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DNS Query Code in C with linux sockets
//DNS Query Program on Linux
//Author : Silver Moon (m00n.silv3r@gmail.com)
//Dated : 29/4/2009
//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';
}
@hiqhan

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@hiqhan hiqhan commented Jun 5, 2018

Nice, but found two problems to report:

  1. Should use free() after using malloc();
  2. line 402: “strcat((char*)host,".");” -- it would overflow here.
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@dmutende dmutende commented Jul 2, 2018

Nice. Thanks mahn!

@morenofloris

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@morenofloris morenofloris commented Aug 27, 2018

Thank you for this code.

On line 197 you allocate N bytes for answer[i].rdata and then on line 204 you write on the Nth byte of answer[i].rdata which is not allocated.
This works on PC linux but the code crashes on other platforms.

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@arunmir arunmir commented Oct 3, 2018

Can you explain the purpose of line 323-328 please?

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@bmulkey bmulkey commented Oct 16, 2018

@markpairdha

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@markpairdha markpairdha commented Mar 17, 2019

what is the purpose of the line 323-327

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@zoeurk zoeurk commented Jul 28, 2020

maybe you have to add:
if ( (int)(offset - 1)>datalen){
reader++;
continue;
}
else your program can segfault
->datalen is the size of your buffer.
I Have try on TXT record and this thing seems to work

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@zoeurk zoeurk commented Jul 29, 2020

I' m think I' Wrong but...

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@teggytan teggytan commented Jul 31, 2020

@zoeurk How did you do with TXT record, there are no TXT mod ?

Is this available for Arduino, I want to use this on a card

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@zoeurk zoeurk commented Jul 31, 2020

I do a small sniffer with the raw socket (https://github.com/zoeurk/sniffer) and I see this code.
I supose that they are problem in this code because offset is sometime outside the buffer. (but I 'm not sure).
If you want to see how I can read buffer with this code, see in dns.c and dns.h

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@zoeurk zoeurk commented Aug 25, 2020

I found the problem: your code don't understand the difference between len = 210 (for exemple: in binary: 11010010) and a pointer...

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@22shubh22 22shubh22 commented May 11, 2021

How can we use this code such that it servers AAAA Ipv6 queries.

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