A multithreaded DNS resolver in PHP using basic pthreads capability. You supply a list of IP addresses, hostnames, or both, and the script will resolve each one to its DNS counterpart.

I often work with large batches of hosts that need to be translated from IP to hostname or vice versa. If you only have a handful of hosts to deal with, a simple script like the following will suffice:

<?php
$ips = array_map('trim', file('ips.txt'));
foreach ($ips as $ip) {
    echo $ip . ':' . gethostbyaddr($ip) . "\n";
}

When you're processing many thousands of hosts at a time, such a script can take hours to complete. I needed a multithreaded solution, so here it is. Using a pool of 16 threads, this script is capable of performing 20,000 lookups in 5 minutes. Your mileage will vary based on your hardware, DNS server cache, network connectivity, the hosts being resolved and their DNS servers...

You must have a version of PHP with the pthreads extension installed. You can compile it into PHP, get it from PECL, or perhaps install it through your OS package manager. This script was tested and is known to work on:

• Linux, PHP 5.6.30 with pthreads 2.0.11
• Linux, PHP 7.1.x with pthreads 3.1.7dev
• Linux, PHP 7.2.x with pthreads 3.2.1dev
• Windows, PHP 5.4.45 with pthreads 2.0.9
• Windows, PHP 5.6.30 with pthreads 2.0.9

This script will use whichever DNS server your operating system is configured for. Many public DNS servers, including those operated by Google (8.8.8.8), Level3 (4.2.2.4), etc. employ a throttling mechanism that refuses or ignores your queries if you send too many too quickly. When resolving hosts in bulk, you're better off using your ISP's DNS server or running a local recursive DNS server.

As an aside, if you do operate your own DNS server(s), running this script against a portion of the Alexa Top 1M list is a good way to prime a freshly rebooted resolver.