C实现DNS ---转载

kathy870513 发表于 2011-12-22 08:53

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<DIV style="MARGIN: 15px" width="100%">   转载地址：<A href="http://hubeihuyanwei.blog.163.com/blog/static/282052842008104115848444/" target=_blank>http://hubeihuyanwei.blog.163.com/blog/static/282052842008104115848444/</A></DIV>
<DIV style="MARGIN: 15px" width="100%">DNS(Domain Name System)域名系统提供了主机名和IP地址之间的转换。通常我们在应用程序中使用库函数gethostbyname()和gethostbyaddr()来完成两者之间的转换。但是为了更深入的学习网络底层知识，有必要从源代码级别来分析和实现 <A href="http://www.faqs.org/rfcs/rfc1034.html" target=_blank>RFC 1034</A>说明了DNS的概念和功能，<A href="http://www.faqs.org/rfcs/rfc1035.html" target=_blank>RFC 1035</A>详细说明了DNS的规范和实现。通过阅读RFC，我们知道明白了，应用程序对DNS的访问是通过解析器来(resolver)完成的，解析器并不像TCP/IP协议那样是OS的内核，而是通过网络访问DNS服务器来得到名字和地址的对应关系。OS的TCP/IP协议簇对DNS一点都知道。 工欲善其事必先利其器，先得进行些基础知识的复习:<A href="http://faq.csdn.net/read/217043.html" target=_blank>《bit与byte的区别》</A>和《<A href="http://www.sn988.com/nzs/2006105205648.html" target=_blank>bit与byte的联系》</A>及《<A href="http://www.programfan.com/article/2694.html" target=_blank>位运算》</A>，一个int是4个byte(十六进制中01 02 03 04转化为十进制为16909060)，一个char是1个byte(十六进制中97转化为字符为a)。例如在十六进制中0x80，用bit来表示就是1000 0000，此时如果我们对它实施位(>>5)运算，得到的结果就是0000 0100，十六进制值为0x04。 在Linux的内核代码中，经常可以看见形如#define do{ }while(0)的宏定义，是否感到疑惑呢？宏定义只是帮助我们进行替换而已，当定义多条语句时，会在if...else...语句中产生歧义，详细解释参考<A href="http://zhidao.baidu.com/question/57329016.html" target=_blank>链接</A>。(小插曲，我在测试中只#include <stdlib.h>，忘记了#include <stdio.h>，然后在后面使用了printf等，结果编译的时候产生警告: </DIV>
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<TD style="BACKGROUND-COLOR: rgb(199,199,199)">warning: implicit declaration of function ‘printf’ warning: incompatible implicit declaration of built-in function ‘printf’</TD></TR></TBODY></TABLE></DIV>
<DIV style="MARGIN: 15px" width="100%">经过查找才知道警告的原因是没有包含printf函数的明确定义，那么就隐式定义了。而编译时库里恰好有这个函数，虽然不会出错，但会给出警告。从代码习惯上讲，所有函数都应该被明确定义，切记) 一般的DNS是基于UDP，报文格式如下图: </DIV>
<DIV style="MARGIN: 15px" align=center width="100%"><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080923204717.jpg" width=500 __1313585478953__="ev_9137600137">500)this.width=500;" border=0> 
<DIV style="TEXT-ALIGN: left">前面是固定的12byte首部，后面是4个长度可变的字段。 从首部开始，0~15位bit刚好是2个byte，由客户程序设置并由服务器返回，客户程序通过它来确定响应是否与查询匹配(例如，客户程序在这里输入的是十六进制的0xD8B4，那么服务器的该字段也会填入相同的值。这个标识又称为Transaction ID，在<A href="http://241072.blog.51cto.com/231072/54307" target=_blank>《DNS欺骗技术原理与安全防范技术》</A>中有更详细的讨论)。 </DIV></DIV>
<DIV style="MARGIN: 15px" width="100%"> 接下来的16~31位bit刚好也是2个byte，用作协议的标志位 </DIV>
<DIV style="MARGIN: 15px" align=center width="100%"><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080924151245.jpg" __1313585478953__="ev_7688679543">500)this.width=500;" border=0> 
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<UL>
<LI>QR是1个bit位:0代表查询报文，1代表相应报文 </LI>
<LI>opcode是4个bit位字段:0代表标准查询，1代表反向查询，2代表服务器状态请求 </LI>
<LI>AA是1个bit位，是Authoritative Answer的缩写，指明名字服务器是授权于该域的 </LI>
<LI>TC是1个bit位，是Truncated的缩写，意为可截断的，指明在UDP中应答报文超过512字节时，只返回512字节 </LI>
<LI>RD是1个bit位，是Recursion Desired的缩写，意为期望递归，期望名字服务器必须处理这个查询，而不是给出一个迭代查询服务器的列表 </LI>
<LI>RA是1个bit位，是Recursion Available的缩写，意为可用递归，如果名字服务器支持递归查询，这会将此位设置为1 </LI>
<LI>zero是3个bit位，设置为0 </LI>
<LI>rcode是4个bit位，表示名字差错，0为无差错，3为有差错。当查询中指定的域不存在的时候，就返回3</LI></UL>现在通过抓包来加深对上面的理解: <IMG src="http://blogimg.chinaunix.net/blog/upfile2/080924172043.gif" __1313585478953__="ev_9701976653"> </DIV></DIV>
<DIV style="MARGIN: 15px" width="100%">通过上图我们可以，二进制格式就为0000 0001 0000 0000，16位bit两个byte，其十六进制值为0x0100，这是一个标准的DNS查询请求的标志位 再接着是4段16位bit: </DIV>
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<UL>
<LI>QuestionCount 查询问题记录数由客户端填写，服务器端按原值返回 </LI>
<LI>AnswerCount 资源记录数由服务器端填写，代表有多少适应这个问题记录的对应IP </LI>
<LI>NameServerCount 授权资源记录数，一般为0 </LI>
<LI>AdditionalCount 额外资源记录数，一般为0</LI></UL></DIV>现在通过抓包来加深对上面的理解: 
<DIV style="TEXT-ALIGN: left"><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080924171026.gif" __1313585478953__="ev_6613115906">500)this.width=500;" border=0></DIV> 分析完报文头，现在该是报文体了。分为四大块: 
<UL>
<LI style="COLOR: rgb(0,128,255); FONT-WEIGHT: bold">查询问题 </LI>
<LI style="COLOR: rgb(0,128,255); FONT-WEIGHT: bold">回答 </LI>
<LI style="COLOR: rgb(0,128,255); FONT-WEIGHT: bold">授权 </LI>
<LI style="COLOR: rgb(0,128,255); FONT-WEIGHT: bold">额外信息</LI></UL>先看查询问题，它通常只有一个问题，当然也可以有多个问题，问题数由QuestionCount确定: 
<DIV style="TEXT-ALIGN: center"><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080924174238.gif" __1313585478953__="ev_9614616063"> </DIV>查询名是要查找的名字，它是一个或多个标识符的序列。每个标识符以首字节的计数值，来说明随后标识符的字节长度，每个名字以最后字节为0结束，长度为0的标识符是根标识符。计数字节的值必须是0~63的数，因为标识符的最大长度仅为63。该字段无需以整32bit边界结束，即无需填充字节。这种编码格式非常象BT协议中bencode编码，例如查询twistedmatrix.com: 
<DIV style="TEXT-ALIGN: center"><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080925172639.gif" __1313585478953__="ev_4322353105">500)this.width=500;" border=0> </DIV>第一位为计数位，从首字母到第一个.号，一共是13位(twistedmatrix)，然后是第二个计数位，值为3，由com计算得到，最后是结束符号0。 接着就是查询类型，该类型就是针对查询问题的，在RFC中有详细的描述，一般使用如下表:
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类型</TD>
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值</TD>
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描述</TD></TR>
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A</TD>
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1</TD>
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IP地址</TD></TR>
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NS</TD>
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2</TD>
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名字服务器</TD></TR>
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MD</TD>
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3</TD>
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邮件目的的(已过时，请用MX)</TD></TR>
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MF</TD>
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4</TD>
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邮件中转站(已过时，请用MX)</TD></TR>
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CNAME</TD>
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5</TD>
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规范名词</TD></TR>
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SOA</TD>
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6</TD>
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 xxx</TD></TR>
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MB</TD>
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7</TD>
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邮箱记录名(实验性质)</TD></TR>
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MG</TD>
<TD style="BORDER-BOTTOM: 1pt solid; BORDER-LEFT: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; WIDTH: 42.55pt; PADDING-RIGHT: 5.4pt; FONT-FAMILY: 黑体; BORDER-TOP: medium none; BORDER-RIGHT: 1pt solid; PADDING-TOP: 0cm" vAlign=top width=57>
8</TD>
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邮件组成员(实验性质)</TD></TR>
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MR</TD>
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9</TD>
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邮件更改后记录名(实验性质)</TD></TR>
<TR>
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NULL</TD>
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10</TD>
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空RR(实验性质)</TD></TR>
<TR>
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WKS</TD>
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11</TD>
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众所皆知的服务描述</TD></TR>
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PTR</TD>
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12</TD>
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指针记录</TD></TR>
<TR>
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HINFO</TD>
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13</TD>
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主机信息</TD></TR>
<TR>
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MINFO</TD>
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14</TD>
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邮箱或者邮件列表信息</TD></TR>
<TR>
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MX</TD>
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15</TD>
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邮件交换记录</TD></TR>
<TR>
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TXT</TD>
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16</TD>
<TD style="BORDER-BOTTOM: 1pt solid; BORDER-LEFT: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; WIDTH: 148.8pt; PADDING-RIGHT: 5.4pt; FONT-FAMILY: 黑体; BORDER-TOP: medium none; BORDER-RIGHT: 1pt solid; PADDING-TOP: 0cm" vAlign=top width=198>
文本字符串</TD></TR></TBODY></TABLE>最常用的查询类型为A，表示期望获得查询名对应的IP地址。最后的查询类，通常是1，指互联网地址 剩下的3个字段是:回答，授权和额外信息，均采用资源记录RR(Resource Record)格式，如下图: 
<DIV align=center>500)this.width=500;" border=0><IMG src="http://blogimg.chinaunix.net/blog/upfile2/080925194234.gif" width=500 __1313585478953__="ev_5370869917"></DIV>域名是记录中资源数据对应的名字。它的格式和前面介绍的查询名字段格式相同。类型说明RR的类型码。它的值和前面介绍的查询类型值是一样的。类通常为1，指Internet数据。生存时间字段是客户程序保留该资源记录的秒数。资源记录通常的生存时间值为2天。资源数据长度说明资源数据的数量。该数据的格式依赖于类型字段的值。对于类型1(A记录)资源数据是4字节的IP地址。 =============================================================================
<DIV style="MARGIN: 15px" width="100%">上文已提过，通常进行域名和IP地址的转换时，使用gethostbyname()和gethostbyaddr()(在W.Richard Stevens的《Unix Network Programming》第11章有详细的说明)。当仔细分析了DNS的协议后，我们自己动手来写写看。 下例执行的环境在Debian4.0上，编译工具为gcc，DNS服务器地址为192.168.1.1(通常该服务的默认监听端口为53)，文件名为DNSClient.c: 
<TABLE style="BORDER-BOTTOM: rgb(153,153,153) 1px solid; BORDER-LEFT: rgb(153,153,153) 1px solid; WIDTH: 80%; FONT-SIZE: 12px; BORDER-TOP: rgb(153,153,153) 1px solid; BORDER-RIGHT: rgb(153,153,153) 1px solid" align=center>
<TBODY>
<TR>
<TD style="BACKGROUND-COLOR: rgb(199,199,199)">
#include <stdio.h>
#include <stdlib.h>
#include <error.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
 
static void printmessage(unsigned char *buf);
static unsigned char *printnamestring(unsigned char *p,unsigned char *buf);
 
#define GETWORD(__w,__p) do{__w=*(__p++)<<8;__w|=*(p++);}while(0)
#define GETLONG(__l,__p) do{__l=*(__p++)<<24;__l|=*(__p++)<<16;__l|=*(__p++)<<8;__l|=*(p++);}while(0)
 
int main(int argc,char* argv[])
{
    if(argc != 2)
    {
       printf("usage: dnsclient <host_name>\n");
       return -1;
    }
 
    time_t ident;
    int fd;
    int rc;
    int serveraddrlent;
    char *q;
    unsigned char *p;
    unsigned char *countp;
    unsigned char reqBuf = {0};
    unsigned char rplBuf = {0};
    struct sockaddr_in serveraddr;
 
    //udp
    fd = socket(AF_INET, SOCK_DGRAM, 0);
    if(fd == -1)
    {
       perror("error create udp socket");
       return -1;
    }
    
    time(&ident);
    //copy
    p = reqBuf;
    //Transaction ID
    *(p++) = ident;
    *(p++) = ident>>8;
    //Header section
    //flag word = 0x0100
    *(p++) = 0x01;
    *(p++) = 0x00;
    //Questions = 0x0001
    //just one query
    *(p++) = 0x00;
    *(p++) = 0x01;
    //Answer RRs = 0x0000
    //no answers in this message
    *(p++) = 0x00;
    *(p++) = 0x00;
    //Authority RRs = 0x0000
    *(p++) = 0x00;
    *(p++) = 0x00;
    //Additional RRs = 0x0000
    *(p++) = 0x00;
    *(p++) = 0x00;
    //Query section
    countp = p;   
    *(p++) = 0;
    for(q=argv; *q!=0; q++)
    {
       if(*q != '.')
       {
           (*countp)++;
           *(p++) = *q;
       }
       else if(*countp != 0)
       {
           countp = p;
           *(p++) = 0;
       }
    }
    if(*countp != 0)
       *(p++) = 0;
 
    //Type=1(A):host address
    *(p++)=0;
    *(p++)=1;
    //Class=1(IN):internet
    *(p++)=0;
    *(p++)=1;
 
    printf("\nRequest:\n");
    printmessage(reqBuf);
 
    //fill 
    bzero(&serveraddr, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_port = htons(53);
    serveraddr.sin_addr.s_addr = inet_addr("192.168.1.1");
 
    //send to DNS Serv
    if(sendto(fd,reqBuf,p-reqBuf,0,(void *)&serveraddr,sizeof(serveraddr)) < 0)
    {
       perror("error sending request");
       return -1;
    }
 
    //recev the reply
    bzero(&serveraddr,sizeof(serveraddr));
    serveraddrlent = sizeof(serveraddr);
    rc = recvfrom(fd,&rplBuf,sizeof(rplBuf),0,(void *)&serveraddr,&serveraddrlent);
    if(rc < 0)
    {
       perror("error receiving request\n");
       return -1;
    }   
 
    //print out results
    printf("\nReply:\n");
    printmessage(rplBuf);
 
    //exit
    printf("Program Exit\n");
    return 0;  
}
 
static void printmessage(unsigned char *buf)
{
    unsigned char *p;
    unsigned int ident,flags,qdcount,ancount,nscount,arcount;
    unsigned int i,j,type,class,ttl,rdlength;
 
    p = buf;
    GETWORD(ident,p);
    printf("ident=%#x\n",ident);
 
    GETWORD(flags,p);
    printf("flags=%#x\n",flags);
    //printf("qr=%u\n",(flags>>15)&1);
    printf("qr=%u\n",flags>>15);
 
    printf("opcode=%u\n",(flags>>11)&15);
    printf("aa=%u\n",(flags>>10)&1);
    printf("tc=%u\n",(flags>>9)&1);
    printf("rd=%u\n",(flags>>8)&1);
    printf("ra=%u\n",(flags>>7)&1);
    printf("z=%u\n",(flags>>4)&7);
    printf("rcode=%u\n",flags&15);  
 
    GETWORD(qdcount,p);
    printf("qdcount=%u\n",qdcount);
 
    GETWORD(ancount,p);
    printf("ancount=%u\n",ancount);
 
    GETWORD(nscount,p);
    printf("nscount=%u\n",nscount);
 
    GETWORD(arcount,p);
    printf("arcount=%u\n",arcount);
 
    for(i=0; i<qdcount; i++)
    {
       printf("qd[%u]:\n",i);
       while(*p!=0)
       {
           p = printnamestring(p,buf);
           if(*p != 0)
              printf(".");
       }
       p++;
       printf("\n");
       GETWORD(type,p);
       printf("type=%u\n",type);
       GETWORD(class,p);
       printf("class=%u\n",class);
    }
 
    for(i=0; i<ancount; i++)
    {
       printf("an[%u]:\n",i);
       p = printnamestring(p,buf);
       printf("\n");
       GETWORD(type,p);
       printf("type=%u\n",type);
       GETWORD(class,p);
       printf("class=%u\n",class);
       GETLONG(ttl,p);
       printf("ttl=%u\n",ttl);
       GETWORD(rdlength,p);
       printf("rdlength=%u\n",rdlength);
       printf("rd=");
       for(j=0; j<rdlength; j++)
       {
           printf("%2.2x(%u)",*p,*p);
           p++;
       }
       printf("\n");
    }
}
 
static unsigned char *printnamestring(unsigned char *p,unsigned char *buf)
{
    unsigned int nchars,offset;
 
    nchars = *(p++);
    if((nchars & 0xc0) == 0xc0)
    {
       offset = (nchars & 0x3f) << 8;
       offset |= *(p++);
       nchars = buf;
       printf("%*.*s",nchars,nchars,buf+offset);
    }
    else
    {
       printf("%*.*s",nchars,nchars,p);
       p += nchars;
    }
 
    return (p);
}
 </TD></TR></TBODY></TABLE>编译命令为 lsj@debian007:~$ gcc -g -Wall -o DNSClient DNSClient.c 然后执行: lsj@debian007:~$ ./DNSClient bigdogchina.cublog.cn 就可以看见结果啦,这里使用的是UDP的数据格式，我们知道UDP的头部有一个16bit的长度，那么能表示的最大长度为2的16次方65536，再减去包头20，所以UDP包最大长度为65536-20=65516，但是在实际应用中，最好不要超过1K</DIV></DIV>

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