linux中查询dns示例

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dns.c

/*
* DNS Query Program on Linux
*
* Author : ismdeep@live.com
*
* */
//Header Files
#include<stdio.h> //printf
#include<string.h> //strlen
#include<stdlib.h> //malloc
#include<sys/socket.h> //you know what this is for
#include<arpa/inet.h> //inet_addr , inet_ntoa , ntohs etc
#include<netinet/in.h>
#include<unistd.h> //getpid

//List of DNS Servers registered on the system
char dns_servers[10][100];
int dns_server_count = 0;
//Types of DNS resource records :)

#define T_A 1 //Ipv4 address
#define T_NS 2 //Nameserver
#define T_CNAME 5 // canonical name
#define T_SOA 6 /* start of authority zone */
#define T_PTR 12 /* domain name pointer */
#define T_MX 15 //Mail server

//Function Prototypes
void ngethostbyname (unsigned char* , int);
void ChangetoDnsNameFormat (unsigned char*,unsigned char*);
unsigned char* ReadName (unsigned char*,unsigned char*,int*);
void get_dns_servers();

//DNS header structure
struct DNS_HEADER
{
unsigned short id; // identification number

unsigned char rd :1; // recursion desired
unsigned char tc :1; // truncated message
unsigned char aa :1; // authoritive answer
unsigned char opcode :4; // purpose of message
unsigned char qr :1; // query/response flag

unsigned char rcode :4; // response code
unsigned char cd :1; // checking disabled
unsigned char ad :1; // authenticated data
unsigned char z :1; // its z! reserved
unsigned char ra :1; // recursion available

unsigned short q_count; // number of question entries
unsigned short ans_count; // number of answer entries
unsigned short auth_count; // number of authority entries
unsigned short add_count; // number of resource entries
};

//Constant sized fields of query structure
struct QUESTION
{
unsigned short qtype;
unsigned short qclass;
};

//Constant sized fields of the resource record structure
#pragma pack(push, 1)
struct R_DATA
{
unsigned short type;
unsigned short _class;
unsigned int ttl;
unsigned short data_len;
};
#pragma pack(pop)

//Pointers to resource record contents
struct RES_RECORD
{
unsigned char *name;
struct R_DATA *resource;
unsigned char *rdata;
};

//Structure of a Query
typedef struct
{
unsigned char *name;
struct QUESTION *ques;
} QUERY;

int main( int argc , char *argv[])
{
unsigned char hostname[100];

//Get the DNS servers from the resolv.conf file
get_dns_servers();

//Get the hostname from the terminal
printf("Enter Hostname to Lookup : ");
scanf("%s" , hostname);

//Now get the ip of this hostname , A record
ngethostbyname(hostname , T_A);

return 0;
}

/*
* Perform a DNS query by sending a packet
* */
void ngethostbyname(unsigned char *host , int query_type)
{
unsigned char buf[65536],*qname,*reader;
int i , j , stop , s;

struct sockaddr_in a;

struct RES_RECORD answers[20],auth[20],addit[20]; //the replies from the DNS server
struct sockaddr_in dest;

struct DNS_HEADER *dns = NULL;
struct QUESTION *qinfo = NULL;

printf("Resolving %s" , host);

s = socket(AF_INET , SOCK_DGRAM , IPPROTO_UDP); //UDP packet for DNS queries

dest.sin_family = AF_INET;
dest.sin_port = htons(53);
dest.sin_addr.s_addr = inet_addr(dns_servers[0]); //dns servers

//Set the DNS structure to standard queries
dns = (struct DNS_HEADER *)&buf;

dns->id = (unsigned short) htons(getpid());
dns->qr = 0; //This is a query
dns->opcode = 0; //This is a standard query
dns->aa = 0; //Not Authoritative
dns->tc = 0; //This message is not truncated
dns->rd = 1; //Recursion Desired
dns->ra = 0; //Recursion not available! hey we dont have it (lol)
dns->z = 0;
dns->ad = 0;
dns->cd = 0;
dns->rcode = 0;
dns->q_count = htons(1); //we have only 1 question
dns->ans_count = 0;
dns->auth_count = 0;
dns->add_count = 0;

//point to the query portion
qname =(unsigned char*)&buf[sizeof(struct DNS_HEADER)];

ChangetoDnsNameFormat(qname , host);
qinfo =(struct QUESTION*)&buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname) + 1)]; //fill it

qinfo->qtype = htons( query_type ); //type of the query , A , MX , CNAME , NS etc
qinfo->qclass = htons(1); //its internet (lol)

printf("\nSending Packet...");
if( sendto(s,(char*)buf,sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION),0,(struct sockaddr*)&dest,sizeof(dest)) < 0)
{
perror("sendto failed");
}
printf("Done");

//Receive the answer
i = sizeof dest;
printf("\nReceiving answer...");
if(recvfrom (s,(char*)buf , 65536 , 0 , (struct sockaddr*)&dest , (socklen_t*)&i ) < 0)
{
perror("recvfrom failed");
}
printf("Done");

dns = (struct DNS_HEADER*) buf;

//move ahead of the dns header and the query field
reader = &buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION)];

printf("\nThe response contains : ");
printf("\n %d Questions.",ntohs(dns->q_count));
printf("\n %d Answers.",ntohs(dns->ans_count));
printf("\n %d Authoritative Servers.",ntohs(dns->auth_count));
printf("\n %d Additional records.\n\n",ntohs(dns->add_count));

//Start reading answers
stop=0;

for(i=0;i<ntohs(dns->ans_count);i++)
{
answers[i].name=ReadName(reader,buf,&stop);
reader = reader + stop;

answers[i].resource = (struct R_DATA*)(reader);
reader = reader + sizeof(struct R_DATA);

  if(ntohs(answers[i].resource->type) == 1) //if its an ipv4 address
  {
   answers[i].rdata = (unsigned char*)malloc(ntohs(answers[i].resource->data_len));

   for(j=0 ; j<ntohs(answers[i].resource->data_len) ; j++)
   {
    answers[i].rdata[j]=reader[j];
   }

answers[i].rdata[ntohs(answers[i].resource->data_len)] = '\0';

   reader = reader + ntohs(answers[i].resource->data_len);
  }
  else
  {
   answers[i].rdata = ReadName(reader,buf,&stop);
   reader = reader + stop;
  }
}

//read authorities
for(i=0;i<ntohs(dns->auth_count);i++)
{
auth[i].name=ReadName(reader,buf,&stop);
reader+=stop;

auth[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);

auth[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}

//read additional
for(i=0;i<ntohs(dns->add_count);i++)
{
addit[i].name=ReadName(reader,buf,&stop);
reader+=stop;

addit[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);

  if(ntohs(addit[i].resource->type)==1)
  {
   addit[i].rdata = (unsigned char*)malloc(ntohs(addit[i].resource->data_len));
   for(j=0;j<ntohs(addit[i].resource->data_len);j++)
   addit[i].rdata[j]=reader[j];

   addit[i].rdata[ntohs(addit[i].resource->data_len)]='\0';
   reader+=ntohs(addit[i].resource->data_len);
  }
  else
  {
   addit[i].rdata=ReadName(reader,buf,&stop);
   reader+=stop;
  }
}

//print answers
printf("\nAnswer Records : %d \n" , ntohs(dns->ans_count) );
for(i=0 ; i < ntohs(dns->ans_count) ; i++)
{
printf("Name : %s ",answers[i].name);

  if( ntohs(answers[i].resource->type) == T_A) //IPv4 address
  {
   long *p;
   p=(long*)answers[i].rdata;
   a.sin_addr.s_addr=(*p); //working without ntohl
   printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
  }

  if(ntohs(answers[i].resource->type)==5)
  {
   //Canonical name for an alias
   printf("has alias name : %s",answers[i].rdata);
  }

printf("\n");
}

//print authorities
printf("\nAuthoritive Records : %d \n" , ntohs(dns->auth_count) );
for( i=0 ; i < ntohs(dns->auth_count) ; i++)
{

  printf("Name : %s ",auth[i].name);
  if(ntohs(auth[i].resource->type)==2)
  {
   printf("has nameserver : %s",auth[i].rdata);
  }
  printf("\n");
}

//print additional resource records
printf("\nAdditional Records : %d \n" , ntohs(dns->add_count) );
for(i=0; i < ntohs(dns->add_count) ; i++)
{
  printf("Name : %s ",addit[i].name);
  if(ntohs(addit[i].resource->type)==1)
  {
   long *p;
   p=(long*)addit[i].rdata;
   a.sin_addr.s_addr=(*p);
   printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
  }
  printf("\n");
}
return;
}

/*
*
* */
u_char* ReadName(unsigned char* reader,unsigned char* buffer,int* count)
{
unsigned char *name;
unsigned int p=0,jumped=0,offset;
int i , j;

*count = 1;
name = (unsigned char*)malloc(256);

name[0]='\0';

//read the names in 3www6google3com format
while(*reader!=0)
{
  if(*reader>=192)
  {
   offset = (*reader)*256 + *(reader+1) - 49152; //49152 = 11000000 00000000 ;)
   reader = buffer + offset - 1;
   jumped = 1; //we have jumped to another location so counting wont go up!
  }
  else
  {
   name[p++]=*reader;
  }

reader = reader+1;

  if(jumped==0)
  {
   *count = *count + 1; //if we havent jumped to another location then we can count up
  }
}

name[p]='\0'; //string complete
if(jumped==1)
{
*count = *count + 1; //number of steps we actually moved forward in the packet
}

//now convert 3www6google3com0 to www.google.com
for(i=0;i<(int)strlen((const char*)name);i++)
{
  p=name[i];
  for(j=0;j<(int)p;j++)
  {
   name[i]=name[i+1];
   i=i+1;
  }
  name[i]='.';
}
name[i-1]='\0'; //remove the last dot
return name;
}

/*
* Get the DNS servers from /etc/resolv.conf file on Linux
* */
void get_dns_servers()
{
FILE *fp;
char line[200] , *p;
if((fp = fopen("/etc/resolv.conf" , "r")) == NULL)
{
  printf("Failed opening /etc/resolv.conf file \n");
}

while(fgets(line , 200 , fp))
{
  if(line[0] == '#')
  {
   continue;
  }
  if(strncmp(line , "nameserver" , 10) == 0)
  {
   p = strtok(line , " ");
   p = strtok(NULL , " ");

   //p now is the dns ip :)
   //????
  }
}

strcpy(dns_servers[0] , "208.67.222.222");
strcpy(dns_servers[1] , "208.67.220.220");
}

/*
* This will convert www.google.com to 3www6google3com
* got it :)
* */
void ChangetoDnsNameFormat(unsigned char* dns,unsigned char* host)
{
int lock = 0 , i;
strcat((char*)host,".");

for(i = 0 ; i < strlen((char*)host) ; i++)
{
  if(host[i]=='.')
  {
   *dns++ = i-lock;
   for(;lock<i;lock++)
   {
    *dns++=host[lock];
   }
   lock++; //or lock=i+1;
  }
}
*dns++='\0';
}

dns.c

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