dns.c
//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