MQTT troubleshooting

Cannot connect to the broker

Behavior: docker containers are waiting indefinitely for the mqtt_broker.

1. check if your mqtt_broker container is up and running.
2. check if it actually listens to the tcp port: netstat -tln | grep 8883. Adapt if you changed the port.
3. check if your mqtt_broker says it listen to tcp ports, for example if you have only something like:

mqtt_broker_1          | 1617352477: mosquitto version 2.0.8 starting
mqtt_broker_1          | 1617352477: Config loaded from /mosquitto/config/mosquitto.conf.

Something is wrong, you should have something like:

mqtt_broker_1          | 1617352704: mosquitto version 2.0.7 starting
mqtt_broker_1          | 1617352704: Config loaded from /mosquitto/config/mosquitto.conf.
mqtt_broker_1          | 1617352704: Opening ipv4 listen socket on port 8883.
mqtt_broker_1          | 1617352704: Opening ipv6 listen socket on port 8883.
mqtt_broker_1          | 1617352704: mosquitto version 2.0.7 running

Verify your passwd file

I got this issue because my passwd was not a file for some reason but a folder. Take a look at your mosquitto passwd file to verify that it's a like containing a hashed password and nothing else. If you have doubt delete it and regenerate it.

That is incredible... mosquitto does not crash, does not send any error messages, just silently don't run if your passwd file is corrupted.