Question

I’m trying to add a new protocol with scapy, and I’m running into difficulties building packets that store other BitEnumField “packets” that are under one byte of length. I was wondering if there is a workaround to make it work (without bundling packets into full byte fields). Here is the example:

from scapy.packet import Packet
from scapy.fields import *    

class Status(Packet):
    name = '4 bit status'

    fields_desc = [
        BitEnumField('a', 0, 1, {0:'Disabled', 1:'Enabled'}),
        BitEnumField('b', 0, 1, {0:'Disabled', 1:'Active'}),
        BitEnumField('c', 0, 1, {0:'Disabled', 1:'Active'}),
        BitEnumField('d', 0, 1, {0:'Disabled', 1:'Active'}),
        ]

    #this is added for debug purposes only
    def post_build(self, pkt,pay):
        print "pkt:", pkt, "type:", type(pkt)
        return pkt+pay

Now I can understand why Status().show2() fails with pkt being only 4 bits long. but this one dies too (I guess because each packet is formed independently):

class TotalStatus(Packet):
    name = "8 bit status"

    fields_desc = [
        PacketField('axis0', Status(), Status),
        PacketField('axis1', Status(), Status),
        ]

TotalStatus().show2() gives you lengthy traceback that ends in self.post_build() unable to cat pkt tuple and the rest of payload, which is empty string. I.e.:>>> TypeError: can only concatenate tuple (not "str") to tuple

Is there any way to avoid bundling of bit fields into full bytes?

Answer 1

I guess that a Packet is always byte aligned so you may have to use a field somehow, e.g:

class StatusField(FlagsField):
  name = '4 bit status'
  def __init__(self, name):
    FlagsField.__init__(self, name, 0, 4, ["a", "b", "c", "d"])

class TotalStatus(Packet):
  name = "8 bit status"
  fields_desc = [
    StatusField("axis0"),
    StatusField("axis1"),
  ]