PHP script to retrieve a DNS record from a custom nameserver

dns_get_record_from.php

<?php

/**
 * Make a DNS a request to a custom nameserver, this is similar to dns_get_record, but allows you to query any nameserver
 * Usage: dns_get_record_from('ns.server.tld', 'A', 'mydomain.tld');
 * => ['42.42.42.42']
 * @author bohwaz
 */
function dns_get_record_from(string $server, string $type, string $record): array
{
	// Source: https://github.com/metaregistrar/php-dns-client/blob/master/DNS/dnsData/dnsTypes.php
	static $types = [
		1   => 'A',
		2   => 'NS',
		5   => 'CNAME',
		6   => 'SOA',
		12  => 'PTR',
		15  => 'MX',
		16  => 'TXT',
		28  => 'AAAA',
		255 => 'ANY',
	];

	$typeid = array_search($type, $types, true);

	if (!$typeid) {
		throw new \InvalidArgumentException('Invalid type');
	}

	$host = 'udp://' . $server;

	if (!$socket = @fsockopen($host, 53, $errno, $errstr, 10)) {
		throw new \RuntimeException('Failed to open socket to ' . $host);
	}

	$labels = explode('.', $record);
	$question_binary = '';

	foreach ($labels as $label) {
		$question_binary .= pack("C", strlen($label)); // size byte first
		$question_binary .= $label; // then the label
	}

	$question_binary .= pack("C", 0); // end it off

	$id = rand(1,255)|(rand(0,255)<<8);	// generate the ID

	// Set standard codes and flags
	$flags = (0x0100 & 0x0300) | 0x0020; // recursion & queryspecmask | authenticated data

	$opcode = 0x0000; // opcode

	// Build the header
	$header = "";
	$header .= pack("n", $id);
	$header .= pack("n", $opcode | $flags);
	$header .= pack("nnnn", 1, 0, 0, 0);
	$header .= $question_binary;
	$header .= pack("n", $typeid);
	$header .= pack("n", 0x0001); // internet class
	$headersize = strlen($header);
	$headersizebin = pack("n", $headersize);

	$request_size = fwrite($socket, $header, $headersize);
	$rawbuffer = fread($socket, 4096);
	fclose($socket);

	if (strlen($rawbuffer) < 12) {
		throw new \RuntimeException("DNS query return buffer too small");
	}

	$pos = 0;

	$read = function ($len) use (&$pos, $rawbuffer) {
		$out = substr($rawbuffer, $pos, $len);
		$pos += $len;
		return $out;
	};

	$read_name_pos = function ($offset_orig, $max_len=65536) use ($rawbuffer) {
		$out = [];
		$offset = $offset_orig;

		while (($len = ord(substr($rawbuffer, $offset, 1))) && $len > 0 && ($offset+$len < $offset_orig+$max_len ) ) {
			$out[] = substr($rawbuffer, $offset + 1, $len);
			$offset += $len + 1;
		}

		return $out;
	};

	$read_name = function() use (&$read, $read_name_pos) {
		$out = [];

		while (($len = ord($read(1))) && $len > 0) {
			if ($len >= 64) {
				$offset = (($len & 0x3f) << 8) + ord($read(1));
				$out = array_merge($out, $read_name_pos($offset));
				break;
			}
			else {
				$out[] = $read($len);
			}
		}

		return implode('.', $out);
	};

	$header = unpack("nid/nflags/nqdcount/nancount/nnscount/narcount", $read(12));
	$flags = sprintf("%016b\n", $header['flags']);

	// No answers
	if (!$header['ancount']) {
		return [];
	}

	$is_authorative = $flags[5] == 1;

	// Question section
	if ($header['qdcount']) {
		// Skip name
		$read_name();

		// skip question part
		$pos += 4; // 4 => QTYPE + QCLASS
	}

	$responses = [];

	for ($a = 0; $a < $header['ancount']; $a++) {
		$read_name(); // Skip name
		$ans_header = unpack("ntype/nclass/Nttl/nlength", $read(10));

		$t = $types[$ans_header['type']] ?? null;

		if ($type != 'ANY' && $t != $type) {
			// Skip type that was not requested
			$t = null;
		}

		switch ($t) {
			case 'A':
				$responses[] = implode(".", unpack("Ca/Cb/Cc/Cd", $read(4)));
				break;
			case 'AAAA':
				$responses[] = implode(':', unpack("H4a/H4b/H4c/H4d/H4e/H4f/H4g/H4h", $read(16)));
				break;
			case 'MX':
				$prio = unpack('nprio', $read(2)); // priority
				$responses[$prio['prio']] = $read_name();
				break;
			case 'NS':
			case 'CNAME':
			case 'PTR':
				$responses[] = $read_name();
				break;
			case 'TXT':
				$responses[] = implode($read_name_pos($pos, $ans_header['length']));
				$read($ans_header['length']);
				break;
			default:
				// Skip
				$read($ans_header['length']);
				break;
		}
	}

	return $responses;
}

I can confirm that for github.com the issue is that the response is truncated (TC) and should be upgraded to TCP. I'll take a look if I can.

My comments:

1. A possible fix for the TXT queries
   https://gist.github.com/LatinSuD/1fffa7f3ec9dce5f0dbbed22dbc9266a
   I reused the function read_name_pos() to read a series of character-strings, but imposing as a limit the size of record.

2. The code may still not handle multiple levels of compression scenarios. See https://serverfault.com/questions/1007283/multiple-levels-of-dns-name-compression.
   If you try to handle that beware of the risks (eg: infinite loops). You may want to check rfc9267 for good practices.

3. Line 95, the check of $len < 64 could be changed with $len < 0xc0. Although it probably doesn't matter, as it is a reserved value.

Thank you @LatinSuD I merged your changes, this fixes the TXT issues for northskytech.com.

I started some work on a function that upgrades to UDP for TC records but its not finished.