复制套接字缓冲区时，使用ctypes的Python数据包嗅探器崩溃

我试图捕获IP数据包的前20个字节（全包减去期权），填充struct的ctype成员，并打印我想在屏幕上（协议，源和目的地址）的信息。我的IP类别如下：

import socket
import os
import struct
from ctypes import *

# host to listen on
host = "192.168.0.187"
# our IP header
class IP(Structure):
        _fields_ = [
            ("ihl", c_ubyte, 4),
            ("version", c_ubyte, 4),
            ("tos", c_ubyte),
            ("len", c_ushort),
            ("id", c_ushort),
            ("offset", c_ushort),
            ("ttl", c_ubyte),
            ("protocol_num", c_ubyte),
            ("sum", c_ushort),
            ("src", c_ulong),
            ("dst", c_ulong)
        ]
    def __new__(self, socket_buffer=None):
        return self.from_buffer_copy(socket_buffer)
    def __init__(self, socket_buffer=None):
        # map protocol constants to their names
        self.protocol_map = {1:"ICMP", 6:"TCP", 17:"UDP"}
        # human readable IP addresses
        self.src_address = socket.inet_ntoa(struct.pack("<L",self.src))
        self.dst_address = socket.inet_ntoa(struct.pack("<L",self.dst))
        # human readable protocol
        try:
            self.protocol = self.protocol_map[self.protocol_num]
        except:
            self.protocol = str(self.protocol_num)

现在，我创建套接字，将其绑定到主机，然后循环以获取数据包：

# create socket and bind to public interface (os dependent)
if os.name == "nt":
    socket_protocol = socket.IPPROTO_IP
else:
    socket_protocol = socket.IPPROTO_ICMP

# create raw socket
sniffer = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket_protocol)
sniffer.bind((host, 0))
# include header information
sniffer.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)

if os.name == "nt":
    sniffer.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)
    
try:
    while True:
        # read in a packet
        raw_buffer = sniffer.recvfrom(65565)[0]
        # create an IP header from the first 20 bytes of the buffer
        ip_header = IP(raw_buffer[0:20])
        # print out the protocol that was detected and the hosts
        print "Protocol: %s %s -> %s" % (ip_header.protocol, ip_header.src_¬
                                address, ip_header.dst_address)
# handle CTRL-C
except KeyboardInterrupt:
    # if we're using Windows, turn off promiscuous mode
    if os.name == "nt":
        sniffer.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)

当使用和成员c_ulong作为数据类型运行时，出现以下错误（在一个终端上运行，而在另一个终端上对nostarch.com进行ping操作）："src""dst" _fields_ struct

我假设的大小可能c_ulong大于一个字节，从而使我对前20个字节的要求不成立（我对python来说是很新的）。然后我改变了c_ulong对c_ushort并再次运行它：

实际的ping路径：

因此，尽管脚本运行没有错误，但它切断了src和dst地址。

当我告诉我只想要前20个字节时，为什么它要求至少32个字节？

（我在使用Python 2.7在Win7主机上运行的Kali64 VBox VM中）

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