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 NetworkManager nameserver 208.67.222.222 nameserver 208.67.220.220So 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 :
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 //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' ; }Compile
$ gcc dns.cOutput
$ ./a.out Enter Hostname to Lookup : www.google.com Resolving www.google.com Sending Packet...Done Receiving answer...Done The 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.com Name : www.l.google.com has IPv4 address : 74.125.235.16 Name : www.l.google.com has IPv4 address : 74.125.235.17 Name : www.l.google.com has IPv4 address : 74.125.235.18 Name : www.l.google.com has IPv4 address : 74.125.235.19 Name : www.l.google.com has IPv4 address : 74.125.235.20 Authoritive Records : 4 Name : google.com has nameserver : ns2.google.com Name : google.com has nameserver : ns1.google.com Name : google.com has nameserver : ns3.google.com Name : google.com has nameserver : ns4.google.com Additional Records : 4 Name : ns1.google.com has IPv4 address : 216.239.32.10 Name : ns2.google.com has IPv4 address : 216.239.34.10 Name : ns3.google.com has IPv4 address : 216.239.36.10 Name : 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.
