sudo apt-get install mosquitto mosquitto-clients
sudo mosquitto_passwd -c /etc/mosquitto/passwd <mqtt_user> (set passwd here)
sudo vi /etc/mosquitto/conf.d/default.conf # new file with
  allow_anonymous false # disable anonymous user
  listener 1883 0.0.0.0 # open to all IP, open to localhost by default.
  password_file /etc/mosquitto/passwd # make password work
sudo /etc/init.d/mosquitto restart

mosquitto_sub  -h localhost -u netro -P netrohome -t 'test'
mosquitto_pub -h localhost -u netro -P netrohome -t 'test' -m 'hello'

The default max open file is 1024, which may not be enough for large MQTT service, increase it to 65535
# vi /etc/systemd/system.conf
DefaultLimitNOFILE=65536:524288
sudo systemctl daemon-reexec
sudo /etc/init.d/mosquitto restart
# cat /proc/<pid>/limits should be: 
Max open files            65535                524288               files
# count the number of file opened by: 
sudo ls /proc/<pid>/fd | wc -l