Maintenance flag bypass via IP whitelist file var/maintenance-whitelist.txt for Magento v1.x

Magento_1_ip_whitelist_for_mainteance_mode.php

if (file_exists($maintenanceFile)) {
    $whitelistIps = [];
    if (file_exists('var/maintenance-whitelist.txt')) {
        $whitelistIps = explode("\n", trim(file_get_contents('var/maintenance-whitelist.txt')));
        $whitelistIps = array_map(function($value) {
            return preg_replace('%^\s*([0-9\.]+).*$%', '$1', $value);
        }, $whitelistIps);
    }
    if (!in_array($_SERVER['REMOTE_ADDR'], $whitelistIps)) {
        include_once dirname(__FILE__) . '/errors/503.php';
        exit;
    }
}