Download zip files
Extract files at your magento installation
For white listed ip addresses. Replace or backup your original index.php then use and rename the index.php.simple to index.php.
or, use and insert the code below in your index.php for whitelisted ip

/**
 * Maintenance mode
 * note! insert me after `define('MAGENTO_ROOT', getcwd());`
 */
$allowed_ip = array('127.0.0.12', '60.241.193.31');
if (file_exists(MAGENTO_ROOT.'/maintenance.flag') && !in_array($_SERVER['REMOTE_ADDR'], $allowed_ip)) {
	include_once __DIR__.'/errors/503.php';
	exit();
}

License

This project is open-sourced software licensed under the MIT license.