sudo vim /etc/init.d/mosquitto

Change /bin/sh to /bin/bash for fixing "ulimit -u illegal option" error

set_ulimit () {

ulimit -f unlimited
ulimit -t unlimited
ulimit -v unlimited
ulimit -n 1048576
ulimit -m unlimited
ulimit -u 1048576

}

start)

...
# Update ulimit config in start command
set_ulimit
...
;;

stop)

After changed the config, reload systemctl config and restart mosquitto

sudo systemctl daemon-reload
sudo service mosquitto stop
sudo service mosquitto start

https://stackoverflow.com/questions/40199152/mosquitto-increase-max-connections