How do I subscribe to all topics of a MQTT broker

Subscribing to # gives you a subscription to everything except for topics that start with a $ (these are normally control topics anyway).

It is better to know what you are subscribing to first though, of course, and note that some broker configurations may disallow subscribing to # explicitly.

You can use mosquitto_sub (which is part of the mosquitto-clients package) and subscribe to the wildcard topic #:

mosquitto_sub -v -h broker_ip -p 1883 -t '#'

Concrete example

mosquitto.org is very active (at the time of this posting). This is a nice smoke test for a MQTT subscriber linux device:

mosquitto_sub -h test.mosquitto.org -t "#" -v

The "#" is a wildcard for topics and returns all messages (topics): the server had a lot of traffic, so it returned a 'firehose' of messages.

If your MQTT device publishes a topic of irisys/V4D-19230005/ to the test MQTT broker , then you could filter the messages:

mosquitto_sub -h test.mosquitto.org -t "irisys/V4D-19230005/#" -v

Options:

• -h the hostname (default MQTT port = 1883)
• -t precedes the topic

Use the wildcard "#" but beware that at some point you will have to somehow understand the data passing through the bus!