check_external_commands=1
command_check_interval=15s
command_file=/var/lib/nagios3/rw/nagios.cmd
Add www-data user to nagios group
usermod www-data -G nagios
dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios3/rw
dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios
Restart Nagios