A script to create byte sequence of an arp packet. This can not emit packet onto the network.

arp.rb

#!/usr/bin/env ruby
# asks for IP address of a MAC address
#
# Copyright (c) 2011 zunda <zunda at freeshell.org>
# 
# Permission is granted for use, copying, modification,
# distribution, and distribution of modified versions of this
# work as long as the above copyright notice is included.
#

# ARP message over Ethenet ARP packet
# http://en.wikipedia.org/wiki/Address_Resolution_Protocol
class ARPMessage
	attr_accessor :htype	# Hardware type
	attr_accessor :ptype	# Protocol type
	attr_accessor :hlen	# Hardware address length
	attr_accessor :plen	# Protocol address length
	attr_accessor :oper	# Operation
	attr_accessor :sha	# Sender hardware address
	attr_accessor :spa	# Sender protocol address
	attr_accessor :tha	# Target hardware address
	attr_accessor :tpa	# Target protocol address

	# http://www.javvin.com/protocolARP.html
	OperCode = {
		:ARP_request => 1,
		:ARP_response => 2,
		:RARP_response => 3,
		:RARP_response => 4,
		:Dynamic_RARP_response => 5,
		:Dynamic_RARP_response => 6,
		:Dynamic_RARP_error => 7,
		:InARP_request => 8,
		:InARP_response => 9,
	}

	def initialize(htype = 1,	ptype = 0x0800, hlen = 6, plen = 4)
		@htype = htype	# 1:Ethernet
		@ptype = ptype	# 0x0800:IPv4
		@hlen = hlen	# 6:Ethernet
		@plen = plen	# 4:IPv4
	end

	def msg
		raise 'oper must be set' unless @oper
		result = [@htype, @ptype, @hlen, @plen, @oper].pack('n2C2n')
		result += @sha.bytes.to_a.pack("C#{@hlen}")
		result += (@spa || "\x00"*@plen).bytes.to_a.pack("C#{@plen}")
		result += (@tha || "\x00"*@hlen).bytes.to_a.pack("C#{@hlen}")
		result += (@tpa || "\x00"*@plen).bytes.to_a.pack("C#{@plen}")
		return result
	end

	def ARPMessage.parse(message)
		result = ARPMessage.new
		result.htype, result.ptype, result.hlen, result.plen, result.oper = message.slice!(0,8).unpack('n2C2n')
		result.sha = message.slice!(0,result.hlen)
		result.spa = message.slice!(0,result.plen)
		result.tha = message.slice!(0,result.hlen)
		result.tpa = message.slice!(0,result.plen)
		return result
	end
end

if __FILE__ == $0
  require 'test/unit'
  class TestARPMessage < Test::Unit::TestCase
    def setup
      @oper = ARPMessage::OperCode[:ARP_request]
      @sha = "\x00\x00\x00\x12\x34\x56"
      @spa = "\xc0\xa8\x01\x01"
      @tpa = "\xc0\xa8\x01\x40"
      @packet = ARPMessage.new
      @packet.oper = @oper
      @packet.sha = @sha
      @packet.spa = @spa
      @packet.tpa = @tpa
    end

    def test_creation
      assert_equal("\x00\x01\x08\x00\x06\x04\x00\x01\x00\x00\x00\x12\x34\x56\xc0\xa8\x01\x01\x00\x00\x00\x00\x00\x00\xc0\xa8\x01\x40", @packet.msg)
    end

    def test_parse
      packet = ARPMessage.parse(@packet.msg)
      assert_equal(1, packet.htype, 'htype')
      assert_equal(@oper, packet.oper, 'oper')
      assert_equal(@sha, packet.sha, 'sha')
      assert_equal(@spa, packet.spa, 'spa')
      assert_equal("\x00"*6, packet.tha, 'tha')
      assert_equal(@tpa, packet.tpa, 'tpa')
    end
  end
end