c++ - header 结构中的 DNS 查询格式小-大端问题

Question

我对 DNS 查询格式有疑问......

我知道这个结构是DNS查询头的结构......

typedef struct _DNS_HEADER {
    unsigned short id;       // identification number
    unsigned char qr :1;     // query/response flag
    unsigned char opcode :4; // purpose of message
    unsigned char aa :1;     // authoritative answer
    unsigned char tc :1;     // truncated message
    unsigned char rd :1;     // recursion desired
    unsigned char ra :1;     // recursion available
    unsigned char z :1;      // its z! reserved
    unsigned char ad :1;     // authenticated data
    unsigned char cd :1;     // checking disabled
    unsigned char rcode :4;  // response code
    unsigned short q_count;  // number of question entries
    unsigned short ans_count; // number of answer entries
    unsigned short auth_count; // number of authority entries
    unsigned short add_count; // number of resource entries
} DNS_HEADER;

+问题查询和...

但是这里有一个问题:我创建了一个套接字 socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP) 并创建了一个查询请求:

id = htons(10000);
qr = 0;
opcode = 0
aa = 0
tc = 0
rd = 1
ra = 0
z = 0 
ad = 0
cd = 0
q_count = 1
ans_count = 0
auth_count = 0
add_count = 0

和我的问题 ->

3,'w','w','w',4,'s','i','t','e',3,'c','o','m',0

TYPE = htons(1)
CLASS = htons(1)

我将此请求发送到我的特殊 DNS 服务器(绑定(bind))，我认为存在问题。有时 DNS 服务器没有响应我，即使响应，它也不是真正的响应(一切都是 0，除了相同的 header (12 字节)之外没有收到任何内容)

我搜索了一下，发现我的 DNS header 有问题，而且一定是这样的 ->

typedef struct _DNS_HEADER {
    unsigned short id;       // identification number

    unsigned char rd :1;     // recursion desired
    unsigned char tc :1;     // truncated message
    unsigned char aa :1;     // authoritative answer
    unsigned char opcode :4; // purpose of message
    unsigned char qr :1;     // query/response flag

    unsigned char rcode :4;  // response code
    unsigned char cd :1;     // checking disabled
    unsigned char ad :1;     // authenticated data
    unsigned char z :1;      // its z! reserved
    unsigned char ra :1;     // recursion available

    unsigned short q_count;  // number of question entries
    unsigned short ans_count; // number of answer entries
    unsigned short auth_count; // number of authority entries
    unsigned short add_count; // number of resource entries
} DNS_HEADER;

关于小-大端的一些事情。我将我的结构更改为这个结构，然后发送请求并完成 - 一切正常，没有任何问题，但这是我的问题:我不明白这是什么。是的，我读了一点关于小-大端的内容，但我仍然不明白:

1. 为什么其他教授 DNS 格式的网站没有提到这一点？他们用第一种结构进行教学。
2. 这是什么？为什么要把第二个16位的高8位改成第二个16位的低8位？

Answer 1

Why haven't other websites that teaches about DNS format mentioned this? They teach with the first structure.

我不可能回答这个问题。我遇到的大多数网络文献都提到了网络字节顺序及其动机。

Why we must change the higher 8 bits of the second 16 bits to be the lower 8 bits of the second 16 bits?

其动机可在“[IEN-137] Cohen, D., “ON HOLY WARS AND A PLEA FOR PEACE”, IEN 137, USC/ISI, April 1980 中找到。互联网工程任务组草案中提到:Network Byte Order

Background and Motivation

The document "ON HOLY WARS AND A PLEA FOR PEACE" [IEN-137] writtenin 1980 argues that the industry should settle on a single byteorder. Since then, the IETF has largely settled on a single byteorder known as "Network Byte Order" and this memo is intended torecord that rough concensus. Unfortunately, the "holy war"continues among CPU manufacturers.

最后一句话就是这句话的意思。

某些 CPU 型号使用 Big-Endian其他人则使用 Little-Endian。甚至还有一些使用混合的。

两个字节(二进制形式)代表 std::uint16_t :

大尾数:10000000 00000000表示 32768(十进制)。
小尾数:10000000 00000000表示 128(十进制)。

如果对于如何解释这两个字节没有达成一致，沟通将会非常困难。