server.rb

#!/usr/bin/env ruby

require "socket"
require "pry"
require "pry-byebug"

Socket.udp_server_loop(53) do |msg, msg_src|
  id, flags, qdcount, ancount, nscount, arcount = msg.unpack("nnnnnn")

  qr =     (flags & 0x8000) >> 15
  opcode = (flags & 0x7800) >> 11
  aa =     (flags & 0x0400) >> 10
  tc =     (flags & 0x0200) >> 9
  rd =     (flags & 0x0100) >> 8
  ra =     (flags & 0x0080) >> 7
  z =      (flags & 0x0070) >> 4
  rcode =   flags & 0x000f
  rest = msg[12..-1]

  questions = msg.bytes[12..-1]
  labels = []
  idx = 0
  while idx < questions.length-5
    size = questions[idx]
    labels << questions[idx+1..idx+size].pack('c*')
    idx += size+1
  end
  hostname = labels.join('.')

  puts "Received Query for : #{hostname}"

  if hostname == "example.good.com"
    id = id
    qr = 1
    opcode = opcode
    aa = 0
    tc = 0
    rd = rd
    ra = 1
    z = 0
    rcode = 0
    qdcount = 1
    ancount = 1
    nscount = 0
    arcount = 0
    word2 = (qr << 15) |
            (opcode << 11) |
            (aa << 10) |
            (tc << 9) |
            (rd << 8) |
            (ra << 7) |
            (z << 4) |
            rcode
    msg = [id, word2, qdcount, ancount, nscount, arcount].pack("nnnnnn")
    msg << questions.pack('c*')
    type = 1
    klass = 1
    ttl = 3600
    rdlength = 4
    rdata = [52,0,2,1].pack("CCCC")
    rr = [0xc00c, type, klass, ttl, rdlength, rdata].pack("nnnNna*")
    msg << rr
    rdata = [52,0,2,2].pack("CCCC")
    rr = [0xc00c, type, klass, ttl, rdlength, rdata].pack("nnnNna*")
    msg << rr

    puts "Returning single response with records"
    msg_src.reply msg
  else
    id = id
    qr = 1
    opcode = opcode
    aa = 0
    tc = 0
    rd = rd
    ra = 1
    z = 0
    rcode = 3
    qdcount = 1
    ancount = 0
    nscount = 0
    arcount = 0
    word2 = (qr << 15) |
            (opcode << 11) |
            (aa << 10) |
            (tc << 9) |
            (rd << 8) |
            (ra << 7) |
            (z << 4) |
            rcode
    msg = [id, word2, qdcount, ancount, nscount, arcount].pack("nnnnnn")
    msg << questions.pack('c*')

    puts "Returning duplicate responses with \"Not found\" RCODE"
    msg_src.reply msg
    msg_src.reply msg
  end
end