mosquitto is an Open Source MQTT server. MQTT is a machine to machine or IOT (Internet of Things) protocol.
To install run
apt-get install mosquitto mosquitto-clients
To test, run
mosquitto_sub -h localhost -t test
To create a user, run
mosquitto_passwd -c /etc/mosquitto/passwd USER_NAME
Config file available at
/etc/mosquitto/conf.d/default.conf
To restart, use
systemctl restart mosquitto
To connect with user and password, use
mosquitto_sub -h localhost -t test -u "USER_NAME" -P "PASSWORD"