Linux C/C++实现DNS请求器（UDP编程）

admin • 2022-01-13 09:04 • 运维

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文章目录

前言
一、DNS
- 1.DNS是什么
- 1.DNS协议报文格式
二、代码实现
总结
参考资料

前言

我们今天要实现一个DNS请求器，运用到的知识主要是UDP编程，以及了解DNS协议，以及协议解析。

一、DNS

1.DNS是什么

域名系统（英文：Domain Name System，缩写：DNS）是互联网的一项服务。它作为将域名和 IP 地址相互映射的一个分布式数据库，能够使人更方便地访问互联网。DNS 使用 TCP 和 UDP 端口 53。当前，对于每一级域名长度的限制是 63 个字符，域名总长度则不能超过 253 个字符。

域名系统（英文：Domain Name System，缩写：DNS）的作用是将人类可读的域名(如，www.example.com) 转换为机器可读的 IP 地址 (如，192.0.2.44)。

域名是由一串用点分隔符 . 组成的互联网上某一台计算机或计算机组的名称，用于在数据传输时标识计算机的方位。域名可以说是一个 IP 地址的代称，目的是为了便于记忆后者。例如，www.0voice.com 是一个域名，和 IP 地址 122.152.222.180 相对应。人们可以直接访问www.0voice.com 来代替 IP 地址，然后域名系统（DNS）就会将它转化成便于机器识别的 IP 地址。这样，人们只需要记忆 www.0voice.com 这一串带有特殊含义的字符，而不需要记忆没有含义的数字。

1.DNS协议报文格式

1. 头部

会话标识（2 字节）：是 DNS 报文的 ID 标识，对于请求报文和其对应的应答报文，这个字段是相同的，通过它可以区分 DNS 应答报文是哪个请求的响

标志（2 字节）

QR（1bit）查询/响应标志，0 为查询，1 为响应
opcode（4bit）0 表示标准查询，1 表示反向查询，2 表示服务器状态请求
AA（1bit）表示授权回答
TC（1bit）表示可截断的
RD（1bit）表示期望递归
RA（1bit）表示可用递归
rcode（4bit）表示返回码，0 表示没有差错，3 表示名字差错，2 表示服务器错误（Server Failure）

数量字段（总共 8 字节）：Questions、Answer RRs、Authority RRs、Additional RRs 各自表示后面的四个区域的数目。Questions 表示查询问题区域节的数量，Answers 表示回答区域的数量，Authoritative namesversers 表示授权区域的数量，Additional recoreds 表示附加区域的数量

2.Queries

查询名：长度不固定，且不使用填充字节，一般该字段表示的就是需要查询的域名（如果是
反向查询，则为 IP，反向查询即由 IP 地址反查域名），一般的格式如下图所示。

3.查询类型

4.查询类

通常为 1，表明是 Internet 数据

二、代码实现

1.DNS请求头的定义

我们可以通过Wireshark捕获www.baidu.com
可以看到：

代码如下（示例）：

struct dns_header {

	unsigned short id;			//会话标识
	unsigned short flags;		//标志

	unsigned short questions;	//问题数
	unsigned short answer;		//回答 资源记录数

	unsigned short authority;	//授权 资源记录数
	unsigned short additional;	//附加 资源记录数

};


struct dns_queries {

	int length;
	unsigned short qtype;
	unsigned short qclass;
	unsigned char* name;

};


struct dns_item {//解析

	char* domain;
	char* ip;

};

2.头部数据

代码如下（示例）：

int dns_create_header(struct dns_header* header) {

	if (header == NULL)return -1;
	memset(header, 0, sizeof(struct dns_header));

	//random
	srandom(time(NULL));
	header->id = random();

	header->flags = htons(0x0100);//转化成网络字节序
	header->questions = htons(1);

}

3.queries数据

代码如下（示例）：

int dns_create_queries(struct dns_queries* question, const char* hostname) {

	if (question == NULL || hostname == NULL)return -1;
	memset(question, 0, sizeof(struct dns_queries));

	question->name = (char*)malloc(strlen(hostname) + 2);
	if (question->name == NULL) {
		return -2;
	}

	question->length = strlen(hostname) + 2;
	
	question->qtype = htons(1);
	question->qclass = htons(1);


	const char delim[2] = ".";
	char* qname = question->name;

	char* hostname_dup = strdup(hostname); //strdup -->malloc
	char* token = strtok(hostname_dup, delim);

	while (token != NULL) {

		size_t len = strlen(token);

		*qname = len;
		qname++;

		strncpy(qname, token, len + 1);
		qname += len;

		token = strtok(NULL, delim);

	}

	free(hostname_dup);

}

4.将头部和queries合并

代码如下（示例）：

int dns_build_request(struct dns_header* header, struct dns_queries* question, char* request,int rlen) {

	if (header == NULL || question == NULL || request == NULL)return -1;

	int offset = 0;

	memset(request, 0, rlen);

	memcpy(request, header, sizeof(struct dns_header));
	offset = sizeof(struct dns_header);

	memcpy(request + offset, question->name, question->length);
	offset += question->length;

	memcpy(request + offset, &question->qtype, sizeof(question->qtype));
	offset += sizeof(question->qtype);

	memcpy(request + offset, &question->qclass, sizeof(question->qclass));
	offset += sizeof(question->qclass);

	return offset;

}

5.解析数据

代码如下（示例）：

static int is_pointer(int in) {
	return ((in & 0xC0) == 0xC0);
}


static void dns_parse_name(unsigned char* chunk, unsigned char* ptr, char* out, int* len) {

	int flag = 0, n = 0, alen = 0;
	char* pos = out + (*len);

	while (1) {

		flag = (int)ptr[0];
		if (flag == 0) break;

		if (is_pointer(flag)) {

			n = (int)ptr[1];
			ptr = chunk + n;
			dns_parse_name(chunk, ptr, out, len);
			break;

		}
		else {

			ptr++;
			memcpy(pos, ptr, flag);
			pos += flag;
			ptr += flag;

			*len += flag;
			if ((int)ptr[0] != 0) {
				memcpy(pos, ".", 1);
				pos += 1;
				(*len) += 1;
			}
		}

	}

}


static int dns_parse_response(char* buffer, struct dns_item** domains) {

	int i = 0;
	unsigned char* ptr = buffer;

	ptr += 4;
	int querys = ntohs(*(unsigned short*)ptr);

	ptr += 2;
	int answers = ntohs(*(unsigned short*)ptr);

	ptr += 6;
	for (i = 0; i < querys; i++) {
		while (1) {
			int flag = (int)ptr[0];
			ptr += (flag + 1);

			if (flag == 0) break;
		}
		ptr += 4;
	}

	char cname[128], aname[128], ip[20], netip[4];
	int len, type, ttl, datalen;

	int cnt = 0;
	struct dns_item* list = (struct dns_item*)calloc(answers, sizeof(struct dns_item));
	if (list == NULL) {
		return -1;
	}

	for (i = 0; i < answers; i++) {

		bzero(aname, sizeof(aname));
		len = 0;

		dns_parse_name(buffer, ptr, aname, &len);
		ptr += 2;

		type = htons(*(unsigned short*)ptr);
		ptr += 4;

		ttl = htons(*(unsigned short*)ptr);
		ptr += 4;

		datalen = ntohs(*(unsigned short*)ptr);
		ptr += 2;

		if (type == DNS_CNAME) {

			bzero(cname, sizeof(cname));
			len = 0;
			dns_parse_name(buffer, ptr, cname, &len);
			ptr += datalen;

		}
		else if (type == DNS_HOST) {

			bzero(ip, sizeof(ip));

			if (datalen == 4) {
				memcpy(netip, ptr, datalen);
				inet_ntop(AF_INET, netip, ip, sizeof(struct sockaddr));

				printf("%s has address %sn", aname, ip);
				printf("tTime to live: %d minutes , %d secondsn", ttl / 60, ttl % 60);

				list[cnt].domain = (char*)calloc(strlen(aname) + 1, 1);
				memcpy(list[cnt].domain, aname, strlen(aname));

				list[cnt].ip = (char*)calloc(strlen(ip) + 1, 1);
				memcpy(list[cnt].ip, ip, strlen(ip));

				cnt++;
			}

			ptr += datalen;
		}
	}

	*domains = list;
	ptr += 2;

	return cnt;

}

6.发送请求和获取结果

代码如下（示例）：

int dns_client_commit(const char* domain) {

	int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
	if (sockfd < 0) {
		return -1;
	}

	struct sockaddr_in servaddr = { 0 };
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons(DNS_SERVER_PORT);
	servaddr.sin_addr.s_addr = inet_addr(DNS_SERVER_IP);

	int ret = connect(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr));
	//printf("cooect: %dn", ret);

	struct dns_header header = { 0 };
	dns_create_header(&header);

	struct dns_queries question = { 0 };
	dns_create_queries(&question, domain);


	char request[1024] = { 0 };
	int length = dns_build_request(&header, &question, request,1024);

	//request
	int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));

	//recvfrom
	char response[1024] = { 0 };
	struct sockaddr_in addr = { 0 };
	size_t addr_len = sizeof(struct sockaddr_in);


	int n = recvfrom(sockfd, response, sizeof(response), 0, (struct sockaddr*)&addr, (socklen_t*)&addr_len);

	//printf("recvfrom: %d,%sn", n, response);

	struct dns_item* dns_domain = NULL;
	dns_parse_response(response, &dns_domain);

	free(dns_domain);
	

	return n;
}

7.全部代码

代码如下（示例）：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#include<sys/socket.h>
#include<netinet/in.h>
#include<unistd.h>


#define DNS_SERVER_PORT		53
#define DNS_SERVER_IP		"114.114.114.114"

#define DNS_HOST			0x01
#define DNS_CNAME			0x05



struct dns_header {

	unsigned short id;			//会话标识
	unsigned short flags;		//标志

	unsigned short questions;	//问题数
	unsigned short answer;		//回答 资源记录数

	unsigned short authority;	//授权 资源记录数
	unsigned short additional;	//附加 资源记录数

};


struct dns_queries {

	int length;
	unsigned short qtype;
	unsigned short qclass;
	unsigned char* name;

};


struct dns_item {

	char* domain;
	char* ip;

};


//client sendto dns server


int dns_create_header(struct dns_header* header) {

	if (header == NULL)return -1;
	memset(header, 0, sizeof(struct dns_header));

	//random
	srandom(time(NULL));
	header->id = random();

	header->flags = htons(0x0100);//转化成网络字节序
	header->questions = htons(1);

}


//hostname:  www.baidu.com

//name:		3www5baidu3com0

int dns_create_queries(struct dns_queries* question, const char* hostname) {

	if (question == NULL || hostname == NULL)return -1;
	memset(question, 0, sizeof(struct dns_queries));

	question->name = (char*)malloc(strlen(hostname) + 2);
	if (question->name == NULL) {
		return -2;
	}

	question->length = strlen(hostname) + 2;
	
	question->qtype = htons(1);
	question->qclass = htons(1);


	const char delim[2] = ".";
	char* qname = question->name;

	char* hostname_dup = strdup(hostname); //strdup -->malloc
	char* token = strtok(hostname_dup, delim);

	while (token != NULL) {

		size_t len = strlen(token);

		*qname = len;
		qname++;

		strncpy(qname, token, len + 1);
		qname += len;

		token = strtok(NULL, delim);

	}

	free(hostname_dup);

}


int dns_build_request(struct dns_header* header, struct dns_queries* question, char* request,int rlen) {

	if (header == NULL || question == NULL || request == NULL)return -1;

	int offset = 0;

	memset(request, 0, rlen);

	memcpy(request, header, sizeof(struct dns_header));
	offset = sizeof(struct dns_header);

	memcpy(request + offset, question->name, question->length);
	offset += question->length;

	memcpy(request + offset, &question->qtype, sizeof(question->qtype));
	offset += sizeof(question->qtype);

	memcpy(request + offset, &question->qclass, sizeof(question->qclass));
	offset += sizeof(question->qclass);

	return offset;

}


static int is_pointer(int in) {
	return ((in & 0xC0) == 0xC0);
}


static void dns_parse_name(unsigned char* chunk, unsigned char* ptr, char* out, int* len) {

	int flag = 0, n = 0, alen = 0;
	char* pos = out + (*len);

	while (1) {

		flag = (int)ptr[0];
		if (flag == 0) break;

		if (is_pointer(flag)) {

			n = (int)ptr[1];
			ptr = chunk + n;
			dns_parse_name(chunk, ptr, out, len);
			break;

		}
		else {

			ptr++;
			memcpy(pos, ptr, flag);
			pos += flag;
			ptr += flag;

			*len += flag;
			if ((int)ptr[0] != 0) {
				memcpy(pos, ".", 1);
				pos += 1;
				(*len) += 1;
			}
		}

	}

}


static int dns_parse_response(char* buffer, struct dns_item** domains) {

	int i = 0;
	unsigned char* ptr = buffer;

	ptr += 4;
	int querys = ntohs(*(unsigned short*)ptr);

	ptr += 2;
	int answers = ntohs(*(unsigned short*)ptr);

	ptr += 6;
	for (i = 0; i < querys; i++) {
		while (1) {
			int flag = (int)ptr[0];
			ptr += (flag + 1);

			if (flag == 0) break;
		}
		ptr += 4;
	}

	char cname[128], aname[128], ip[20], netip[4];
	int len, type, ttl, datalen;

	int cnt = 0;
	struct dns_item* list = (struct dns_item*)calloc(answers, sizeof(struct dns_item));
	if (list == NULL) {
		return -1;
	}

	for (i = 0; i < answers; i++) {

		bzero(aname, sizeof(aname));
		len = 0;

		dns_parse_name(buffer, ptr, aname, &len);
		ptr += 2;

		type = htons(*(unsigned short*)ptr);
		ptr += 4;

		ttl = htons(*(unsigned short*)ptr);
		ptr += 4;

		datalen = ntohs(*(unsigned short*)ptr);
		ptr += 2;

		if (type == DNS_CNAME) {

			bzero(cname, sizeof(cname));
			len = 0;
			dns_parse_name(buffer, ptr, cname, &len);
			ptr += datalen;

		}
		else if (type == DNS_HOST) {

			bzero(ip, sizeof(ip));

			if (datalen == 4) {
				memcpy(netip, ptr, datalen);
				inet_ntop(AF_INET, netip, ip, sizeof(struct sockaddr));

				printf("%s has address %sn", aname, ip);
				printf("tTime to live: %d minutes , %d secondsn", ttl / 60, ttl % 60);

				list[cnt].domain = (char*)calloc(strlen(aname) + 1, 1);
				memcpy(list[cnt].domain, aname, strlen(aname));

				list[cnt].ip = (char*)calloc(strlen(ip) + 1, 1);
				memcpy(list[cnt].ip, ip, strlen(ip));

				cnt++;
			}

			ptr += datalen;
		}
	}

	*domains = list;
	ptr += 2;

	return cnt;

}


int dns_client_commit(const char* domain) {

	int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
	if (sockfd < 0) {
		return -1;
	}

	struct sockaddr_in servaddr = { 0 };
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons(DNS_SERVER_PORT);
	servaddr.sin_addr.s_addr = inet_addr(DNS_SERVER_IP);

	int ret = connect(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr));
	//printf("cooect: %dn", ret);

	struct dns_header header = { 0 };
	dns_create_header(&header);

	struct dns_queries question = { 0 };
	dns_create_queries(&question, domain);


	char request[1024] = { 0 };
	int length = dns_build_request(&header, &question, request,1024);

	//request
	int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));

	//recvfrom
	char response[1024] = { 0 };
	struct sockaddr_in addr = { 0 };
	size_t addr_len = sizeof(struct sockaddr_in);


	int n = recvfrom(sockfd, response, sizeof(response), 0, (struct sockaddr*)&addr, (socklen_t*)&addr_len);

	//printf("recvfrom: %d,%sn", n, response);

	struct dns_item* dns_domain = NULL;
	dns_parse_response(response, &dns_domain);

	free(dns_domain);
	

	return n;
}


int main(int argc,char* argv[]) {

	if (argc < 2)return -1;

	dns_client_commit(argv[1]);


}

总结

关于今天的学习，我们主要了解的DNS协议，以及协议的解析，主要使用了UDP的编程。

参考资料

C/C++ Linux高级开发课程

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文章目录

前言

一、DNS

1.DNS是什么

1.DNS协议报文格式

二、代码实现

1.DNS请求头的定义

2.头部数据

3.queries数据

4.将头部和queries合并

5.解析数据

6.发送请求和获取结果

7.全部代码

总结

参考资料

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