DNS Query Code in C with linux sockets

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The Linux version has some changes. On Linux the dns server ips are stored in a file called /etc/resolv.conf.
So the get_dns_servers function will open this file and pickup the dns server ip addresses.

A typical /etc/resolv.conf file can look like this :

# Generated by NetworkManagernameserver 208.67.222.222nameserver 208.67.220.220

So the lines starting with nameserver can be picked up and broken into parts using strtok.

Rest of the code is very much similar to the winsock version. A udp socket is used to send a UDP packet and the response is analysed.

Code :

//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
chardns_servers[10][100];
intdns_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
voidngethostbyname (unsigned char* , int);
voidChangetoDnsNameFormat (unsigned char*,unsignedchar*);
unsignedchar* ReadName (unsigned char*,unsignedchar*,int*);
voidget_dns_servers();
 
//DNS header structure
structDNS_HEADER
{
    unsignedshortid; // identification number
 
    unsignedcharrd :1; // recursion desired
    unsignedchartc :1; // truncated message
    unsignedcharaa :1; // authoritive answer
    unsignedcharopcode :4; // purpose of message
    unsignedcharqr :1; // query/response flag
 
    unsignedcharrcode :4; // response code
    unsignedcharcd :1; // checking disabled
    unsignedcharad :1; // authenticated data
    unsignedcharz :1; // its z! reserved
    unsignedcharra :1; // recursion available
 
    unsignedshortq_count; // number of question entries
    unsignedshortans_count; // number of answer entries
    unsignedshortauth_count; // number of authority entries
    unsignedshortadd_count; // number of resource entries
};
 
//Constant sized fields of query structure
structQUESTION
{
    unsignedshortqtype;
    unsignedshortqclass;
};
 
//Constant sized fields of the resource record structure
#pragma pack(push, 1)
structR_DATA
{
    unsignedshorttype;
    unsignedshort_class;
    unsignedintttl;
    unsignedshortdata_len;
};
#pragma pack(pop)
 
//Pointers to resource record contents
structRES_RECORD
{
    unsignedchar*name;
    structR_DATA *resource;
    unsignedchar*rdata;
};
 
//Structure of a Query
typedefstruct
{
    unsignedchar*name;
    structQUESTION *ques;
} QUERY;
 
intmain( intargc , char*argv[])
{
    unsignedcharhostname[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);
 
    return0;
}
 
/*
 * Perform a DNS query by sending a packet
 * */
voidngethostbyname(unsigned char*host , intquery_type)
{
    unsignedcharbuf[65536],*qname,*reader;
    inti , j , stop , s;
 
    structsockaddr_in a;
 
    structRES_RECORD answers[20],auth[20],addit[20]; //the replies from the DNS server
    structsockaddr_in dest;
 
    structDNS_HEADER *dns = NULL;
    structQUESTION *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 = (structDNS_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(structDNS_HEADER)];
 
    ChangetoDnsNameFormat(qname , host);
    qinfo =(structQUESTION*)&buf[sizeof(structDNS_HEADER) + (strlen((constchar*)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(structDNS_HEADER) + (strlen((constchar*)qname)+1) + sizeof(structQUESTION),0,(structsockaddr*)&dest,sizeof(dest)) < 0)
    {
        perror("sendto failed");
    }
    printf("Done");
     
    //Receive the answer
    i = sizeofdest;
    printf("\nReceiving answer...");
    if(recvfrom (s,(char*)buf , 65536 , 0 , (structsockaddr*)&dest , (socklen_t*)&i ) < 0)
    {
        perror("recvfrom failed");
    }
    printf("Done");
 
    dns = (structDNS_HEADER*) buf;
 
    //move ahead of the dns header and the query field
    reader = &buf[sizeof(structDNS_HEADER) + (strlen((constchar*)qname)+1) + sizeof(structQUESTION)];
 
    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 = (structR_DATA*)(reader);
        reader = reader + sizeof(structR_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=(structR_DATA*)(reader);
        reader+=sizeof(structR_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=(structR_DATA*)(reader);
        reader+=sizeof(structR_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)
{
    unsignedchar*name;
    unsignedintp=0,jumped=0,offset;
    inti , 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((constchar*)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
    returnname;
}
 
/*
 * Get the DNS servers from /etc/resolv.conf file on Linux
 * */
voidget_dns_servers()
{
    FILE*fp;
    charline[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 :)
 * */
voidChangetoDnsNameFormat(unsigned char* dns,unsigned char* host) 
{
    intlock = 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';
}
Compile
$ gcc dns.cOutput
$ ./a.out Enter Hostname to Lookup : www.google.comResolving www.google.comSending Packet...DoneReceiving answer...DoneThe response contains :  1 Questions. 6 Answers. 4 Authoritative Servers. 4 Additional records.Answer Records : 6 Name : www.google.com has alias name : www.l.google.comName : www.l.google.com has IPv4 address : 74.125.235.16Name : www.l.google.com has IPv4 address : 74.125.235.17Name : www.l.google.com has IPv4 address : 74.125.235.18Name : www.l.google.com has IPv4 address : 74.125.235.19Name : www.l.google.com has IPv4 address : 74.125.235.20Authoritive Records : 4 Name : google.com has nameserver : ns2.google.comName : google.com has nameserver : ns1.google.comName : google.com has nameserver : ns3.google.comName : google.com has nameserver : ns4.google.comAdditional Records : 4 Name : ns1.google.com has IPv4 address : 216.239.32.10Name : ns2.google.com has IPv4 address : 216.239.34.10Name : ns3.google.com has IPv4 address : 216.239.36.10Name : ns4.google.com has IPv4 address : 216.239.38.10The same code can be used to fetch different kinds of dns records like Nameserver , MX , CNAME , SOA etc.

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