networking - 为什么 DNS 请求中句点的十六进制值不是 0x2E，为什么会发生变化？

Question

在 wireshark 中查看 www.google.com 的 DNS 请求，它的十六进制是 03 77 77 77 06 67 6f 6f 67 6c 65 03 63 6f 6d 00

有点困惑，为什么第一个句点是 03(以及为什么它在那里)，第二个句点是 06，最后一个句点是 03

Answer 1

DNS 协议(protocol)层在RFC 1035 中定义.引用自“3.1. namespace 定义”:

Domain names in messages are expressed in terms of a sequence of labels. Each label is represented as a one octet length field followed by that number of octets. Since every domain name ends with the null label of the root, a domain name is terminated by a length byte of zero.

因此 www.google.com 在 DNS 数据包中被编码为:

03 77 77 77                  length 3, "www"
06 67 6f 6f 67 6c 65         length 6, "google"
03 63 6f 6d                  length 3, "com"
00                           length 0 (end of label)