Is it possible to block Tor users?

Question

Would it be possible to block Tor users? (https://www.torproject.org/)

Due to the nature of the site I run I should do all I can to stop multiple accounts and block cert

Answer 1

You can use the TorDNSEL service to perform a live query about whether a specific IP address is a Tor exit node. You query the service via a specially-formed DNS request.

Here is some sample PHP code that performs the lookup:

function isTorExitNode() {
    $serverPort = $_SERVER['SERVER_PORT'];
    $remoteAddr = reverseIp(getClientIp());
    $serverAddr = reverseIp($_SERVER['SERVER_ADDR']);
    $placeholders = '%s.%s.%s.ip-port.exitlist.torproject.org';
    $name = sprintf($placeholders, $remoteAddr, $serverPort, $serverAddr);
    return ( gethostbyname($name) === '127.0.0.2' );
}

function getClientIp() {
    if (isset($_SERVER['HTTP_CF_CONNECTING_IP'])) {
        return $_SERVER['HTTP_CF_CONNECTING_IP'];
    }
    return $_SERVER['REMOTE_ADDR'];
}

function reverseIp($ip) {
    $ipParts = explode('.', $ip);
    return $ipParts[3] . '.' . $ipParts[2] . '.' .
           $ipParts[1] . '.' . $ipParts[0];
}

if (!isTorExitNode()) {
    // Do nothing
} else {
    Die("Sorry, You cannot use TOR network!!!");
}

Important Notes:

• This example supports IPv4 addresses only, not IPv6.

• It could take a couple of seconds to get a response, so be careful about introducing delays into your site rendering.