MQTT server sending a message to a client declaring unauthorization

Question

So, there's this MQTT client that sends a CONNECT packet the MQTT broker with a username and the password. 

If the client is not authorized to connect, the broker closes the connection. 

Since the CONNACK packet does not allow any payloads, I was wondering how the MQTT broker tells the client that the reason for closing the connection is no authorization.

Answer 1

It would return a CONNACK packet with the Connect Return code set to 5. 

Refer section 3.2.2.3 of the MQTT v3.1.1 spec.

http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718033