Install the packages:

# apt-get install mosquitto
# apt-get install mosquitto-clients

Update the config file:

# nano /etc/mosquitto/mosquitto.conf
------------------------------------
pid_file /run/mosquitto/mosquitto.pid

persistence true
persistence_location /var/lib/mosquitto/

log_dest file /var/log/mosquitto.log

include_dir /etc/mosquitto/conf.d

allow_anonymous true
log_type error
log_type warning
#log_type notice
#log_type information

connection_messages true
log_timestamp true

Check status:

# systemctl status mosquitto.service
------------------------------------
● mosquitto.service - Mosquitto MQTT Broker
     Loaded: loaded (/lib/systemd/system/mosquitto.service; enabled; preset: enabled)
     Active: active (running) since Fri 2023-10-20 08:29:19 CEST; 1s ago
       Docs: man:mosquitto.conf(5)
             man:mosquitto(8)
    Process: 1120 ExecStartPre=/bin/mkdir -m 740 -p /var/log/mosquitto (code=exited, status=0/SUCCESS)
    Process: 1122 ExecStartPre=/bin/chown mosquitto /var/log/mosquitto (code=exited, status=0/SUCCESS)
    Process: 1123 ExecStartPre=/bin/mkdir -m 740 -p /run/mosquitto (code=exited, status=0/SUCCESS)
    Process: 1124 ExecStartPre=/bin/chown mosquitto /run/mosquitto (code=exited, status=0/SUCCESS)
   Main PID: 1125 (mosquitto)
      Tasks: 1 (limit: 1571)
        CPU: 110ms
     CGroup: /system.slice/mosquitto.service
             └─1125 /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf

okt 20 08:29:19 raspberrypi systemd[1]: Starting mosquitto.service - Mosquitto MQTT Broker...
okt 20 08:29:19 raspberrypi mosquitto[1125]: 1697783359: Error: Unable to open log file /var/log/mosquitto.log for writing.
okt 20 08:29:19 raspberrypi systemd[1]: Started mosquitto.service - Mosquitto MQTT Broker.

In your existing terminal, subscribe to the “testtopic” topic

# mosquitto_sub -h localhost -t "testtopic" -v

Then open another terminal and send a message on that topic

# mosquitto_pub -h localhost -t "testtopic" -m "Testing"