MQTT monitoring Daemon

Proven on:

11 (bullseye)

An example of what's possible...

Requirements:

• apt install mosquitto-clients
• apt install jq

The systemd Service Unit

/etc/systemd/system/MQTTmon.service

[Unit]

Description=MQTT Monitoring Daemon
After=network.target
Requires=network.target

[Service]
Type=simple
User=root
ExecStart=/usr/bin/MQTTmon
Restart=always

[Install]
WantedBy=multi-user.target

• Any time you create or edit a service unit:
  • systemctl daemon-reload
  • systemctl restart MQTTmon.service
• Check on how it's going...
  • systemctl status MQTTmon.service

The Script

/usr/bin/MQTTmon

#!/bin/bash

BROKER="LXC-Mosquitto"
TOPIC="turd"

MQTT=`/usr/bin/mosquitto_sub -C 1 -h $BROKER -t $TOPIC`

TARGET=`echo $MQTT {{!}} jq '.Target'`
MESSAGE=`echo $MQTT {{!}} jq '.Message'`
COMMAND=`echo ${MQTT} {{!}} jq '.Command'`

HOST=\"`hostname`\"

if [[ $TARGET == $HOST ]]
  then
    /root/Logit $TARGET , $COMMAND , $MESSAGE
    if [[ $COMMAND == "\"MSG\"" ]] ; then
      /root/Logit "Message is: " $MESSAGE
    fi
    if [[ $COMMAND == "\"WALL\"" ]] ; then
      wall -n $MESSAGE
    fi
    if [[ $COMMAND == "\"DIE\"" ]] ; then
      shutdown
    fi
  else
    /root/Logit $TARGET " got a message too, but this is " $HOST " so we don't GAF."
fi

Sending it messages

In these examples, the machine (an LXC) used for testng is named poop2

• Telling it to shut down...
  • mosquitto_pub -h LXC-Mosquitto -t turd -m '{"Target":"poop2","Message": "It is time to DIE","Command":"DIE"}'
• Send a message to all logged-in users terminals
  • mosquitto_pub -h LXC-Mosquitto -t turd -m '{"Target":"poop2","Message": "This is a TURD","Command":"WALL"}'
• Just log a message (using random logger for testing)
  • mosquitto_pub -h LXC-Mosquitto -t turd -m '{"Target":"poop2","Message": "This is another TURD","Command":"MSG"}'
• Send a message to a different machine
  • mosquitto_pub -h LXC-Mosquitto -t turd -m '{"Target":"poop1","Message": "This is a TURD too","Command":"DoIt"}'