DNS是什么：

DNS是域名系统,Domain Name System的缩写,是一个服务。

作用：

DNS就是把域名解析为IP地址，提供我们上网，我们能够上网最终是找到IP地址。

DNS请求报文格式：

分别包含

• Transaction ID：会话标识
• Flags：标志
• Questions：问题数量
• Answer RRs：回答数量
• Authority RRs：授权数量
• Additional RRs：附加数量
• Queries：查询区域

再细分展开Flags与Queries：
Flags：

其包含：

• Response: 0 为查询，1 为响应
• Opcode: 0 表示标准查询，1 表示反向查询，2 表示服务器状态请求
• Truncated: 表示可截断的
• Recursion desired: 表示期望递归
• Z: 保留
• Non-authenticated data: 是否为未经验证的数据

Queries：

其包含1个问题，问题内容为：

• Name: m.baidu.com
• Type: A (Host Address) (1)
• Class: IN (0x0001)

type数值依据：

Class(查询类)
通常为 1，是 Internet 数据

构建DNS ：

header ：

struct header {unsigned short id;			unsigned short flags;		unsigned short questions;	unsigned short answer;		unsigned short authority;unsigned short additional;
};

queries：

struct queries {int length;unsigned short qtype;unsigned short qclass;unsigned char* name;
};

创建结构体存储ip

struct dns_item {char* ip;

};
header创建：

int create_header(struct header* header) {if (header == NULL)
{ 
return -1;
}memset(header, 0, sizeof(struct header));srandom(time(NULL));header->id = random();header->flags = htons(0x0100);//16位主机字节顺序转化成网络字节序 0000 0001 0000 0000 =十进制256header->questions = htons(1);//0000 0001 转换}

创建Queries

int create_queries(struct queries* question, const char* hostname) {if (question == NULL || hostname == NULL){return -1;}memset(question, 0, sizeof(struct queries));question->name = (char*)malloc(strlen(hostname) + 2);if (question->name == NULL) {return -1;}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); 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);
}

将header和Queries组包：

int dns_build_request(struct header* header, struct 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 header));offset = sizeof(struct 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;
}

DNS回复报文格式：

下面是Queris的展开：

下面是Answer的展开（没用截图到回复的DNS_HOST类型，只有CNAME，逻辑可以参考有回复IP的）：

服务器回复后的解析包：

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);printf("------问题数querys:%#x------------\n",querys);ptr += 2;int answers = ntohs(*(unsigned short*)ptr);printf("-----回复数answers:%#x------------\n",answers);ptr += 6;//queriesfor (i = 0; i < querys; i++) {while (1) {int flag = (int)ptr[0];ptr += (flag + 1);if (flag == 0) break;}ptr += 4;}char  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++) {//ptr answerslen = 0;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) {printf("--------DNS_CNAME--------\n");ptr += datalen;}else if (type == DNS_HOST) {printf("--------DNS_HOST--------\n");bzero(ip, sizeof(ip));if (datalen == 4) {memcpy(netip, ptr, datalen);inet_ntop(AF_INET, netip, ip, sizeof(ip));list[cnt].ip = (char*)calloc(strlen(ip) + 1, 1);memcpy(list[cnt].ip, ip, strlen(ip));printf("ip address %s\n", list[cnt].ip);cnt++;}ptr += datalen;}}*domains = list;ptr += 2;return cnt;}

socket请求

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));//绑定struct header header = { 0 };dns_create_header(&header);struct queries question = { 0 };dns_create_queries(&question, domain);char request[1024] = { 0 };int length = dns_build_request(&header, &question, request,1024);int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));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);struct dns_item* dns_domain = NULL;dns_parse_response(response, &dns_domain);free(dns_domain);return 1;
}

main函数（查询百度dns）：

int main(void) {dns_client_commit("www.baidu.com");
}

所有的代码

#include
#include
#include
#include
#include
#include
#include #define DNS_SERVER_PORT		53
#define DNS_SERVER_IP		"114.114.114.114"#define DNS_HOST			0x01
#define DNS_CNAME			0x05struct header {unsigned short id;			unsigned short flags;		unsigned short questions;	unsigned short answer;		unsigned short authority;unsigned short additional;
};struct queries {int length;unsigned short qtype;unsigned short qclass;unsigned char* name;
};
struct dns_item {char* ip;int create_header(struct header* header) {if (header == NULL)
{ 
return -1;
}memset(header, 0, sizeof(struct header));srandom(time(NULL));header->id = random();header->flags = htons(0x0100);//16位主机字节顺序转化成网络字节序 0000 0001 0000 0000 =十进制256header->questions = htons(1);//0000 0001 转换}int create_queries(struct queries* question, const char* hostname) {if (question == NULL || hostname == NULL){return -1;}memset(question, 0, sizeof(struct queries));question->name = (char*)malloc(strlen(hostname) + 2);if (question->name == NULL) {return -1;}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); 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 header* header, struct 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 header));offset = sizeof(struct 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 dns_parse_response(char* buffer, struct dns_item** domains) {int i = 0;unsigned char* ptr = buffer;ptr += 4;int querys = ntohs(*(unsigned short*)ptr);printf("------问题数querys:%#x------------\n",querys);ptr += 2;int answers = ntohs(*(unsigned short*)ptr);printf("-----回复数answers:%#x------------\n",answers);ptr += 6;//queriesfor (i = 0; i < querys; i++) {while (1) {int flag = (int)ptr[0];ptr += (flag + 1);if (flag == 0) break;}ptr += 4;}char  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++) {//ptr answerslen = 0;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) {printf("--------DNS_CNAME--------\n");ptr += datalen;}else if (type == DNS_HOST) {printf("--------DNS_HOST--------\n");bzero(ip, sizeof(ip));if (datalen == 4) {memcpy(netip, ptr, datalen);inet_ntop(AF_INET, netip, ip, sizeof(ip));list[cnt].ip = (char*)calloc(strlen(ip) + 1, 1);memcpy(list[cnt].ip, ip, strlen(ip));printf("ip address %s\n", list[cnt].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));//绑定struct header header = { 0 };dns_create_header(&header);struct queries question = { 0 };dns_create_queries(&question, domain);char request[1024] = { 0 };int length = dns_build_request(&header, &question, request,1024);int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));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);struct dns_item* dns_domain = NULL;dns_parse_response(response, &dns_domain);free(dns_domain);return 1;
}
int main(void) {dns_client_commit("www.baidu.com");
}

结语

属于应用层的编程，对于我i们后续的协议理解是很有帮助的。上面提供的所有代码可以直接编译运行，执行的时候请检查电脑是否有网络，否则无法正常执行。