Merge pull request #12352 from rabbitmq/mqtt-max-packet-size

Decrease default MQTT Maximum Packet Size
rabbitmq · Sep 21, 2024 · bfdf4fb · bfdf4fb
2 parents 83f2875 + bddc546
commit bfdf4fb
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Showing 4 changed files with 11 additions and 2 deletions.
diff --git a/deps/rabbitmq_mqtt/BUILD.bazel b/deps/rabbitmq_mqtt/BUILD.bazel
@@ -49,7 +49,8 @@ APP_ENV = """[
 	    {mailbox_soft_limit, 200},
 	    {max_packet_size_unauthenticated, 65536},
 	    %% 256 MB is upper limit defined by MQTT spec
-	    {max_packet_size_authenticated, 268435455},
+	    %% We set 16 MB as defined in deps/rabbit/Makefile max_message_size
+	    {max_packet_size_authenticated, 16777216},
 	    {topic_alias_maximum, 16}
 	  ]
 """

diff --git a/deps/rabbitmq_mqtt/Makefile b/deps/rabbitmq_mqtt/Makefile
@@ -27,7 +27,8 @@ define PROJECT_ENV
 	    {mailbox_soft_limit, 200},
 	    {max_packet_size_unauthenticated, 65536},
 	    %% 256 MB is upper limit defined by MQTT spec
-	    {max_packet_size_authenticated, 268435455},
+	    %% We set 16 MB as defined in deps/rabbit/Makefile max_message_size
+	    {max_packet_size_authenticated, 16777216},
 	    {topic_alias_maximum, 16}
 	  ]
 endef

diff --git a/deps/rabbitmq_mqtt/src/rabbit_mqtt.erl b/deps/rabbitmq_mqtt/src/rabbit_mqtt.erl
@@ -112,6 +112,8 @@ persist_static_configuration() ->
 
     {ok, MaxSizeAuth} = application:get_env(?APP_NAME, max_packet_size_authenticated),
     assert_valid_max_packet_size(MaxSizeAuth),
+    {ok, MaxMsgSize} = application:get_env(rabbit, max_message_size),
+    ?assert(MaxSizeAuth =< MaxMsgSize),
     ok = persistent_term:put(?PERSISTENT_TERM_MAX_PACKET_SIZE_AUTHENTICATED, MaxSizeAuth).
 
 assert_valid_max_packet_size(Val) ->

diff --git a/release-notes/4.1.0.md b/release-notes/4.1.0.md
@@ -0,0 +1,5 @@
+## RabbitMQ 4.1.0
+
+## Potential incompatibilities
+
+* The default MQTT [Maximum Packet Size](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901086) changed from 256 MiB to 16 MiB. This default can be overriden by [configuring](https://www.rabbitmq.com/docs/configure#config-file) `mqtt.max_packet_size_authenticated`. Note that this value must not be greater than `max_message_size` (which also defaults to 16 MiB).