Question

我试图通过使用Wireshark检查UDP数据包的校验和值来验证其有效性。在这个特定的数据包中，UDP标头的值如下：

源端口：53 (0000 0000 0011 0101)

目标端口：64992 (1111 1101 1110 0000)

长度：64 (0000 0000 0100 0000)

现在，如果添加这些值，则总和为65109 (1111 1110 0101 0101)

所以我希望校验和值为426 (0001 1010 1010)，它是总和的1的补码。

但是在Wireshark中，校验和值为0x63c7，它表示此校验和是正确的。

我想知道我在哪里弄错了。任何帮助或朝正确方向的推动将不胜感激。

谢谢。

Answer 1

如果您引用RFC 768，则会找到正确计算校验和所需的详细信息：

Checksum is the 16-bit one's complement of the one's complement sum of a
pseudo header of information from the IP header, the UDP header, and the
data,  padded  with zero octets  at the end (if  necessary)  to  make  a
multiple of two octets.

The pseudo  header  conceptually prefixed to the UDP header contains the
source  address,  the destination  address,  the protocol,  and the  UDP
length.   This information gives protection against misrouted datagrams.
This checksum procedure is the same as is used in TCP.

                  0      7 8     15 16    23 24    31
                 +--------+--------+--------+--------+
                 |          source address           |
                 +--------+--------+--------+--------+
                 |        destination address        |
                 +--------+--------+--------+--------+
                 |  zero  |protocol|   UDP length    |
                 +--------+--------+--------+--------+

If the computed  checksum  is zero,  it is transmitted  as all ones (the
equivalent  in one's complement  arithmetic).   An all zero  transmitted
checksum  value means that the transmitter  generated  no checksum  (for
debugging or for higher level protocols that don't care).

如果您想了解Wireshark的UDP解剖器如何处理它，可以查看packet-udp.c的源代码。基本上，在正确设置数据输入后，它实际上只是调用in_cksum.c文件中的in_cksum()函数来计算它。

您可能还想看看RFC 1071，“计算Internet校验和”。

通过Wireshark验证校验和值

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