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 = $config['nagios_plugins'] . "/check_dns -H ".$nsquery." -s ".$resolver;