Building a Basic DNS Server in PHP/Python: A Beginner's Guide

Creating a full-fledged DNS server in PHP is not recommended for production environments due to performance and security concerns. DNS servers require low-level networking capabilities and efficient handling of DNS protocol messages, which PHP is not optimized for. However, for educational purposes or experimental projects, you can implement a basic DNS server in PHP. Here's a very simplified example using UDP sockets:

<?php

// Define DNS records (for demonstration purposes)
$dnsRecords = [
    'example.com' => '192.168.1.100',
    'sub.example.com' => '192.168.1.101',
    'another.example.com' => '192.168.1.102',
];

// Create UDP socket
$socket = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_bind($socket, '0.0.0.0', 53); // Bind to port 53

// Handle DNS queries
while (true) {
    // Receive DNS query
    socket_recvfrom($socket, $buffer, 512, 0, $clientIP, $clientPort);

    // Parse DNS query
    $dnsQuery = dns_parse_message($buffer);

    // Prepare DNS response
    $response = '';
    foreach ($dnsQuery['questions'] as $question) {
        $domain = $question['qname'];

        if (isset($dnsRecords[$domain])) {
            // Construct DNS response
            $response .= dns_build_response($question, $dnsRecords[$domain]);
        }
    }

    // Send DNS response
    socket_sendto($socket, $response, strlen($response), 0, $clientIP, $clientPort);
}

// Function to parse DNS query
function dns_parse_message($data) {
    $dns = [];

    $dns['header'] = unpack('nid/nflags/nqdcount/nancount/nnscount/narcount', substr($data, 0, 12));
    $offset = 12;

    // Parse questions
    for ($i = 0; $i < $dns['header']['qdcount']; $i++) {
        $dns['questions'][] = dns_parse_question($data, $offset);
    }

    return $dns;
}

// Function to parse DNS question
function dns_parse_question($data, &$offset) {
    $question = [];
    $question['qname'] = '';
    $len = ord($data[$offset]);

    while ($len != 0) {
        $question['qname'] .= substr($data, $offset + 1, $len) . '.';
        $offset += $len + 1;
        $len = ord($data[$offset]);
    }

    $question['qtype'] = unpack('nid/n', substr($data, $offset + 1, 2));
    $offset += 4; // Jump over QTYPE and QCLASS

    return $question;
}

// Function to build DNS response
function dns_build_response($question, $ip) {
    $response = pack('nid*', 0x8180, 0x0001, 0x0001);

    // Write question
    $response .= dns_encode_domain($question['qname']);
    $response .= pack('nid*', 0x0001, 0x0001);

    // Write answer
    $response .= dns_encode_domain($question['qname']);
    $response .= pack('nid*', 0x0001, 0x0001);
    $response .= pack('Nn*', 60, strlen($ip));
    $response .= inet_pton($ip);

    return $response;
}

// Function to encode domain name for DNS response
function dns_encode_domain($domain) {
    $labels = explode('.', $domain);
    $encoded = '';

    foreach ($labels as $label) {
        $encoded .= chr(strlen($label)) . $label;
    }

    $encoded .= chr(0);

    return $encoded;
}

Please note that this is a very basic and limited implementation for educational purposes. In a real-world scenario, you'd need to handle many more DNS record types, support TCP-based communication for large responses, implement caching, handle concurrent requests, and ensure security measures like input validation and sanitization to prevent DNS-related vulnerabilities like cache poisoning and amplification attacks.

Below is based in Python

from socket import socket, AF_INET, SOCK_DGRAM
from dnslib import DNSRecord, RR

# Define DNS records (for demonstration purposes)
dns_records = {
    'example.com.': '192.168.1.100',
    'sub.example.com.': '192.168.1.101',
    'another.example.com.': '192.168.1.102',
}

# Create UDP socket
sock = socket(AF_INET, SOCK_DGRAM)
sock.bind(('0.0.0.0', 53))  # Bind to port 53

# Handle DNS queries
while True:
    data, addr = sock.recvfrom(512)  # Buffer size is 512 bytes
    dns_request = DNSRecord.parse(data)

    # Prepare DNS response
    dns_response = DNSRecord()
    dns_response.add_question(dns_request.q.qname)

    for question in dns_request.questions:
        qname = str(question.qname)
        if qname in dns_records:
            dns_response.add_answer(RR.fromZone(f"{qname} IN A {dns_records[qname]}"))

    # Send DNS response
    sock.sendto(dns_response.pack(), addr)