librenms-librenms/includes/services/check_domain.inc.php

<?php

//Get check_domain from https://raw.githubusercontent.com/glensc/nagios-plugin-check_domain/master/check_domain.sh
$check_cmd = \LibreNMS\Config::get('nagios_plugins') . '/check_domain -d ';
if ($service['service_ip']) {
    $check_cmd .= $service['service_ip'];
} else {
    $check_cmd .= $service['hostname'];
}
$check_cmd .= ' ' . $service['service_param'];