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DNS Lookup - Implementation in C
/**dnsclient.c**/
#include"stdio.h"
#include"stdlib.h"
#include"sys/types.h"
#include"sys/socket.h"
#include"string.h"
#include"netinet/in.h"
#include"netdb.h"
#include"arpa/inet.h"
#define BUF_SIZE 512
#define SERVER "8.8.8.8"
#define PORT 53
#define WIDTH 16
int createSocket();
void createRequest(char * url);
void hexdump (char *desc, void *addr, int len);
void lookUp();
struct query {
uint16_t length;
char * url;
unsigned char request[BUF_SIZE];
uint16_t reqType;
};
struct query dnsQuery = {
.length = 12,
.url = "",
.reqType = 0x01,
.request = { 0xDB, 0x42, 0x01, 0x00, 0x00, 0x01,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }
};
int main(int argc, char** argv) {
char * url;
if (argc != 2) {
fprintf(stderr,"usage: %s hostnameToResolve\n", argv[0]);
exit(1);
}
url = argv[1];
createRequest(url);
lookUp();
}
int createSocket() {
int sockfd;
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
if (sockfd < 0) {
printf("Error creating socket!\n");
exit(1);
}
//printf("Socket created...\n");
return sockfd;
}
void createRequest(char * url) {
char * word;
int i;
printf("Asking DNS server %s about %s\n", SERVER, url);
dnsQuery.url = strdup(url);
dnsQuery.reqType = 0x01;
word = strtok(url, ".");
while (word) {
printf("parsing hostname: \"%s\" is %d characters\n", word,
strlen(word));
dnsQuery.request[dnsQuery.length++] = strlen(word);
for (i = 0; i < strlen(word); i++) {
dnsQuery.request[dnsQuery.length++] = word[i];
}
word = strtok(NULL, ".");
}
dnsQuery.request[dnsQuery.length++] = 0x00; // End of the host name
dnsQuery.request[dnsQuery.length++] = 0x00; // 0x0001 - Query is a Type A
// query (host address)
dnsQuery.request[dnsQuery.length++] = dnsQuery.reqType;
dnsQuery.request[dnsQuery.length++] = 0x00; // 0x0001 - Query is class IN
// (Internet address)
dnsQuery.request[dnsQuery.length++] = 0x01;
}
// http://stackoverflow.com/questions/7775991/how-to-get-hexdump-of-a-structure-data
void hexdump (char *desc, void *addr, int len) {
int i;
unsigned char buff[17];
unsigned char *pc = addr;
// Output description if given.
if (desc != NULL)
printf ("%s:\n", desc);
// Process every byte in the data.
for (i = 0; i < len; i++) {
// Multiple of 16 means new line (with line offset).
if ((i % 16) == 0) {
// Just don't print ASCII for the zeroth line.
if (i != 0)
printf (" %s\n", buff);
// Output the offset.
printf (" %04x ", i);
}
// Now the hex code for the specific character.
printf (" %02x", pc[i]);
// And store a printable ASCII character for later.
if ((pc[i] < 0x20) || (pc[i] > 0x7e))
buff[i % 16] = '.';
else
buff[i % 16] = pc[i];
buff[(i % 16) + 1] = '\0';
}
// Pad out last line if not exactly 16 characters.
while ((i % 16) != 0) {
printf (" ");
i++;
}
// And print the final ASCII bit.
printf (" %s\n", buff);
}
void lookUp() {
struct sockaddr_in addr;
int socket;
int ret, rcode, size, i;
int ip = 0, dom = 0;
int length;
unsigned char buffer[BUF_SIZE];
unsigned char tempBuf[3];
uint16_t QDCOUNT; //No. of items in Question Section
uint16_t ANCOUNT; //No. of items in Answer Section
uint16_t NSCOUNT; //No. of items in Authority Section
uint16_t ARCOUNT; //No. of items in Additional Section
uint16_t QCLASS; //Specifies the class of the query
uint16_t ATYPE; //Specifies the meaning of the data in the RDATA field
uint16_t ACLASS; //Specifies the class of the data in the RDATA field
uint32_t TTL; //The number of seconds the results can be cached
uint16_t RDLENGTH; //The length of the RDATA field
uint16_t MSGID;
socket = createSocket();
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(SERVER);
addr.sin_port = htons(PORT);
size = sizeof(addr);
hexdump("sending packet", &dnsQuery.request, dnsQuery.length);
ret = sendto(socket, dnsQuery.request, dnsQuery.length, 0,
(struct sockaddr*)&addr, size);
if (ret < 0) {
printf("Error Sending Request");
exit(1);
}
//printf("Sent\n");
memset(&buffer, 0, BUF_SIZE);
ret = recvfrom(socket, buffer, BUF_SIZE, 0, (struct sockaddr*)&addr,
&size);
if (ret < 0) {
printf("Error Receiving Response");
exit(1);
}
hexdump("received packet", &buffer, ret);
close(socket);
rcode = (buffer[3] & 0x0F);
//tempBuf[0] = buffer[4];
//tempBuf[1] = buffer[5];
//tempBuf[2] = '\0';
//printf("%0x %0x %0x %0x\n", buffer[4], buffer[5], tempBuf[0], tempBuf[1]);
//QDCOUNT = (uint16_t) strtol(tempBuf, NULL, 16);
QDCOUNT = (uint16_t) buffer[4] * 0x100 + buffer[5];
printf("entries in question section: %u\n", QDCOUNT);
ANCOUNT = (uint16_t) buffer[6] * 0x100 + buffer[7];
printf("records in answer section: %u\n", ANCOUNT);
NSCOUNT = (uint16_t) buffer[8] * 0x100 + buffer[9];
printf("name server resource record count: %u\n", NSCOUNT);
ARCOUNT = (uint16_t) buffer[10] * 0x100 + buffer[11];
printf("additional records count: %u\n", ARCOUNT);
printf("query type: %u\n", dnsQuery.reqType);
QCLASS = (uint16_t) dnsQuery.request[dnsQuery.length - 2] * 0x100 +
dnsQuery.request[dnsQuery.length - 1];
printf("query class: %u\n", QCLASS);
length = dnsQuery.length + 1; // to skip 0xc00c
ATYPE = (uint16_t) buffer[length + 1] * 0x100 + buffer[length + 2];
printf("answer type: %u\n", ATYPE);
ACLASS = (uint16_t) buffer[length + 3] * 0x100 + buffer[length + 4];
printf("answer class: %u\n", ACLASS);
TTL = (uint32_t) buffer[length + 5] * 0x1000000 + buffer[length + 6] *
0x10000 + buffer[length + 7] * 0x100 + buffer[length + 8];
printf("seconds to cache: %u\n", TTL);
RDLENGTH = (uint16_t) buffer[length + 9] * 0x100 + buffer[length + 10];
printf("bytes in answer: %u\n", RDLENGTH);
MSGID = (uint16_t) buffer[0] * 0x100 + buffer[1];
printf("answer msg id: %u\n", MSGID);
if (rcode == 2) {
printf("nameserver %s returned SERVFAIL:\n", SERVER);
printf(" the name server was unable to process this query due to"
+ " a\n problem with the name server.\n");
exit(1);
} else if (rcode == 3) {
printf("nameserver %s returned NXDOMAIN for %s:\n", SERVER,
dnsQuery.url);
printf(" the domain name referenced in the query does not exist\n");
exit(1);
}
/* search for and print IPv4 addresses */
if (dnsQuery.reqType == 0x01) {
printf("DNS server's answer is: (type#=%u):", ATYPE);
//printf("IPv4 address(es) for %s:\n", dnsQuery.url);
for (i = 0 ; i < ret ; i++) {
if (buffer[i] == 0xC0 && buffer[i+3] == 0x01) {
ip++; i += 12; /* ! += buf[i+1]; */
printf(" %u.%u.%u.%u\n", buffer[i], buffer[i+1],
buffer[i+2], buffer[i+3]);
}
}
if (!ip) {
printf(" No IPv4 address found in the DNS response!\n");
exit(1);
}
}
}
Output
dhanoopbhaskar@dhanoop-laptop:~/workspace/Copy/_works$ gcc dnsclient.c
dhanoopbhaskar@dhanoop-laptop:~/workspace/Copy/_works$ ./a.out www.theinsanetechie.in
Asking DNS server 8.8.8.8 about www.theinsanetechie.in
parsing hostname: "www" is 3 characters
parsing hostname: "theinsanetechie" is 15 characters
parsing hostname: "in" is 2 characters
sending packet:
0000 db 42 01 00 00 01 00 00 00 00 00 00 03 77 77 77 .B...........www
0010 0f 74 68 65 69 6e 73 61 6e 65 74 65 63 68 69 65 .theinsanetechie
0020 02 69 6e 00 00 01 00 01 .in.....
received packet:
0000 db 42 81 80 00 01 00 03 00 00 00 00 03 77 77 77 .B...........www
0010 0f 74 68 65 69 6e 73 61 6e 65 74 65 63 68 69 65 .theinsanetechie
0020 02 69 6e 00 00 01 00 01 c0 0c 00 05 00 01 00 00 .in.............
0030 32 c7 00 10 03 67 68 73 06 67 6f 6f 67 6c 65 03 2....ghs.google.
0040 63 6f 6d 00 c0 34 00 05 00 01 00 00 54 5f 00 08 com..4......T_..
0050 03 67 68 73 01 6c c0 38 c0 50 00 01 00 01 00 00 .ghs.l.8.P......
0060 01 2b 00 04 4a 7d 44 79 .+..J}Dy
entries in question section: 1
records in answer section: 3
name server resource record count: 0
additional records count: 0
query type: 1
query class: 1
answer type: 5
answer class: 1
seconds to cache: 12999
bytes in answer: 16
answer msg id: 56130
DNS server's answer is: (type#=5): 74.125.68.121
dhanoopbhaskar@dhanoop-laptop:~/workspace/Copy/_works$
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