标签：des class blog code http tar
本来是一次计算机网络的实验，但是还没有完全写好，DNS的响应请求报文的冗余信息太多了，不只有IP地址。所以这次的实验主要就是解析DNS报文。同时也需要正确的填充请求报文。如果代码有什么bug，欢迎指正啊。代码排版有点乱。。。
本文有以下内容
DNS报文的填充和解析
利用socket API传输信息
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：
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/* 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：
下面是填充域名为百度(www.baidu.com)的代码
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/* 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：
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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)。
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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+13],recvMsg[pos+14]); } } 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：
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/************************************************************************* > File Name: MyFiles/C和C++程序/socket/getIP.c > Author: mr_zys > Mail: 247629929@qq.com > 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 53void
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+13],recvMsg[pos+14]); } } 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);} |
总结：刚开始感觉无从下手
暂时就这么多吧！
感谢下面的博客：
http://blog.csdn.net/ericzhong83/article/details/8108103介绍DNS报文
http://blogfeifei.iteye.com/blog/1213628更详细的介绍
http://blog.csdn.net/kevinshq/article/details/7199573windows系统下的实现，不过没试过，但是也给了启发
Linux socket编程 DNS查询IP地址,布布扣,bubuko.com
标签：des class blog code http tar
原文地址：http://www.cnblogs.com/mr-zys/p/3784629.html
