Starting mosquitto from the command line, you can use the ‘-v’ (or ‘–verbose’) option for showing verbose messages…this will override the logtype option in the config file. So first step, when something is not going right, start the MQTT server manually with mosquitto -v and watch what the server does while you debug the problem.
If you don’t have direct access to the Broker/Server, you can also check out MQTTBox http://workswithweb.com/mqttbox.html, works on most platforms…great tip from David B., thank you.
Create a new MQTT Client, with some unique MQTT Client Name (MeBox will work), select the protocol you are using in your application, and enter the hostname or IP address for the Host. Enter Username and Password, if used, and check or uncheck all the other boxes that are application to your setup. Then click Save and selecting the “MeBox” client in the list, will allow you to add Publishers to Publish to a Topic, or Subscribers to Subscribe to one or more Topics, with the wildcards (Single Level ‘+’ and Multilevel ‘#’) , as usual.
Very easy to monitor and interact with the MQTT Server: a Published message has a “Publish again” arrow, avoiding quite a but of typing.; restarting MQTTBox will automatically connect (if the option was selected), but you will need to click the Subscribe button in each Subscriber window.