How does a MQTT server send a message to a client saying that its not authorized to connect?

In the MQTT (Message Queuing Telemetry Transport) protocol, communication between the server (broker) and client follows a defined process. When a client attempts to connect to an MQTT server, if the server determines the client lacks authorization, it notifies the client by returning a specific connection response message. The steps are as follows:

1. Client sends connection request: The client requests connection to the server by sending a CONNECT message. This message includes the client identifier, username, password, and keep-alive time.

2. Server processes connection request: Upon receiving the CONNECT message, the server validates the provided information. This includes verifying the username and password, checking the client identifier, and potentially checking the client's IP address or other security policies.

3. Server sends connection response:

• If validation succeeds, the server sends a CONNACK message with return code 0 (indicating successful connection).
• If validation fails, for example due to incorrect username or password, or lack of authorization, the server sends a CONNACK message with a return code indicating the specific error. For instance, return code 5 indicates 'Unauthorized', meaning the client lacks authorization.

1. Client processes CONNACK message: Upon receiving the CONNACK message, the client checks the return code. If the return code is not 0, the client typically takes appropriate actions based on the error code, such as retrying the connection, prompting the user with an error message, or terminating the connection attempt.

Example scenario:

Suppose a client attempts to connect to an MQTT server but provides incorrect username and password. The following is a simplified interaction example:

• Client sends CONNECT message:

  shell
  CONNECT
  ClientId: client123
  Username: user_example
  Password: wrong_password

• Server processes and returns CONNACK message:

  shell
  CONNACK
  Return Code: 4 (Bad user name or password)

• Client receives CONNACK and processes: The client checks the return code as 4, realizing the username or password is incorrect, and may prompt the user to re-enter or log an error indicating connection failure.

This process ensures that only clients with correct credentials and authorization can successfully connect to the MQTT server, thereby maintaining system security.