reverse

#!/usr/bin/env ruby

require 'socket'

def get_name(addr)
  info = Socket.getnameinfo(Socket.sockaddr_in('80', addr))
  info[0]
end

ADDR_REG_EXP = /\b((?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))\b/
list = {}

while line = gets
  if line =~ ADDR_REG_EXP
    addr = $1
    name = nil
    if list[addr]
      name = list[addr]
    else
      name = get_name(addr)
      list[addr] = name
    end
    print line.gsub(addr, name)
  end
end