为什么在 python 中使用套接字嗅探 wifi 帧时我得到以太网帧（不是 802.11）

Question

I am trying to write a program that will display all wifi networks and their clients. The code looks as follows:

import os
import socket
import struct

def unpack_ethernet_frame(data):
    dest_mac, src_mac, proto = struct.unpack('! 6s 6s H', data[:14])  # Big-endian, 6bytes, 6bytes, 1 short
    return format_mac(dest_mac), format_mac(src_mac), socket.htons(proto), data[14:]

def format_mac(bytes_addr):
    bytes_s = map('{:02x}'.format, bytes_addr) 
    return ":".join(bytes_s).upper()

def format_ip(bytes_addr):
    return ".".join(map(str, bytes_addr))

def unpack_ipv4_frame(data):
    header_len = (data[0] & 15) * 4  # last 4 bits of first byte * 4
    ttl, prot, src, dst = struct.unpack("! 8x B B 2x 4s 4s", data[:20])
    return format_ip(src), format_ip(dst), prot, data[header_len:]

def unpack_tcp(data):
    src_port, dst_port = struct.unpack("! H H", data[:4])
    return src_port, dst_port

def unpack_udp(data):
    src_port, dst_port, size = struct.unpack("! H H 2x H", data[:8])
    return src_port, dst_port, size, data[8:]

if __name__ == '__main__':
    conn = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(3))

    while True:
        raw_data, addr = conn.recvfrom(65536)
        dest, src, prot, data = unpack_ethernet_frame(raw_data)
        if prot == 8:  # IP
            print("Dest {0}, Src {1}, prot {2}".format(dest, src, prot))
            src_id, dst_ip, prot, data = unpack_ipv4_frame(data)
            print("From {0} to {1}".format(src_id, dst_ip))
            if prot == 6:  # TCP
                src_port, dst_port = unpack_tcp(data)
                print("src_port {0} dst_port {1}".format(src_port, dst_port))
            if prot == 17:  # UdP
                src_port, dst_port, size, data = unpack_udp(data)
                print("src_port {0} dst_port {1} size {2}".format(src_port, dst_port, size))

I do not know why, but "unpack_ethernet_frame" function receives Ethernet frames, not MAC frames. If I receive wireless frames then I should rather get MAC frames? Mostly what I am trying to do is to find out which frames are beacon frames and get their SSID.

EDIT : I am trying this code with Monitor mode enabled

Answer 1

When you are getting your data from a socket , the IEEE802 -frame has already been unpacked and the Ethernet -frame within it is what you see. There are historical reasons why this is done and the whole IEEE802 -layer is usually not visible even to the OS. In order to receive unmodified IEEE802 -frames, you need to

• turn your WiFi-device into "Monitor mode", which not all devices support. This will cause all Ethernet- (and by extension, all TCP/UDP-) connections to get dropped.
• receive your data not from a socket but directly from the device, eg via libpcap .

Also see this answer and this link regarding scapy .

Answer 2

You've connected to the wrong interface. You shall specify which network interface you want to connect.

if __name__ == '__main__':
    conn = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(3))
    conn.bind(("wlan0mon", 0x0003))
    while True:
        raw_data, addr = conn.recvfrom(65536)

This worked for me