使用wiresh的校验和icmp python

Question

在过去的几天里，我对ICMP协议很感兴趣，我发现了一个python函数来计算它的校验和：

def carry_around_add(a, b):
    c = a + b
    return (c & 0xffff) + (c >> 16)

def checksum(msg):
   s = 0
   for i in range(0, len(msg), 2):
       w = ord(msg[i]) + (ord(msg[i+1]) << 8)
       s = carry_around_add(s, w)
   return ~s & 0xffff

print checksum("abcdefghijklmnopqrst")

在这张截获的wireshark照片中：

校验和是“0xcfcb”，但我测试中的函数返回“55 245->；0xd7cd”。为什么？在

非常感谢=）

Answer 1

wireshark转储显示ICMP校验和，但是（wikipedia）：

^{bq}$

测试中校验和例程的输入只是ASCII有效负载部分。您必须提供整个ICMP输入。在

---

例如：

def carry_around_add(a, b):
    c = a + b
    return (c & 0xffff) + (c >> 16)

def checksum(msg):
   s = 0
   for i in range(0, len(msg), 2):
       w = ord(msg[i]) + (ord(msg[i+1]) << 8)
       s = carry_around_add(s, w)
   return ~s & 0xffff

payload_body = "abcdefghijklmnopqrst"
chk = checksum(payload_body)
print chk, '{:x}'.format(chk), '(host byte order)'

msg_type = '\x08' # ICMP Echo Request
msg_code = '\x00' # must be zero
msg_checksum_padding = '\x00\x00' # "...with value 0 substituted for this field..."
rest_header = '\x00\x01\x00\x01' # from pcap
entire_message = msg_type + msg_code + msg_checksum_padding + rest_header + payload_body
entire_chk = checksum(entire_message)
print entire_chk, '{:x}'.format(entire_chk), '(host byte order)'

当我在我的（little endian）机器上运行这个程序时，我得到：

^{pr2}$