fake DNS server

fakednsserver.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-
require 'socket'

class DNSQuery
  attr_reader :domain_io
  def initialize data
    @data = data
    @domain_io = ''

    tipo = data[2].ord >> 3 & 15 # Opcode bits
    if tipo == 0                 # Standard query
      ini = 12
      lon = data[ini].ord
      while lon != 0
        @domain_io = data[(ini+1)...(ini+lon+1)] + '.'
        ini += lon + 1
        lon = data[ini].ord
      end
    end
  end

  def response ip
    packet = ''
    unless @domain_io.empty?
      packet += @data[0..1] + "\x81\x80"
      packet += self.data[4..5] + self.data[4..5] + '\x00\x00\x00\x00' # Questions and Answers Counts
      packet += self.data[12..-1]                                      # Original Domain Name Question
      packet += '\xc0\x0c'                                             # Pointer to domain name
      packet += '\x00\x01\x00\x01\x00\x00\x00\x3c\x00\x04'             # Response type, ttl and resource data length -> 4 bytes
      packet += ip.split('.').map{|x| x.to_i.to_chr }.join             # 4bytes of IP
    end
    packet
  end
end


if __FILE__ == $0
  ip = '192.168.1.1'
  puts "minifakeDNS :: dom.query. 60 IN A #{ip}"

  udps = Socket.new(Socket::AF_INET, Socket::SOCK_DGRAM, 0)
  sockaddr = Socket::sockaddr_in(53, "localhost")
  udps.bind(sockaddr)

  begin
    while true
      data, sockaddr = udps.recvfrom(1024)
      q = DNSQuery.new(data)
      udps.send(q.response(ip), 0, sockaddr)
      puts 'Response: #{q.domain_io} -> #{ip}'
    end
  rescue KeyboardInterrupt
    puts "finish"
  ensure
    udps.close
  end
end