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

<?php

// provide some sane default
if ($service['service_param']) {
    $nsquery  = $service['service_param'];
} else {
    $nsquery  = "localhost";
}
if ($service['service_ip']) {
    $resolver = $service['service_ip'];
} else {
    $resolver = $service['hostname'];
}

$check_cmd = \LibreNMS\Config::get('nagios_plugins') . "/check_dns -H " . $nsquery . " -s " . $resolver;