计算TCP序列号

#!/usr/bin/python
#coding=utf-8
from scapy.all import *

def calTSN(tgt):
    seqNum = 0
    preNum = 0
    diffSeq = 0
    # 重复4次操作
    for x in range(1,5):
        # 若不是第一次发送SYN包，则设置前一个序列号值为上一次SYN/ACK包的序列号值
        # 逻辑出现问题
        # if preNum != 0:
        if seqNum != 0:
            preNum = seqNum
        # 构造并发送TCP SYN包
        pkt = IP(dst=tgt) / TCP()

        ans = sr1(pkt, verbose=0)
        # 读取SYN/ACK包的TCP序列号
        seqNum = ans.getlayer(TCP).seq
        if preNum != 0:
            diffSeq = seqNum - preNum
            print "[*] preNum: %d  seqNum: %d" % (preNum, seqNum)
            print "[+] TCP Seq Difference: " + str(diffSeq)
            print
    return seqNum + diffSeq

def main():
    parser = optparse.OptionParser('[*] Usage : ./countTCPSerial.py -t <target>')
    parser.add_option('-t',dest='target',type='string',help='specify target address')
    (options,args) = parser.parse_args()
    target = options.target

    if target == None:
        print parser.usage
        exit(0)
    # tgt = "192.168.31.165"
    seq = calTSN(target)
    print "[+] Next TCP Sequence Numbet to ACK is: " + str(seqNum + 1)