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#include "dns.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>
#include <fcntl.h>
struct header {
unsigned short id;
char a;
/*
int is_response: 1;
int opcode: 4;
int is_authanswer: 1;
int is_recursion_desired: 1;
*/
char b;
/*
int is_recursion_available: 1;
int: 3;
int response_code: 4;
*/
unsigned short questions;
unsigned short answers;
unsigned short authorities;
unsigned short additionals;
};
struct question_header {
unsigned short type;
unsigned short class;
};
struct resource_header {
unsigned short type;
unsigned short class;
unsigned int ttl;
unsigned short datalen;
};
static void fill_header(char **writer)
{
struct header *head = (struct header *) *writer;
memset(head, 0, sizeof(*head));
head->id = (unsigned short) htons(getpid());
head->a = 0b00000001;
head->questions = htons(1);
*writer += sizeof(*head);
}
static void fill_name(char **writer, const char *host)
{
char *last = *writer;
do {
if (*host == '.' || !*host) {
*last = *writer - last;
last = ++*writer;
} else {
*++*writer = *host;
}
} while (*host++);
*last = 0;
++*writer;
}
static void fill_question(char **writer, int query_type)
{
struct question_header *q = (struct question_header *) *writer;
q->type = htons(query_type);
q->class = htons(1);
*writer += sizeof(*q);
}
static int ask(const char *s, const char *h, int qt, struct sockaddr_in *a)
{
int sent;
int plen = sizeof(struct header) + strlen(h) + 2 +
sizeof(struct question_header);
char *packet = malloc(plen);
char *writer = packet;
int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
a->sin_family = AF_INET;
a->sin_port = htons(53);
a->sin_addr.s_addr = inet_addr(s);
fill_header(&writer);
fill_name(&writer, h);
fill_question(&writer, qt);
sent = sendto(sock, packet, plen, 0, (struct sockaddr *) a, sizeof(*a));
free(packet);
if (sent != plen) {
return 0;
}
return sock;
}
static void skip_questions(char **reader, int num)
{
for (; num; --num) {
*reader += strlen(*reader) + 1;
*reader += sizeof(struct question_header);
}
}
static char *read_name(char **extrareader, char *packet)
{
char *name = malloc(256); /* cannot be more =) */
char *reader = *extrareader;
char *writer = name;
int jumped = 0;
if (!*reader) {
*name = 0;
return name;
}
while (*reader) {
unsigned char len = *reader++;
if (len >= 192) {
int jump = ((unsigned char) *reader) + (len - 192) * 256;
if (!jumped) {
*extrareader += 2;
jumped = 1;
}
reader = packet + jump;
continue;
}
memcpy(writer, reader, len);
writer += len;
reader += len;
if (!jumped) {
*extrareader += len + 1;
}
*writer++ = '.';
}
if (!jumped) {
++*extrareader;
}
*--writer = 0;
return name;
}
static struct dns_answers *get_answers(char **reader, char *packet, int num)
{
struct dns_answers *res;
struct resource_header *head;
if (!num) {
return 0;
}
res = malloc(sizeof(*res));
res->host = read_name(reader, packet);
head = (struct resource_header *) *reader;
res->type = ntohs(head->type);
res->size = ntohs(head->datalen);
*reader += 10;
if (res->type == dns_type_cname) {
res->size--;
res->data = read_name(reader, packet);
} else if (res->type == dns_type_mx) {
char *name;
unsigned short pref = ntohs(*((unsigned short *) *reader));
*reader += 2;
name = read_name(reader, packet);
res->data = malloc(3 + strlen(name));
*((unsigned short *) res->data) = pref;
memcpy(res->data + 2, name, strlen(name) + 1);
free(name);
} else {
if (res->type == dns_type_txt) {
res->size--;
++*reader;
}
res->data = malloc(res->size);
memcpy(res->data, *reader, res->size);
*reader += res->size;
}
res->next = get_answers(reader, packet, num - 1);
return res;
}
static struct dns_answers *get(int sock, struct sockaddr_in *a)
{
char packet[512];
char *reader = packet;
struct header *head;
socklen_t slen = sizeof(*a);
if (!sock) {
return 0;
}
if (recvfrom(sock, packet, 512, 0, (struct sockaddr *) a, &slen) < 0) {
return 0;
}
head = (struct header *) reader;
head->questions = ntohs(head->questions);
head->answers = ntohs(head->answers);
reader += sizeof(*head);
skip_questions(&reader, head->questions);
return get_answers(&reader, packet, head->answers);
}
int dns_mx_preference(void *data)
{
return *((unsigned short *) data);
}
char *dns_mx_server(void *data)
{
return ((char *) data) + 2;
}
struct dns_answers *dns_get(const char *server, const char *host, int query)
{
struct sockaddr_in addr;
return get(ask(server, host, query, &addr), &addr);
}
void dns_free(struct dns_answers *answers)
{
if (answers) {
dns_free(answers->next);
free(answers->host);
free(answers->data);
free(answers);
}
}
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