Mosquitto

Mosquitto is an open-source message broker that implements the MQTT (Message Queuing Telemetry Transport) protocol. It is designed to be lightweight and efficient, making it suitable for low-resource devices and Internet of Things deployments. Developed originally by Roger Light, the project is now maintained under the Eclipse Foundation as part of the Eclipse IoT ecosystem.

Mosquitto supports MQTT protocol versions 3.1, 3.1.1, and 5.0. It provides features commonly used in IoT deployments, including TLS/SSL encryption, username/password authentication via a password file, access control lists, and support for WebSocket connections. It offers retained messages, Last Will and Testament, and QoS levels 0–2, as well as the ability to bridge connections to other MQTT brokers. The broker can be configured to persist messages to disk.

Architecture and configuration: Mosquitto runs as a lightweight daemon on various platforms, including Linux, Windows, and macOS, and can be used in embedded environments. Configuration is done with a plain text file (mosquitto.conf) that defines listeners (ports and interfaces), authentication methods, TLS certificates, logging, and other options. Common listeners include port 1883 for MQTT, 8883 for TLS, and 9001 for WebSockets. The broker can be extended with plugins and can be integrated with external authentication backends.

Usage: Mosquitto ships with command-line clients mosquitto_pub and mosquitto_sub for publishing and subscribing. It also provides mosquitto_passwd to create password files and mosquitto.conf examples to start quickly. It is distributed as free and open-source software and is widely used in a variety of IoT projects.