DNS Query Code in C with linux sockets

dns.c

//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';
}

Nice, but found two problems to report:

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

Nice. Thanks mahn!

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.

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

@arunmir, those lines deal with dns compression techniques. See http://www.tcpipguide.com/free/t_DNSNameNotationandMessageCompressionTechnique-2.htm

what is the purpose of the line 323-327

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

I' m think I' Wrong but...

@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

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

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

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