Scapy: Getting trailer field in the dissector

问题

I'm using scapy to read and pretty print a trace file. The PDUs I'm reading (My_Packet) have the format:

| header | payload | crc |
--------------------------

The payload field contains another PDU inside. I'm using the following code to strip the crc part, but unfortunately something does not work as expected (I cannot figure out what's wrong).

#!/usr/bin/env python

from scapy.all import *

class My_Packet(Packet):
    '''My_Packet PDU'''

    name = "My Packet"
    CRC_SIZE = 4

    fields_desc=[ LEShortField("len", 0) ]

    def __init__(self, *args, **kargs):
        """Build My_Packet"""
        self.crc = []
        super(My_Packet, self).__init__(*args, **kargs)

    def get_crc(self):
        """Get the CRC"""
        return self.crc

    def post_dissect(self, p):
        """Remove trailer/CRC"""
        self.crc = p[-self.CRC_SIZE:]
        return p[:self.len - self.CRC_SIZE]

    def extract_padding(self, p):
        """Remove trailer for the upper layer"""
        return p[:self.len - self.CRC_SIZE], p[self.len - self.CRC_SIZE:]

if __name__ == '__main__':

    bind_layers(UDP, My_Packet, sport=5001, dport=5001)

    packets = rdpcap("./tcpdump_5501.bin")

    # iterate through PDUs
    for pkt in packets:
        if pkt.haslayer(UDP):
            pld = pkt[My_Packet].load
            print("PDU: 0x{}".format(pld.encode("hex")))

            crc = pkt[My_Packet].get_crc()
            print("CRC: 0x{}".format(crc.encode("hex")))

The runtime error is:

Traceback (most recent call last):
  File "./so.py", line 41, in <module>
    pld = pkt[My_Packet].load
  File "/usr/local/lib/python2.7/dist-packages/scapy/packet.py", line 964, in __getitem__
    raise IndexError("Layer [%s] not found" % lname)
IndexError: Layer [My_Packet] not found

Scapy version: Version git-archive.dev9fc949739
Python version: Python 2.7.12

来源：https://stackoverflow.com/questions/48763072/scapy-getting-trailer-field-in-the-dissector