token_request.py b/synapse/rest/client/login_token_request.py
index b1629f94a5..d189a923b5 100644
--- a/synapse/rest/client/login_token_request.py
+++ b/synapse/rest/client/login_token_request.py
@@ -16,6 +16,7 @@ import logging
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.ratelimiting import Ratelimiter
+from synapse.config.ratelimiting import RatelimitSettings
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.http.site import SynapseRequest
@@ -66,15 +67,18 @@ class LoginTokenRequestServlet(RestServlet):
         self.token_timeout = hs.config.auth.login_via_existing_token_timeout
         self._require_ui_auth = hs.config.auth.login_via_existing_require_ui_auth
 
-        # Ratelimit aggressively to a maxmimum of 1 request per minute.
+        # Ratelimit aggressively to a maximum of 1 request per minute.
         #
         # This endpoint can be used to spawn additional sessions and could be
         # abused by a malicious client to create many sessions.
         self._ratelimiter = Ratelimiter(
             store=self._main_store,
             clock=hs.get_clock(),
-            rate_hz=1 / 60,
-            burst_count=1,
+            cfg=RatelimitSettings(
+                key="<login token request>",
+                per_second=1 / 60,
+                burst_count=1,
+            ),
         )
 
     @interactive_auth_handler
diff --git a/synapse/rest/client/register.py b/synapse/rest/client/register.py
index 77e3b91b79..132623462a 100644
--- a/synapse/rest/client/register.py
+++ b/synapse/rest/client/register.py
@@ -376,8 +376,7 @@ class RegistrationTokenValidityRestServlet(RestServlet):
         self.ratelimiter = Ratelimiter(
             store=self.store,
             clock=hs.get_clock(),
-            rate_hz=hs.config.ratelimiting.rc_registration_token_validity.per_second,
-            burst_count=hs.config.ratelimiting.rc_registration_token_validity.burst_count,
+            cfg=hs.config.ratelimiting.rc_registration_token_validity,
         )
 
     async def on_GET(self, request: Request) -> Tuple[int, JsonDict]:
diff --git a/synapse/server.py b/synapse/server.py
index 7cdd3ea3c2..fd16dacd0d 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -408,8 +408,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         return Ratelimiter(
             store=self.get_datastores().main,
             clock=self.get_clock(),
-            rate_hz=self.config.ratelimiting.rc_registration.per_second,
-            burst_count=self.config.ratelimiting.rc_registration.burst_count,
+            cfg=self.config.ratelimiting.rc_registration,
         )
 
     @cache_in_self
diff --git a/synapse/util/ratelimitutils.py b/synapse/util/ratelimitutils.py
index cde4a0780f..f693ba2a8c 100644
--- a/synapse/util/ratelimitutils.py
+++ b/synapse/util/ratelimitutils.py
@@ -291,7 +291,8 @@ class _PerHostRatelimiter:
             if self.metrics_name:
                 rate_limit_reject_counter.labels(self.metrics_name).inc()
             raise LimitExceededError(
-                retry_after_ms=int(self.window_size / self.sleep_limit)
+                limiter_name="rc_federation",
+                retry_after_ms=int(self.window_size / self.sleep_limit),
             )
 
         self.request_times.append(time_now)
diff --git a/tests/api/test_errors.py b/tests/api/test_errors.py
index 319abfe63d..8e159029d9 100644
--- a/tests/api/test_errors.py
+++ b/tests/api/test_errors.py
@@ -1,6 +1,5 @@
 # Copyright 2023 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
@@ -13,24 +12,32 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
+
 from synapse.api.errors import LimitExceededError
 
 from tests import unittest
 
 
-class ErrorsTestCase(unittest.TestCase):
+class LimitExceededErrorTestCase(unittest.TestCase):
+    def test_key_appears_in_context_but_not_error_dict(self) -> None:
+        err = LimitExceededError("needle")
+        serialised = json.dumps(err.error_dict(None))
+        self.assertIn("needle", err.debug_context)
+        self.assertNotIn("needle", serialised)
+
     # Create a sub-class to avoid mutating the class-level property.
     class LimitExceededErrorHeaders(LimitExceededError):
         include_retry_after_header = True
 
     def test_limit_exceeded_header(self) -> None:
-        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=100)
+        err = self.LimitExceededErrorHeaders(limiter_name="test", retry_after_ms=100)
         self.assertEqual(err.error_dict(None).get("retry_after_ms"), 100)
         assert err.headers is not None
         self.assertEqual(err.headers.get("Retry-After"), "1")
 
     def test_limit_exceeded_rounding(self) -> None:
-        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=3001)
+        err = self.LimitExceededErrorHeaders(limiter_name="test", retry_after_ms=3001)
         self.assertEqual(err.error_dict(None).get("retry_after_ms"), 3001)
         assert err.headers is not None
         self.assertEqual(err.headers.get("Retry-After"), "4")
diff --git a/tests/api/test_ratelimiting.py b/tests/api/test_ratelimiting.py
index fa6c1c02ce..a24638c9ef 100644
--- a/tests/api/test_ratelimiting.py
+++ b/tests/api/test_ratelimiting.py
@@ -1,5 +1,6 @@
 from synapse.api.ratelimiting import LimitExceededError, Ratelimiter
 from synapse.appservice import ApplicationService
+from synapse.config.ratelimiting import RatelimitSettings
 from synapse.types import create_requester
 
 from tests import unittest
@@ -10,8 +11,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(key="", per_second=0.1, burst_count=1),
         )
         allowed, time_allowed = self.get_success_or_raise(
             limiter.can_do_action(None, key="test_id", _time_now_s=0)
@@ -43,8 +43,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(
+                key="",
+                per_second=0.1,
+                burst_count=1,
+            ),
         )
         allowed, time_allowed = self.get_success_or_raise(
             limiter.can_do_action(as_requester, _time_now_s=0)
@@ -76,8 +79,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(
+                key="",
+                per_second=0.1,
+                burst_count=1,
+            ),
         )
         allowed, time_allowed = self.get_success_or_raise(
             limiter.can_do_action(as_requester, _time_now_s=0)
@@ -101,8 +107,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(key="", per_second=0.1, burst_count=1),
         )
 
         # Shouldn't raise
@@ -128,8 +133,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(key="", per_second=0.1, burst_count=1),
         )
 
         # First attempt should be allowed
@@ -177,8 +181,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(key="", per_second=0.1, burst_count=1),
         )
 
         # First attempt should be allowed
@@ -208,8 +211,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=1,
+            cfg=RatelimitSettings(key="", per_second=0.1, burst_count=1),
         )
         self.get_success_or_raise(
             limiter.can_do_action(None, key="test_id_1", _time_now_s=0)
@@ -244,7 +246,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
             )
         )
 
-        limiter = Ratelimiter(store=store, clock=self.clock, rate_hz=0.1, burst_count=1)
+        limiter = Ratelimiter(
+            store=store,
+            clock=self.clock,
+            cfg=RatelimitSettings("", per_second=0.1, burst_count=1),
+        )
 
         # Shouldn't raise
         for _ in range(20):
@@ -254,8 +260,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=3,
+            cfg=RatelimitSettings(
+                key="",
+                per_second=0.1,
+                burst_count=3,
+            ),
         )
         # Test that 4 actions aren't allowed with a maximum burst of 3.
         allowed, time_allowed = self.get_success_or_raise(
@@ -321,8 +330,7 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=3,
+            cfg=RatelimitSettings("", per_second=0.1, burst_count=3),
         )
 
         def consume_at(time: float) -> bool:
@@ -346,8 +354,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=3,
+            cfg=RatelimitSettings(
+                "",
+                per_second=0.1,
+                burst_count=3,
+            ),
         )
 
         # Observe two actions, leaving room in the bucket for one more.
@@ -369,8 +380,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=3,
+            cfg=RatelimitSettings(
+                "",
+                per_second=0.1,
+                burst_count=3,
+            ),
         )
 
         # Observe three actions, filling up the bucket.
@@ -398,8 +412,11 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         limiter = Ratelimiter(
             store=self.hs.get_datastores().main,
             clock=self.clock,
-            rate_hz=0.1,
-            burst_count=3,
+            cfg=RatelimitSettings(
+                "",
+                per_second=0.1,
+                burst_count=3,
+            ),
         )
 
         # Observe four actions, exceeding the bucket.
diff --git a/tests/config/test_ratelimiting.py b/tests/config/test_ratelimiting.py
index f12147eaa0..0c27dd21e2 100644
--- a/tests/config/test_ratelimiting.py
+++ b/tests/config/test_ratelimiting.py
@@ -12,11 +12,42 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from synapse.config.homeserver import HomeServerConfig
+from synapse.config.ratelimiting import RatelimitSettings
 
 from tests.unittest import TestCase
 from tests.utils import default_config
 
 
+class ParseRatelimitSettingsTestcase(TestCase):
+    def test_depth_1(self) -> None:
+        cfg = {
+            "a": {
+                "per_second": 5,
+                "burst_count": 10,
+            }
+        }
+        parsed = RatelimitSettings.parse(cfg, "a")
+        self.assertEqual(parsed, RatelimitSettings("a", 5, 10))
+
+    def test_depth_2(self) -> None:
+        cfg = {
+            "a": {
+                "b": {
+                    "per_second": 5,
+                    "burst_count": 10,
+                },
+            }
+        }
+        parsed = RatelimitSettings.parse(cfg, "a.b")
+        self.assertEqual(parsed, RatelimitSettings("a.b", 5, 10))
+
+    def test_missing(self) -> None:
+        parsed = RatelimitSettings.parse(
+            {}, "a", defaults={"per_second": 5, "burst_count": 10}
+        )
+        self.assertEqual(parsed, RatelimitSettings("a", 5, 10))
+
+
 class RatelimitConfigTestCase(TestCase):
     def test_parse_rc_federation(self) -> None:
         config_dict = default_config("test")