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）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)。
internet在命名时采用了层次结构的命名方法。每个域名都是一个标号序列(labels)，用字母，数字，连接符组成。它用点号分割成一个个标号，每个标号可以看成是一个个的层级，级别低的写在左边，级别高的写在右边，域名服务主要基于UDP实现。
顶级域 ---- com edu gov net
第二级域---- baidu 163
第三级域---- www gitlab
在有了域名结构之后需要有机器对其进行解析，而不同层级的根据高低级别由根域名服务器，顶级域名服务器，权限域名服务器，本地域名服务器。域名解析过程分为两大步骤：第一个步骤是本机向本地域名服务器发出一个DNS请求报文，报文携带要查询的域名，第二个步骤是本地域名服务器向本机回应一个DNS响应报文，包含域名对应的IP地址。
以百度为例具体流程描述如下：
递归查询：本机向本地域名服务器发出一次查询请求，就静待最终的结果。如果本地域名服务器无法解析，自己会以 DNS 客户机的身份向其它域名服务器查询，直到得到最
终的 IP 地址告诉本机
迭代查询：本地域名服务器向根域名服务器查询，根域名服务器告诉它下一步到哪里去查询，然后它再去查，每次它都是以客户机的身份去各个服务器查询
报文分为头部和正文：
会话标识（2 字节）：是 DNS 报文的 ID 标识，请求和应答是相同的。
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。
Queries
查询名：长度不固定，不填充字节，一般表示反向查询，即由IP地址反查域名格式如下
5baidu3com0
5表示后面域名的长度，3同理，最后必须为0
查询类型：
查询类：通常为1，表明internet数据
原纪录（RR）：包括回答区域，授权区域和附加区域
#include <stdio.h>
#include <string.h>
#include <stdlib.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;
unsigned short flags;
unsigned short questions;
unsigned short answer;
unsigned short authority;
unsigned short additional;
};
struct dns_question{
int length;
unsigned short qtype;
unsigned short qclass;
unsigned char *name;
};
struct dns_item{
char *domain;
char *ip;
};
//client send to 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);;
header->questions = htons(1);
return 0;
}
//hostname: www.baidu.com
//www
//baidu
//com
//name: 3www5baidu3com0
int dns_creat_questions(struct dns_question *question, const char *hostname){
if(question == NULL || hostname==NULL) return -1;
memset(question,0,sizeof(struct dns_question));
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);
//name
const char delim[2] = ".";
char *qname = question->name;
char *hostname_dup = strdup(hostname); //strdup --> malloc
char *token = strtok(hostname_dup,delim); //www.0voice.com
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_question *question, char *request, int rlen){
if(header == NULL || question == NULL || request == NULL) return -1;
memset(request, 0, rlen);
//header --> server
memcpy(request,header, sizeof(struct dns_header));
int offset = sizeof(struct dns_header);
//question -->request
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(i){
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 %s\n" , aname, ip);
printf("\tTime to live: %d minutes , %d seconds\n", 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("connect: %d\n", ret);
struct dns_header header = {0};
dns_create_header(&header);
struct dns_question question = {0};
dns_creat_questions(&question, domain);
char request[1024] = {0};
int length = dns_build_request(&header, &question, request, 1024);
//requst
int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));
printf("requst: %d\n", slen);
//recvfrom
char response[1024] = {0};
struct sockaddr_in addr;
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\n", n);
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]);
}
_in);
int n = recvfrom(sockfd, response, sizeof(response), 0, (struct sockaddr*)&addr, (socklen_t*)&addr_len);
printf("recvfrom: %d\n", n);
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]);
}