I'm using Mosquitto for my MQTT Broker.
I was wondering if it would be possible to request all published topics?
Thus NOT by subscribing to everything, i.e. #.
EDIT: I don't want to subscribe to every available topic. I'm just looking for a way to retrieve all published topics. The broker could for instance response with a string array containing all the published topics.
Answer
Manageability of MQTT brokers is very immature at this point. I also don't know of a way to retrieve the list of published topics from any broker. But, there are standardization efforts, eg. with the $SYS topic tree https://github.com/mqtt/mqtt.github.io/wiki/SYS-Topics.
Other brokers are manageable via SNMP, eg. MessageSight https://www.ibm.com/support/knowledgecenter/en/SSCGGQ_1.2.0/com.ibm.ism.doc/Monitoring/admin00008_.html . It can list the topics in its WEB interface, but I don't know of a programmatic way to retrieve them.