### Understand the alert

This alert is triggered when there is a significant increase in the number of unsuccessful v3/v5 CONNACK packets sent by the VerneMQ broker within the last minute. A higher-than-normal rate of unsuccessful CONNACKs indicates that clients are experiencing difficulties establishing a connection with the MQTT broker.

### What is a CONNACK packet?

A CONNACK packet is an acknowledgment packet sent by the MQTT broker to a client in response to a CONNECT command. The CONNACK packet informs the client if the connection has been accepted or rejected, which is indicated by the return code. An unsuccessful CONNACK packet indicates a rejected connection.

### Troubleshoot the alert

1. **Check VerneMQ logs**: Inspect the VerneMQ logs for error messages or reasons why the connections are being rejected. By default, these logs are located at `/var/log/vernemq/console.log` and `/var/log/vernemq/error.log`. Look for entries with "CONNACK" and discern the cause of the unsuccessful connections.

2. **Diagnose client configuration issues**: Analyze the rejected connection attempts' client configurations, such as incorrect credentials, unsupported protocol versions, or security settings. Debug the client-side applications, fix the configurations, and try reconnecting to the MQTT broker.

3. **Evaluate broker capacity**: Check the system resources and settings of the VerneMQ broker. An overloaded broker or insufficient system resources, such as CPU and memory, can cause connection rejections. Optimize the VerneMQ configuration, upgrade the broker's hardware, or distribute the load between multiple brokers to resolve the issue.

4. **Assess network issues**: Verify the network topology, firewalls, and router settings to ensure clients can reach the MQTT broker. Network latency or misconfigurations can lead to unsuccessful CONNACKs. Use monitoring tools such as `ping`, `traceroute`, or `netstat` to diagnose network issues and assess connectivity between clients and the broker.

5. **Verify security settings and permissions**: Check the VerneMQ broker's security settings, including access control lists (ACL), user permissions, and authentication/authorization settings. Restricted access or incorrect permissions can lead to connection rejections. Update the security settings accordingly and test the connection again.

