我看不到是什么触发了无效的语法错误

Question

我目前正在尝试在 python 中复制一个数据包嗅探器，它可以解析 udp、tcp 和 ICPM 数据包。 因为现在除了无效的语法错误外，没有其他编译器错误显示。

#packet sniffer for linux in python

import socket, sys
from struct import*

#A function that converts a string of 6 characters of ethernet address into dash seperated hex string
def ethernet_address (string):
    new_String = "%.2x:%.2x:%.2x:%.2x:%.2x:%.2x" % (ord(string(0)), ord(string(1)), ord(string(2)), ord(string(3)), ord(string(4)), ord(string(5)))
    return new_String

#Section that creates the socket
try:
    sock = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.ntohs(0x0003))
except socket.error as msg:
    print ("Socket could not be created, Error : ")
    print (msg)
    sys.exit()

#recieve a packet
while True:
    packet = sock.recvfrom(60000)

    #packet string from tuple
    packet = packet[0]

    #parse ethernet header
    ether_length = 14

    ether_Header = packet[:ether_length]
    ether = unpack('!6s6sh', ether_Header)
    ether_protocol = socket.ntohs(ether[2])
    print ("Destination MAC: " + ethernet_address(packet[0:6]) + "Source MAC: " + ethernet_address(packet[6:12]) + " Protocol: " + str(ether_protocol))

    #This section handles parsing IP packets
    if ether_protocol == 8:
        
        #Parse the IP header
        #take the first 20 character from the header
        ip_header = packet[ether_length:20 + ether_length]

        #now unpack
        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]
        source_address = socket.inet_ntoa( iph[8] )
        destination_address = socket.inet_ntoa( iph[9] )

        print("Version: " + str(version) + " IP header length: " + str(ihl) + " TTL: " + str(ttl) + " Protocol: " + str(protocol + " Source Address: " + str(source_address) + " Destination Address: " + str(destination_address) )


        #This section handles parsing TCP packets
        if protocol == 6 : #This is the line with the error******************************************
            num = iph_length + ether_length
            tcp_header = packet[ num:num+20]

            #unpack
            tcph = unpack('!HHLLBBHHH', tcp_header)

            source_port = tcph[0]
            destination_port = tcph[1]
            sequence = tcph[2]
            acknowledgment = tcph[3]
            doff_reserve = tcph[4]
            tcph_length = doff_reserve >> 4

            print("Source Port: " + str(source_port) + " Destination Port: " + destination_port + " Sequence: " + sequence + " Acknowledgment: " + str(acknowledgment) + " TCP header length: " + str(tcph_length))

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

            #get data
            data = packet[h_size:]

            print ("Data: "+ data)

        #This section Handles parsing ICMP packets
        elif protocol == 1:
            u = iph_length + ether_length
            icmph_length = 4
            icmp_header = packet[u:u+20]

            #unpack
            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 = ether_length + iph_length + icmph_length
            data_size = len(packet) - h_size

            #retrive data
            data = packet[h_size]

            print("Data: " + data)

        #This section handles parsing UDP
        elif protocol == 17:
            u = iph_length +ether_length
            udph_length = 8
            udp_header = packet[u:u+8]

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

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

            print("Source Port: " + str(src_port) + " Destination Port: " + str(dest_port) + " Length: " + str(length) + " Checksum: " + str(checksum) )

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

            #get data
            data = packet[h_size]

            print("Data: " + data)

        else:
            print("Protocol is something other than UDP, ICMP, or TCP")

        print

该错误是由以下行引起的：

if protocol == 6 :

Visual Studio 说这是一个语法错误，但我知道这是怎么回事，因为它的编写方式与其他所有 if 语句一样，并且它们没有语法错误警告。 如果有一个错误是什么？

Answer 1

我相信你缺少一个括号。 print("Version: " + str(version) + " IP header length: " + str(ihl) + " TTL: " + str(ttl) + " Protocol: " + str(protocol + " Source Address: " + str(source_address) + " Destination Address: " + str(destination_address) ))应该可以工作。

注意：我在末尾添加了一个括号，但这可能不是应该为您的代码添加到 function 的位置，尽管删除了错误。 看来print("Version: " + str(version) + " IP header length: " + str(ihl) + " TTL: " + str(ttl) + " Protocol: " + str(protocol) + " Source Address: " + str(source_address) + " Destination Address: " + str(destination_address) )可能是您想要的。

Answer 2

错误在上面的行中找到：

print("Version: " + str(version) + " IP header length: " + str(ihl) + " TTL: " + str(ttl) + " Protocol: " + str(protocol + " Source Address: " + str(source_address) + " Destination Address: " + str(destination_address) )

具体来说，错误来自... str(protocol +... ，应该是... str(protocol) +... 。

Answer 3

您在上一行有一个不匹配的括号：

print("Version: " + 
      str(version) + 
      " IP header length: " + 
      str(ihl) + 
      " TTL: " + 
      str(ttl) + 
      " Protocol: " + 
      str(protocol + 
          " Source Address: " + 
          str(source_address) + 
          " Destination Address: " + 
          str(destination_address)
         )

你可能想要这样的东西：

print("Version: " + 
      str(version) + 
      " IP header length: " + 
      str(ihl) + 
      " TTL: " + 
      str(ttl) + 
      " Protocol: " + 
      str(protocol) + 
      " Source Address: " + 
      str(source_address) + 
      " Destination Address: " + 
      str(destination_address) 
)

另外，考虑使用" ".join(iterable)将str S 与空格连接起来。 连接效率低下（这里不是问题）并且难以阅读（这里肯定是一个问题）。