通过ctypes使用pcap的python分段错误

Question

I am porting my sniffer from C to Python using libpcap via ctypes. Here is the python code:

import ctypes, sys
from ctypes.util import find_library

if sys.platform == "darwin":
    _pcap = ctypes.cdll.LoadLibrary(find_library("libpcap"))
elif sys.platform == "linux2":
    _pcap = ctypes.cdll.LoadLibrary("libpcap.so")

errbuf = ctypes.create_string_buffer(256)

pcap_lookupdev = _pcap.pcap_lookupdev
pcap_lookupdev.restype = ctypes.c_char_p
dev = pcap_lookupdev(errbuf)
print dev

# create handler
pcap_create = _pcap.pcap_create
handle = pcap_create(dev, errbuf)
print handle
if not handle:
    print "failed creating handler:",errbuf
    exit()

# monitor mode
pcap_can_set_rfmon = _pcap.pcap_can_set_rfmon
print "can rfmon:",pcap_can_set_rfmon(handle)

On Linux it works perfectly fine, but on Mac OS X it runs into a segmentation fault when I use handle. The value of handle even is sometimes negative, sometimes positive. I already tried to change the return type of pcap_create to unsigned int, but that didn't help, but I think it returns a wrong type under OS X...

I did a printf("size of pcap_t: %zu\\n", sizeof(pcap_t *)); on both systems in C to get the size of pcap_t handler type. On Linux it says 4 and on OS X 8. But II don't know how to go on from this point...

Or am I on the wrong path? Does anyone have an idea?

Answer 1

Data types matter.

You need to tell ctypes that the return value of pcap_create() is a pointer, and you need to tell it that the argument to pcap_can_set_rfmon() is a pointer.

You do this by doing

# create handler
pcap_create = _pcap.pcap_create
pcap_create.restype = ctypes.c_void_p
handle = pcap_create(dev, errbuf)
print handle
if not handle:
    print "failed creating handler:",errbuf
    exit()

# monitor mode
pcap_can_set_rfmon = _pcap.pcap_can_set_rfmon
pcap_can_set_rfmon.argtypes = [ctypes.c_void_p]
print "can rfmon:",pcap_can_set_rfmon(handle)

The

pcap_create.restype = ctypes.c_void_p

and

pcap_can_set_rfmon.argtypes = [ctypes.c_void_p]

lines are required here. This code will work with both 32-bit and 64-bit pointers, so you can use it on 32-bit and 64-bit Linux and on 32-bit and 64-bit OS X (and 32-bit and 64-bit Solaris and 32-bit and 64-bit FreeBSD and..., with whatever changes are needed to the code to load the library - on most Un*xes shared libraries have names ending with ".so", so if you don't want to use find_library on other Un*xes, the Linux code may suffice).

Answer 2

I really wanted to come here and post since I was having this issue on OS X as well. The issue I found with your code is:

handle = pcap_create(dev, errbuf)

You need to set the restype, here is my example as this may help a few of you down the road: https://github.com/killswitch-GUI/NIX-Sniffer-Examples

    import ctypes
    import threading
    import sys
    import os
    import errno


    OSX_PCAP_DYLIB = '/usr/lib/libpcap.A.dylib'
    OSX_LIBC_DYLIB = '/usr/lib/libSystem.B.dylib'
    PCAP_ERRBUF_SIZE = 256
    packet_count_limit = ctypes.c_int(1)
    timeout_limit = ctypes.c_int(1000) # In milliseconds 
    err_buf = ctypes.create_string_buffer(PCAP_ERRBUF_SIZE)

    class bpf_program(ctypes.Structure):
        _fields_ = [("bf_len", ctypes.c_int),("bf_insns", ctypes.c_void_p)]

    class pcap_pkthdr(ctypes.Structure):
        _fields_ = [("tv_sec", ctypes.c_long), ("tv_usec", ctypes.c_long), ("caplen", ctypes.c_uint), ("len", ctypes.c_uint)]

    class pcap_stat(ctypes.Structure):
        _fields_ = [("ps_recv",ctypes.c_uint), ("ps_drop",ctypes.c_uint), ("ps_ifdrop", ctypes.c_int)]

    def pkthandler(pkthdr,packet):
        print("In callback:")
        print("pkthdr[0:7]:",pkthdr.contents.len)
        print(pkthdr.contents.tv_sec,pkthdr.contents.caplen,pkthdr.contents.len)
        print(packet.contents[:10])
        print()

    print "-------------------------------------------"
    libc = ctypes.CDLL(OSX_LIBC_DYLIB, use_errno=True)
    if not libc:
        print "Error loading C libary: %s" % errno.errorcode[ctypes.get_errno()]
    print "* C runtime libary loaded: %s" % OSX_LIBC_DYLIB
    pcap = ctypes.CDLL(OSX_PCAP_DYLIB, use_errno=True)
    if not pcap:
        print "Error loading C libary: %s" % errno.errorcode[ctypes.get_errno()]
    print "* C runtime libary loaded: %s" % OSX_PCAP_DYLIB
    print "* C runtime handle at: %s" % pcap

    print "-------------------------------------------"
    pcap_lookupdev = pcap.pcap_lookupdev
    pcap_lookupdev.restype = ctypes.c_char_p
    dev = pcap.pcap_lookupdev()
    print "* Device handle at: %s" % dev

    net = ctypes.c_uint()
    mask = ctypes.c_uint()
    pcap.pcap_lookupnet(dev,ctypes.byref(net),ctypes.byref(mask),err_buf)
    print "* Device IP to bind: %s" % net
    print "* Device net mask: %s" % mask

    #pcap_t *pcap_open_live(const char *device, int snaplen,int promisc, int to_ms, char *errbuf)
    pcap_open_live = pcap.pcap_open_live
    pcap_open_live.restype = ctypes.POINTER(ctypes.c_void_p)
    pcap_create = pcap.pcap_create
    pcap_create.restype = ctypes.c_void_p
    #pcap_handle = pcap.pcap_create(dev, err_buf)
    pcap_handle = pcap.pcap_open_live(dev, 1024, packet_count_limit, timeout_limit, err_buf)
    print "* Live capture device handle at: %s" % pcap_handle 

    pcap_can_set_rfmon = pcap.pcap_can_set_rfmon
    pcap_can_set_rfmon.argtypes = [ctypes.c_void_p]
    if (pcap_can_set_rfmon(pcap_handle) == 1):
        print "* Can set interface in monitor mode"

    pcap_pkthdr_p = ctypes.POINTER(pcap_pkthdr)()
    packetdata = ctypes.POINTER(ctypes.c_ubyte*65536)()
    #print pcap.pcap_next(pcap_handle,ctypes.byref(pcap_pkthdr_p))

    if (pcap.pcap_next_ex(pcap_handle, ctypes.byref(pcap_pkthdr_p), ctypes.byref(packetdata)) == 1):
        print "* Packet captured!"
        pkthandler(pcap_pkthdr_p,packetdata)