Code a packet sniffer in python with pcapy exte...

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Code a packet sniffer in python with pcapy extension

Pcapy

In the previous articles we coded packet sniffers in python using raw sockets. Now lets use the libpcap library for the same. Libpcap is the packet capture library for linux and has wrappers for most languages. In python there are multiple libpcap wrappers like pcapy, pypcap etc. In this article we shall use the pcapy python module.

Pcapy is a Python extension module that interfaces with the libpcap packet capture library. Pcapy enables python scripts to capture packets on the network.

Project website :
http://oss.coresecurity.com/projects/pcapy.html

On ubuntu pcapy can be installed directly from synaptic by issuing the following command

$ sudo apt-get install python-pcapy 

Code

Lets code our sniffer right away.

''''''
Packet sniffer in python using the pcapy python library

Project website

http://oss.coresecurity.com/projects/pcapy.html

''''''

import socket
from struct import *
import datetime
import pcapy
import sys

def main(argv):
    #list all devices
    devices = pcapy.findalldevs()
    print devices

    #ask user to enter device name to sniff
    print "Available devices are :"
    for d in devices :
        print d

    dev = raw_input("Enter device name to sniff : ")

    print "Sniffing device " + dev

    ''''''
    open device
    # Arguments here are:
    #   device
    #   snaplen (maximum number of bytes to capture _per_packet_)
    #   promiscious mode (1 for true)
    #   timeout (in milliseconds)
    ''''''
    cap = pcapy.open_live(dev , 65536 , 1 , 0)

    #start sniffing packets
    while(1) :
        (header, packet) = cap.next()
        #print (''%s: captured %d bytes, truncated to %d bytes'' %(datetime.datetime.now(), header.getlen(), header.getcaplen()))
        parse_packet(packet)

#Convert a string of 6 characters of ethernet address into a dash separated hex string
def eth_addr (a) :
    b = "%.2x:%.2x:%.2x:%.2x:%.2x:%.2x" % (ord(a[0]) , ord(a[1]) , ord(a[2]), ord(a[3]), ord(a[4]) , ord(a[5]))
    return b

#function to parse a packet
def parse_packet(packet) :

    #parse ethernet header
    eth_length = 14

    eth_header = packet[:eth_length]
    eth = unpack(''!6s6sH'' , eth_header)
    eth_protocol = socket.ntohs(eth[2])
    print ''Destination MAC : '' + eth_addr(packet[0:6]) + '' Source MAC : '' + eth_addr(packet[6:12]) + '' Protocol : '' + str(eth_protocol)

    #Parse IP packets, IP Protocol number = 8
    if eth_protocol == 8 :
        #Parse IP header
        #take first 20 characters for the ip header
        ip_header = packet[eth_length:20+eth_length]

        #now unpack them :)
        iph = unpack(''!BBHHHBBH4s4s'' , ip_header)

        version_ihl = iph[0]
        version = version_ihl >> 4
        ihl = version_ihl & 0xF

        iph_length = ihl * 4

        ttl = iph[5]
        protocol = iph[6]
        s_addr = socket.inet_ntoa(iph[8]);
        d_addr = socket.inet_ntoa(iph[9]);

        print ''Version : '' + str(version) + '' IP Header Length : '' + str(ihl) + '' TTL : '' + str(ttl) + '' Protocol : '' + str(protocol) + '' Source Address : '' + str(s_addr) + '' Destination Address : '' + str(d_addr)

        #TCP protocol
        if protocol == 6 :
            t = iph_length + eth_length
            tcp_header = packet[t:t+20]

            #now unpack them :)
            tcph = unpack(''!HHLLBBHHH'' , tcp_header)

            source_port = tcph[0]
            dest_port = tcph[1]
            sequence = tcph[2]
            acknowledgement = tcph[3]
            doff_reserved = tcph[4]
            tcph_length = doff_reserved >> 4

            print ''Source Port : '' + str(source_port) + '' Dest Port : '' + str(dest_port) + '' Sequence Number : '' + str(sequence) + '' Acknowledgement : '' + str(acknowledgement) + '' TCP header length : '' + str(tcph_length)

            h_size = eth_length + iph_length + tcph_length * 4
            data_size = len(packet) - h_size

            #get data from the packet
            data = packet[h_size:]

            print ''Data : '' + data

        #ICMP Packets
        elif protocol == 1 :
            u = iph_length + eth_length
            icmph_length = 4
            icmp_header = packet[u:u+4]

            #now unpack them :)
            icmph = unpack(''!BBH'' , icmp_header)

            icmp_type = icmph[0]
            code = icmph[1]
            checksum = icmph[2]

            print ''Type : '' + str(icmp_type) + '' Code : '' + str(code) + '' Checksum : '' + str(checksum)

            h_size = eth_length + iph_length + icmph_length
            data_size = len(packet) - h_size

            #get data from the packet
            data = packet[h_size:]

            print ''Data : '' + data

        #UDP packets
        elif protocol == 17 :
            u = iph_length + eth_length
            udph_length = 8
            udp_header = packet[u:u+8]

            #now unpack them :)
            udph = unpack(''!HHHH'' , udp_header)

            source_port = udph[0]
            dest_port = udph[1]
            length = udph[2]
            checksum = udph[3]

            print ''Source Port : '' + str(source_port) + '' Dest Port : '' + str(dest_port) + '' Length : '' + str(length) + '' Checksum : '' + str(checksum)

            h_size = eth_length + iph_length + udph_length
            data_size = len(packet) - h_size

            #get data from the packet
            data = packet[h_size:]

            print ''Data : '' + data

        #some other IP packet like IGMP
        else :
            print ''Protocol other than TCP/UDP/ICMP''

        print

if __name__ == "__main__":
  main(sys.argv) 

Output

$ sudo python pcapy_sniffer.py 
[''eth0'', ''usbmon1'', ''usbmon2'', ''usbmon3'', ''usbmon4'', ''usbmon5'', ''usbmon6'', ''usbmon7'', ''any'', ''lo'']
Available devices are :
eth0
usbmon1
usbmon2
usbmon3
usbmon4
usbmon5
usbmon6
usbmon7
any
lo
Enter device name to sniff : eth0
Sniffing device eth0
Destination MAC : 00:1c:c0:f8:79:ee Source MAC : 6c:fd:b9:53:6a:21 Protocol : 8
Version : 4 IP Header Length : 5 TTL : 250 Protocol : 17 Source Address : 61.1.96.71 Destination Address : 192.168.1.101
Source Port : 53 Dest Port : 56291 Length : 136 Checksum : 28619
stackexchangecom?ny
                 o@"we?mns3
                           serverfault?%?mns1?N?mns2?N                                                                                                                      

Destination MAC : 6c:fd:b9:53:6a:21 Source MAC : 00:1c:c0:f8:79:ee Protocol : 8                                                                                             
Version : 4 IP Header Length : 5 TTL : 64 Protocol : 1 Source Address : 192.168.1.101 Destination Address : 61.1.96.71                                                      
Type : 3 Code : 3 Checksum : 23788                                                                                                                                          
stackexchangecom?G?e5???o???socketsny                                                                                                                                       
                 o@"we?mns3                                                                                                                                                 
                           serverfault?%?mns1?N?mns2?N 

First the script would list out the available devices and then ask the user to enter the name of the device that is to be sniffed. Alternatively lookupdev function can be used to find a sniffable device without asking user to select anything.

The packet contents are also broken down and parsed. The script can parse only TCP/UDP/ICMP packets.

LeetCode算法题python解法：24. Swap Nodes in Pairs

Given 1->2->3->4, you should return the list as 2->1->4->3.

中文翻译：

给定链表，交换每两个相邻节点并返回其头部。

例：

给定1->2->3->4，您应该将列表作为2->1->4->3。

注意：

• 您的算法应该只使用恒定的额外空间。
• 您可能无法修改列表节点中的值，只能更改节点本身。

解题思路：该题非常简单，给定链表两两对调然后重新链接，返回新链表的头部即可。将链表遍历添加到一个列表linklist中，然后利用索引进行两两对调，最后重新连接这个链表，然后linklist[0]

代码如下：

#!/usr/bin/env python 
# -*- coding:utf-8 -*-
class Solution:
    def swapPairs(self, head):         #思路非常简单，把链表遍历出来放到列表里，然后再两两对调，在重新链接链表。
        linklist = []
        x, y = 0, 1

        while head != None:  #将链表遍历到一个linklist列表中
            linklist.append(head)
            head=head.next
        if len(linklist)<1:   #这里排除一下特殊情况，如果给的链表为空，则直接返回空值
            return linklist

        while y <= len(linklist)-1:      #这里进行两两对调
            linklist[x], linklist[y] = linklist[y], linklist[x]
            x, y = x + 2, y + 2

        for i in range(len(linklist)-1):        #将对调后的linklist链接成新的链表
            linklist[i].next=linklist[i+1]
        linklist[-1].next=None                   #设置链表最后一个值的末端为空值
        return linklist[0]

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