MQTT

Troubleshooting

Client Cannot Connect

Check:

• The MQTT listener port (1883 or 8883)
• TLS configuration if using encrypted MQTT
• Username and password
• Virtual host permissions
• Firewall or network connectivity

LavinMQ authenticates MQTT clients during the MQTT CONNECT phase. Authentication failures typically result in the connection being rejected with a CONNACK authorization error.

---

Subscribers Not Receiving Messages

Verify:

• The published topic matches the subscription filter
• Wildcard usage (+ and #)
• The client is connected to the correct virtual host
• Topic permissions allow reads and writes
• QoS expectations match the client configuration

Internally, LavinMQ routes MQTT messages through the MQTT exchange and subscription tree, so incorrect topic filters are a common source of delivery issues.

---

Retained Messages Not Appearing

Verify:

• Messages are published with the retain flag enabled
• Retained messages were not cleared using an empty retained publish
• The subscriber topic filter matches the retained topic

Retained messages are stored separately from normal message delivery and are sent immediately when a new subscription matches.