Scapy:添加具有复杂字段分组的新协议

Question

我正在尝试使用指定新的数据包格式scapy.在数据包中有一个项目列表,项目由"分组字段"组成."分组字段"是指不同类型的字段的子序列.在scapy中创建我所知道的"分组字段"的唯一方法是使用Packet类和使用FieldLenField/ PacketListField来引用序列的长度和列表成员的类型.这是要走的路吗？看起来像这样的东西:

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

class RepeatingGroupedSequence(Packet):
    name = "Simple group of two fields"

    fields_desc = [IntField('field1', 1), 
                   IntField('field2', 2)]

class TopLayer(Packet):
    name = "Storage for Repeating Sequence"

    fields_desc = [FieldLenField("length", None, count_of='rep_seq'),
                   PacketListField('rep_seq', None, RepeatingGroupedSequence, 
                                   count_from = lambda pkt: pkt.length),
                  ]

#Now here is the problem that I have with assembling PacketListField: 

#craft TopLayer packet
p = TopLayer()

#add two "repeated sequences"
p.rep_seq = [ RepeatingGroupedSequence(), RepeatingGroupedSequence() ]

#both sequences can observed
p.show()

#but the underlying structure of the repeated sequence is #Raw# at this stage
p.show2()

#length is 2
print p.rep_seq, 'length:', len(p.rep_seq)

#but the cloned packet has only one "repeated sequence", the rest is raw
clone = TopLayer(str(p))
clone.show()

#length is 1
print clone.rep_seq, 'length:', len(clone.rep_seq)

这种方法的问题在于,当重新组装分组时,不保留分组的结构.在汇编时,将第二个实例RepeatedSequence视为原始实体,即使count字段为2.如何添加RepeatingSequences这样的结构以便在重组时保留结构？有没有办法将Fields分组而不是Packet作为列表的存储类型？

Answer 1

类RepeatingGroupedSequence需要覆盖extract_padding方法:

def extract_padding(self, s):
    return '', s

默认情况下,每个子数据包都将所有内容视为属于其自己的层,即:

def extract_padding(self, s):
    return s, None

这不是用于分组目的的.有人可以详细说明填充和层分离之间的区别吗？