Scapy：添加新协议/层时出现TypeError

Question

I am trying to build a new layer/protocol in Scapy. I see this error when I am trying to send/show2 it.

I have put breakpoints to see what is happening but somehow internally the str is getting converted to a Tuple. I am not sure what I am missing. Can someone help me nail down the issue?

>>> p1=PCEPOPEN()
>>> p1.show2()
> /usr/local/lib64/python2.6/site-packages/scapy/fields.py(71)addfield()
-> return s+struct.pack(self.fmt, self.i2m(pkt,val))
(Pdb) val
1
(Pdb) p s
''
(Pdb) c
> /usr/local/lib64/python2.6/site-packages/scapy/fields.py(71)addfield()
-> return s+struct.pack(self.fmt, self.i2m(pkt,val))
(Pdb) p s
'\x01\x10'
(Pdb) val
4
(Pdb) p s
'\x01\x10'
(Pdb) c
> /usr/local/lib64/python2.6/site-packages/scapy/fields.py(71)addfield()
-> return s+struct.pack(self.fmt, self.i2m(pkt,val))
(Pdb) val
10
(Pdb) p s
('\x01\x10\x00\x04', 3, 1L)
(Pdb) c
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib64/python2.6/site-packages/scapy/packet.py", line 831, in show2
    self.__class__(str(self)).show()
  File "/usr/local/lib64/python2.6/site-packages/scapy/packet.py", line 262, in __str__
    return self.build()
  File "/usr/local/lib64/python2.6/site-packages/scapy/packet.py", line 321, in build
    p = self.do_build()
  File "/usr/local/lib64/python2.6/site-packages/scapy/packet.py", line 309, in do_build
    pkt = self.self_build()
  File "/usr/local/lib64/python2.6/site-packages/scapy/packet.py", line 300, in self_build
    p = f.addfield(self, p, val)
  File "/usr/local/lib64/python2.6/site-packages/scapy/fields.py", line 71, in addfield
    return s+struct.pack(self.fmt, self.i2m(pkt,val))
TypeError: can only concatenate tuple (not "str") to tuple
>>> p1.show()
###[ OPEN Object for Open message ]###
  oclass= OPEN
  oType= 1
  resflags=
  pflag=
  iflag=
  obLength= 4
  ver= 1
  kalive= 10
  dead= 40
  sid= 1
>>>      


class PCEPOPEN(Packet):
  """OPEN message to establish a PCEP session"""

  name="OPEN Object for Open message"
  fields_desc = [ByteEnumField("oclass",1,_object_class),
                   BitField("oType",1,4),
                   BitField("resflags", 0, 2),
                   FlagsField("pflag", 0x0, 1, "P"),
                   FlagsField("iflag", 0x0, 1, "I"),
                   ShortField("obLength", 4),
                   BitField("ver",1,3),
                   ByteField("kalive",10),
                   ByteField("dead",40),
                   ByteField("sid",1)]

  def post_build(self, pkt, pay):
    if self.obLength is 4:
      olen = len(pkt) + len(pay)
      pkt = pkt[:2]+struct.pack("!h", olen)
    return pkt+pay

Answer 1

Looks like I accidentally deleted a field which caused the problem. I had used BitField and added 3 bits but I was had deleted the line which contained BitField with 5 more bits. Since the packets are Byte aligned scapy was complaining. It was good to learn that :)