通过Wireshark验证校验和值

Question

我试图通过使用Wireshark检查数据包来验证UDP数据包的校验和值的有效性。在我正在查看的这个特定数据包中，UDP标头的值如下：

来源：53 (0000 0000 0011 0101)

目的港：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。

我想知道我错在哪里。任何帮助或推动正确的方向将不胜感激。

提前致谢。

Answer 1

如果您参考RFC 768，您将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).

如果你想看看Wireshark的UDP解析器如何处理它，你可以查看packet-udp.c的源代码。基本上，在正确设置数据输入后，它基本上只调用in_cksum()文件中的in_cksum.c函数来计算它。

您可能还想看看RFC 1071，“计算互联网校验和”。