require 'socket'

def read(sock)
  lenb = sock.readpartial(4)
  len = lenb.unpack("L")[0]
  lenb + sock.readpartial(len)
end

serv = TCPServer.new(1234)

socket = serv.accept
uplink = TCPSocket.new('192.168.0.2', 7777)

loop do
  ret = IO.select([socket, uplink])
  if ret[0]
    ret[0].each do |s|
      buf = read(s)
      if socket == s
        puts ">> #{buf.inspect}"
        uplink.write(buf)
      end

      if uplink == s
        puts "<< #{buf.inspect}"
        socket.write(buf)
      end
    end
  end
end