Simple, scrappy UDP DNS server in Ruby (with protocol annotations)

dnsd.rb

# Simple, scrappy UDP DNS server in Ruby (with protocol annotations)
# By Peter Cooper
#
# MIT license
#
# * Not advised to use in your production environment! ;-)
# * Requires Ruby 1.9
# * Supports A and CNAME records
# * See http://www.ietf.org/rfc/rfc1035.txt for protocol guidance
# * All records get the same TTL

require 'socket'

records = {
  'example.com.' => '1.2.3.4',
  'test.host.' => '127.0.0.2',
  'test.cnames.com.' => 'example.com'
}

class DNSRequest
  attr_reader :server, :data, :domain
  
  def initialize(server, data)
    @server = server
    @data = data
    
    extract_domain
  end
  
  def extract_domain
    @domain = ''
    
    # Check "Opcode" of question header for valid question
    if @data[2].ord & 120 == 0
      # Read QNAME section of question section
      # DNS header section is 12 bytes long, so data starts at offset 12

      idx = 12    
      len = @data[idx].ord
      # Strings are rendered as a byte containing length, then text.. repeat until length of 0
      until len == 0 do
        @domain += @data[idx + 1, len] + '.'
        idx += len + 1
        len = @data[idx].ord
      end
    end
  end
  
  def response(val)
    return empty_response if domain.empty? || !val

    cname = val =~ /[a-z]/
    
    # Valid response header
    response = "#{data[0,2]}\x81\x00#{data[4,2] * 2}\x00\x00\x00\x00"
    
    # Append original question section
    response += data[12..-1]
    
    # Use pointer to refer to domain name in question section
    response += "\xc0\x0c"
    
    # Set response type accordingly
    response += cname ? "\x00\x05" : "\x00\x01"
    
    # Set response class (IN)
    response += "\x00\x01"
          
    # TTL in seconds
    response += [server.ttl].pack("N")
          
    # Calculate RDATA - we need its length in advance
    if cname
      rdata = val.split('.').collect { |a| a.length.chr + a }.join + "\x00"
    else
      # Append IP address as four 8 bit unsigned bytes
      rdata = val.split('.').collect(&:to_i).pack("C*")
    end
    
    # RDATA is 4 bytes
    response += [rdata.length].pack("n")
    response += rdata
  end
  
  def empty_response
    # Empty response header
    # [id * 2, flags, NXDOMAIN, qd count * 2, an count * 2, ns count * 2, ar count * 2]
    response = "#{data[0,2]}\x81\x03#{data[4,2]}\x00\x00\x00\x00\x00\x00"
    
    # Append original question section
    response += data[12..-1]
  end
end

class DNSServer
  attr_reader :port, :ttl
  attr_accessor :records
  
  def initialize(options = {})
    options = {
      port: 53,
      ttl: 60,
      records: {}
    }.merge(options)
    
    @port, @records, @ttl = options[:port], options[:records], options[:ttl]
  end
    
  def run
    Socket.udp_server_loop(@port) do |data, src|
      r = DNSRequest.new(self, data)
      src.reply r.response(@records[r.domain])
    end
  end
end

DNSServer.new(records: records, ttl: 120).run

@johnae very cool

I like the brevity of this!

Am I the only one who looks at this and goes hax? I'm looking for DNS server components which map to the primitives in the Resolv module. This is more like barely scraping the packets...

@tarcieri You might like to try out RubyDNS then.

@peterc Nice work :D

hello
i get this error ................

ru.rb:90:in empty_response': incompatible character encodings: ASCII-8BIT and UTF-8 (Encoding::CompatibilityError) from ru.rb:52:in response'
from ru.rb:114:in block in run' from /usr/lib/ruby/2.3.0/socket.rb:976:in block in udp_server_recv'
from /usr/lib/ruby/2.3.0/socket.rb:966:in each' from /usr/lib/ruby/2.3.0/socket.rb:966:in udp_server_recv'
from /usr/lib/ruby/2.3.0/socket.rb:995:in block in udp_server_loop_on' from /usr/lib/ruby/2.3.0/socket.rb:993:in loop'
from /usr/lib/ruby/2.3.0/socket.rb:993:in udp_server_loop_on' from /usr/lib/ruby/2.3.0/socket.rb:1021:in block in udp_server_loop'
from /usr/lib/ruby/2.3.0/socket.rb:937:in udp_server_sockets' from /usr/lib/ruby/2.3.0/socket.rb:1020:in udp_server_loop'
from ru.rb:112:in run' from ru.rb:119:in

'

...........................
why ??

You need to deal with character encoding mismatches in more modern versions of Ruby. I'd need to grab the code and fix it up myself to say what to do but basically need to use the String#b method in a couple of places to keep things as binary.