Recursive Authoritative Nameserver Lookup in PHP

nslookup.php

<?php
class NSLookup {
	private $root_servers = array(
		'a.root-servers.net.',
		'b.root-servers.net.',
		'c.root-servers.net.',
		'd.root-servers.net.',
		'e.root-servers.net.',
		'f.root-servers.net.',
		'g.root-servers.net.',
		'h.root-servers.net.',
		'i.root-servers.net.',
		'j.root-servers.net.',
		'k.root-servers.net.',
		'l.root-servers.net.',
		'm.root-servers.net.',
	);

	private function get_next_level($domain, $nameservers = null) {
		if ($nameservers == null) {
			$nameservers = $this->root_servers;
		}

		shuffle($nameservers);

		$cli_domain = escapeshellarg($domain);

		$found = false;
		$found_nameservers = array();

		while ($nameserver = escapeshellarg(array_shift($nameservers))) {
			exec("dig +nocomments +noadditional +nocmd +nostats NS {$cli_domain} @{$nameserver}", $output, $status);

			if ($status != 0) {
				continue;
			}

			foreach ($output AS $line) {
				if (preg_match("/^(\S+)\s+\d+\s+IN\s+NS\s+(\S+)$/", $line, $matches)) {
					if ($matches[1] == $domain.'.') {
						$found = true;
					}

					$found_nameservers[] = $matches[2];
				}
			}

			break;
		}

		return array(
			'found' => $found,
			'nameservers' => $found_nameservers
		);
	}

	public function get_nameservers($domain) {
		$result = $this->get_next_level($domain);

		while ($result['found'] == false && count($result['nameservers'])) {
			$result = $this->get_next_level($domain, $result['nameservers']);
		}

		return $result['found'] ? $result['nameservers'] : false;
	}
}