PHP GetHostByAddr() Slow? Make it Faster

Posted on April 16, 2019 at 11:43 pm

A quick workaround to make gethostbyaddr() faster in PHP is:

// Set timeout and retries to 1 to have a max execution time of 1 second for the DNS lookup
putenv('RES_OPTIONS=retrans:1 retry:1 timeout:1 attempts:1');
 
// Now call gethostbyaddr() function
$ip = @gethostbyaddr("1.2.3.4");

Thansk to Dennis for these awesome tips:

http://php.net/manual/en/function.gethostbyname.php#118841

However, you may also use this smart PHP function:

* Uses an UDP socket to talk directly to the DNS server!

function gethostbyaddr_timeout($ip, $dns, $timeout = 1000)
{
    // random transaction number (for routers etc to get the reply back)
    $data = rand(0, 99);
    // trim it to 2 bytes
    $data = substr($data, 0, 2);
    // request header
    $data .= "\1\0\0\1\0\0\0\0\0\0";
    // split IP up
    $bits = explode(".", $ip);
    // error checking
    if (count($bits) != 4) return "ERROR";
    // there is probably a better way to do this bit...
    // loop through each segment
    for ($x=3; $x>=0; $x--)
    {
        // needs a byte to indicate the length of each segment of the request
        switch (strlen($bits[$x]))
        {
            case 1: // 1 byte long segment
                $data .= "\1"; break;
            case 2: // 2 byte long segment
                $data .= "\2"; break;
            case 3: // 3 byte long segment
                $data .= "\3"; break;
            default: // segment is too big, invalid IP
                return "INVALID";
        }
        // and the segment itself
        $data .= $bits[$x];
    }
    // and the final bit of the request
    $data .= "\7in-addr\4arpa\0\0\x0C\0\1";
    // create UDP socket
    $handle = @fsockopen("udp://$dns", 53);
    // send our request (and store request size so we can cheat later)
    $requestsize = @fwrite($handle, $data);
 
    @socket_set_timeout($handle, $timeout - $timeout%1000, $timeout%1000);
    // hope we get a reply
    $response = @fread($handle, 1000);
    @fclose($handle);
    if ($response == "")
        return $ip;
    // find the response type
    $type = @unpack("s", substr($response, $requestsize+2));
    if ($type[1] == 0x0C00)  // answer
    {
        // set up our variables
        $host = "";
        $len = 0;
        // set our pointer at the beginning of the hostname
        // uses the request size from earlier rather than work it out
        $position = $requestsize+12;
        // reconstruct hostname
        do
        {
            // get segment size
            $len = unpack("c", substr($response, $position));
            // null terminated string, so length 0 = finished
            if ($len[1] == 0)
                // return the hostname, without the trailing .
                return substr($host, 0, strlen($host) -1);
            // add segment to our host
            $host .= substr($response, $position+1, $len[1]) . ".";
            // move pointer on to the next segment
            $position += $len[1] + 1;
        }
        while ($len != 0);
        // error - return the hostname we constructed (without the . on the end)
        return $ip;
    }
    return $ip;
}

An example usage is like this:

// Using Google DNS Server (8.8.8.8)
$hostname = gethostbyaddr_timeout("1.2.3.4", "8.8.8.8");

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