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message_size_limit ignores the size of Will message and MQTT5 properties #2022

umorelli opened this issue Jan 15, 2021 · 1 comment


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while performing some tests I noticed that, if setting the message_size_limit in the Mosquitto configuration, clients can bypass it via connect Will messages (with a payload up to 65'535 characters) or, with MQTT5 packets, by specifying large/many user properties (with a limit of 65'535 characters for each property name/value). Is it a bug or the intended behaviour?
- I'm asking because I could not find any reference for Will/Properties limitations in the OASIS MQTT 3/5 specifications and, in any case, Mosquitto supports limiting any MQTT packet (as a whole) via max_packet_size.

To replicate, here is a Paho Python script that sets a 65'535 characters Will message, publish a 1 character payload message (with two user properties of 65'535 x 2 chars) and disconnect with Reason Code
4 (hence publishing the Will message). A subscriber connected with mosquitto_sub -t topic -t will/topic -V 5 -F %%Payload:%p\\n%%Properties:%P\\n will receive both messages and the user properties of the PUBLISH.

Mosquitto is configure to restrict payload size to 1 byte with the following:

listener 1883
allow_anonymous true
message_size_limit 1
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ralight commented Jan 19, 2021

message_size_limit is explicitly for the payload only - it predates MQTT v5.

I would definitely recommend the use of max_packet_size because this controls the whole packet size, which is the more important factor. Going further, I think that message_size_limit should be deprecated in favour of max_packet_size.

Either way, the WIll payload should not be exempt from the message_size_limit and I've now fixed that. it will be part of 2.0.6 next week. Thanks for the nice report.

@ralight ralight added this to the 2.0.6 milestone Jan 19, 2021
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