1. #!/usr/bin/env ruby
  2.  
  3. WANT = 0x44d4     # checksum you want
  4. IS   = 0x50d4     # checksum it is
  5. # your IP header
  6. DATA = [0x4500, 0x0028, 0x1e4f, 0x4000, 0x7b06, 0x0, 0xXXXX, 0xXXXX, 0xXXXX, 0xXXXX]
  7.  
  8. ELEMENT = [
  9.   'version+ihl+tos',
  10.   'packet length',
  11.   'id',
  12.   'fragmentation info',
  13.   'ttl+protocol',
  14.   'checksum',
  15.   'saddr upper',
  16.   'saddr lower',
  17.   'daddr upper',
  18.   'daddr lower',
  19. ]
  20.  
  21. def csum data
  22.   sum = data.inject(0){|r,m|r+m}
  23.   sum1 = (sum & 0xffff0000) >> 16
  24.   sum2 = (sum & 0xffff)
  25.   sum = (sum1+sum2)^0xffff
  26.   sum
  27. end
  28.  
  29. my_data = DATA.dup
  30. offset = nil
  31.  
  32. 65535.times do |x|
  33.   my_data[0] = x
  34.   result = csum(my_data)
  35.   offset = IS-result if result==WANT
  36.   if result==IS
  37.     unless my_data[0] == DATA[0]
  38.       warn 'packet incorrectly entered'
  39.       exit 42
  40.     end
  41.   end
  42. end
  43.  
  44. 10.times do |element|
  45.   next if element == 5 # checksum is 0
  46.   puts '------ element: %d (%s) -------' % [element, ELEMENT[element]]
  47.   x = DATA[element]
  48.   puts 'we have %016b (%x)' % [x, x]
  49.   puts 'we want %016b (%x)' % [x+offset, x+offset]
  50. end