title: trying to build a MODBUS packet with scapy
body: <p>i'm trying to build a MODBUS packet with scapy,
  <strong>but  Wireshark can't recognize it. i mean, the MODBUS interpreted as the TCP data.</strong></p>
  
  <p>here is the python code of Scapy:</p>
  
  <pre><code>from scapy.all import *
  
  
  class Modbus(Packet):
      name = "Modbus/tcp"
      fields_desc = [ ShortField("Transaction Identifier", 1),
                      ShortField("Protocol Identifier", 0),
                      ShortField("Length", 5),
                      XByteField("Unit Identifier",0),
                      ]
  
  def make_test():
      pkt = TCP(sport=502, dport=502)
      return Ether()/IP(src="5.5.5.101",dst="5.5.5.100")/pkt/Modbus()
  
  
  while True:
      send(make_test())
  </code></pre>
  
  <p>someone has an idea how to fix it?</p>