Command Execution

Have a look at the code. Since the domain parameter is not sanitized, you are vulnerable to command injection.

An Insecure Command-Line Call
 <?php
  if (isset($_GET['domain'])) {
    echo '<pre>';
    $domain = $_GET['domain'];
    $lookup = system("nslookup {$domain}");
    echo($lookup);
    echo '</pre>';
  }
?>