一、填充DNS请求报文
随便百度一下，就可以知道DNS报文的格式。所以这里只介绍如何填充DNS报文。
首先是填充报文首部：
/* 填充首部的格式大致相同，下面的填充值是参考他人抓包分析的结果 */
buf[0] = 0x00;
buf[1] = 0;
buf[2] = 0x01;
buf[3] = 0;
buf[4] = 0;
buf[5] = 1;
buf[6] = buf[7] = 0;
buf[8] = buf[9] = buf[10] = buf[11] = 0;然后填充报文的问题部分：
- 域名格式：该部分一数字开始以0结束。
- 查询类型：1代表IP地址、2代表名字服务器、5代表规范名称、12代表指针记录
- 查询类：1代表互联网
下面是填充域名为百度(www.baidu.com)的代码
/* 填充域名 */
buf[12] = 3;
buf[13] = buf[14] = buf[15] = 'w';
buf[16] = 5;
buf[17] = 'b';
buf[18] = 'a';
buf[19] = 'i';
buf[20] = 'd';
buf[21] = 'u';
buf[22] = 3;
buf[23] = 'c';
buf[24] = 'o';
buf[25] = 'm';
buf[26] = 0;
/* 填充查询类型和查询类 */
buf[27] = 0;
buf[28] = 1;
buf[29] = 0;
buf[30] = 1;二、利用socket发送DNS报文
下面是代码：
int sendDNSPacket(unsigned char *buf, int len, char *recvMsg)
{
int s;
struct sockaddr_in sin;
memset(&sin,0,sizeof(sin));
sin.sin_addr.s_addr = inet_addr("127.0.0.1"); /* 本体DNS服务器的地址 */
sin.sin_family = AF_INET;
sin.sin_port = htons(SERVER_PORT); /* 端口为53 */
s = socket(PF_INET,SOCK_DGRAM,0); /* UDP传报文 */
sendto(s,buf,len,0,(struct sockaddr *)&sin,sizeof(sin));
return recv(s,recvMsg,MAX_SIZE,0);
}这部分就是普通的socket的创建、发送和接收过程。
三、解析DNS响应报文
自己错将16进制的数错看为10进制数了，在这里坑了很长时间。注意报文中的指针的偏移量，只有指针的偏移量指的是规范名称该资源记录才是IP地址。在报文中还有很多和IP地址无关的资源记录。下面是代码，不过遇到复杂的DNS报文可能有bug。这里只实验了三个域名：www.ccnu.edu.cn www.baidu.com www.163.com(这个域名有点复杂)。
void resolve(unsigned char *recvMsg, int len, int len_recvMsg)
{
int pos = len;
int cnt = 12;
while(pos < len_recvMsg) {
unsigned char now_pos = recvMsg[pos+1];
unsigned char retype = recvMsg[pos+3];
unsigned char reclass = recvMsg[pos+5];
unsigned char offset = recvMsg[pos+11];
if(retype == 1) {
if(now_pos == cnt && reclass == 1) {
printf("%u.%u.%u.%u\n",recvMsg[pos+12],recvMsg[pos+13],recvMsg[pos+14],recvMsg[pos+15]);
}
}
else if(retype == 5) {
cnt = pos + 12 ;
}
pos = pos + 12 + offset;
}
}四、完整的代码和总结
下面是完整的代码：
/*************************************************************************
> File Name: MyFiles/C和C++程序/socket/getIP.c
> Author: mr_zys
> Mail: [email protected]
> Created Time: Thu 12 Jun 2014 05:22:06 PM CST
> Operating System: Ubuntu 12.04 LTS
> Programming Language: Linux c
> Compiler: gcc
> Description: this is a program with Linux socket APIs to ask DNS server for domain name's IP adress!
************************************************************************/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<errno.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>
#define MAX_SIZE 1024
#define SERVER_PORT 53
void setHead(unsigned char *buf)
{
buf[0] = 0x00;
buf[1] = 0;
buf[2] = 0x01;
buf[3] = 0;
buf[4] = 0;
buf[5] = 1;
buf[6] = 0;
buf[7] = 0;
buf[8] = buf[9] = buf[10] = buf[11] = 0;
}
void setQuery(char *name, unsigned char *buf, int len)
{
strcat(buf+12,name);
int pos = len + 12;
buf[pos] = 0;
buf[pos+1] = 1;
buf[pos+2] = 0;
buf[pos+3] = 1;
}
int changeDN(char *DN,char *name)
{
int i = strlen(DN) - 1;
int j = i + 1;
int k;
name[j+1] = 0;
for(k = 0; i >= 0; i--,j--) {
if(DN[i] == '.') {
name[j] = k;
k = 0;
}
else {
name[j] = DN[i];
k++;
}
}
name[0] = k;
return (strlen(DN) + 2);
}
void printName(int len, char *name)
{
int i;
for(i = 0; i < len; i++) printf("%x.",name[i]);
printf("\n");
}
int sendDNSPacket(unsigned char *buf, int len, char *recvMsg)
{
int s;
struct sockaddr_in sin;
memset(&sin,0,sizeof(sin));
sin.sin_addr.s_addr = inet_addr("127.0.0.1");
sin.sin_family = AF_INET;
sin.sin_port = htons(SERVER_PORT);
s = socket(PF_INET,SOCK_DGRAM,0);
sendto(s,buf,len,0,(struct sockaddr *)&sin,sizeof(sin));
return recv(s,recvMsg,MAX_SIZE,0);
}
void resolve(unsigned char *recvMsg, int len, int len_recvMsg)
{
int pos = len;
int cnt = 12;
while(pos < len_recvMsg) {
unsigned char now_pos = recvMsg[pos+1];
unsigned char retype = recvMsg[pos+3];
unsigned char reclass = recvMsg[pos+5];
unsigned char offset = recvMsg[pos+11];
if(retype == 1) {
if(now_pos == cnt && reclass == 1) {
printf("%u.%u.%u.%u\n",recvMsg[pos+12],recvMsg[pos+13],recvMsg[pos+14],recvMsg[pos+15]);
}
}
else if(retype == 5) {
cnt = pos + 12 ;
}
pos = pos + 12 + offset;
}
}
int main()
{
unsigned char buf[MAX_SIZE]; /* socket发送的数据 */
char DN[MAX_SIZE]; /* 将要解析的域名(www.xxx.xxx) */
char name[MAX_SIZE]; /* 转换为符合DNS报文格式的域名 */
char recvMsg[MAX_SIZE]; /* 接收的数据 */
int len; /* socket发送数据的长度 */
int s; /* socket handler */
printf("输入需要解析的域名：");
scanf("%s",DN);
len = changeDN(DN,name);
//printName(len,name); /* 打印转换后的域名，检测是否转换正确 */
int j;
//printf("len is %d\n",len);
setHead(buf);
setQuery(name,buf,len);
len += 16;
int len_recvMsg = sendDNSPacket(buf,len,recvMsg);
printf("接收的报文长度为 %d 字节\n",len_recvMsg);
printf("下面是接收报文的16进制表示：\n");
int i;
for(i = 0; i < len_recvMsg; i++) {
printf("%x.",(unsigned char)recvMsg[i]);
}
printf("\n");
printf("%s的IP地址为：\n",DN);
resolve(recvMsg,len,len_recvMsg);
}