dns.c

/*
 * DNS Support for Asterisk
 *
 * Written by Thorsten Lockert <tholo@trollphone.org>
 *
 * Funding provided by Troll Phone Networks AS
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <unistd.h>

#include <asterisk/logger.h>
#include <asterisk/channel.h>
#include <asterisk/dns.h>

#define MAX_SIZE 4096

typedef struct {
	unsigned	id :16;		/* query identification number */
#if BYTE_ORDER == BIG_ENDIAN
			/* fields in third byte */
	unsigned	qr: 1;		/* response flag */
	unsigned	opcode: 4;	/* purpose of message */
	unsigned	aa: 1;		/* authoritive answer */
	unsigned	tc: 1;		/* truncated message */
	unsigned	rd: 1;		/* recursion desired */
			/* fields in fourth byte */
	unsigned	ra: 1;		/* recursion available */
	unsigned	unused :1;	/* unused bits (MBZ as of 4.9.3a3) */
	unsigned	ad: 1;		/* authentic data from named */
	unsigned	cd: 1;		/* checking disabled by resolver */
	unsigned	rcode :4;	/* response code */
#endif
#if BYTE_ORDER == LITTLE_ENDIAN || BYTE_ORDER == PDP_ENDIAN
			/* fields in third byte */
	unsigned	rd :1;		/* recursion desired */
	unsigned	tc :1;		/* truncated message */
	unsigned	aa :1;		/* authoritive answer */
	unsigned	opcode :4;	/* purpose of message */
	unsigned	qr :1;		/* response flag */
			/* fields in fourth byte */
	unsigned	rcode :4;	/* response code */
	unsigned	cd: 1;		/* checking disabled by resolver */
	unsigned	ad: 1;		/* authentic data from named */
	unsigned	unused :1;	/* unused bits (MBZ as of 4.9.3a3) */
	unsigned	ra :1;		/* recursion available */
#endif
			/* remaining bytes */
	unsigned	qdcount :16;	/* number of question entries */
	unsigned	ancount :16;	/* number of answer entries */
	unsigned	nscount :16;	/* number of authority entries */
	unsigned	arcount :16;	/* number of resource entries */
} dns_HEADER;

struct dn_answer {
	unsigned short rtype;
	unsigned short class;
	unsigned int ttl;
	unsigned short size;
} __attribute__ ((__packed__));

static int skip_name(u_char *s, int len)
{
	int x = 0;

	while (x < len) {
		if (*s == '\0') {
			s++;
			x++;
			break;
		}
		if ((*s & 0xc0) == 0xc0) {
			s += 2;
			x += 2;
			break;
		}
		x += *s + 1;
		s += *s + 1;
	}
	if (x >= len)
		return -1;
	return x;
}

static int dns_parse_answer(void *context,
							int class, int type, u_char *answer, int len,
							int (*callback)(void *context, u_char *answer, int len, u_char *fullanswer))
{
	u_char *fullanswer = answer;
	struct dn_answer *ans;
	dns_HEADER *h;
	int res;
	int x;

    h = (dns_HEADER *)answer;
	answer += sizeof(dns_HEADER);
	len -= sizeof(dns_HEADER);

	for (x = 0; x < ntohs(h->qdcount); x++) {
		if ((res = skip_name(answer, len)) < 0) {
			ast_log(LOG_WARNING, "Couldn't skip over name\n");
			return -1;
		}
		answer += res + 4;	/* Skip name and QCODE / QCLASS */
		len -= res + 4;
		if (len < 0) {
			ast_log(LOG_WARNING, "Strange query size\n");
			return -1;
		}
	}

	for (x = 0; x < ntohs(h->ancount); x++) {
		if ((res = skip_name(answer, len)) < 0) {
			ast_log(LOG_WARNING, "Failed skipping name\n");
			return -1;
		}
		answer += res;
		len -= res;
		ans = (struct dn_answer *)answer;
		answer += sizeof(struct dn_answer);
		len -= sizeof(struct dn_answer);
		if (len < 0) {
			ast_log(LOG_WARNING, "Strange result size\n");
			return -1;
		}
		if (len < 0) {
			ast_log(LOG_WARNING, "Length exceeds frame\n");
			return -1;
		}

		if (ntohs(ans->class) == class && ntohs(ans->rtype) == type) {
			if (callback) {
				if ((res = callback(context, answer, ntohs(ans->size), fullanswer)) < 0) {
					ast_log(LOG_WARNING, "Failed to parse result\n");
					return -1;
				}
				if (res > 0)
					return 1;
			}
		}
		answer += ntohs(ans->size);
		len -= ntohs(ans->size);
	}
	return 0;
}

#if defined(res_ninit)
#define HAS_RES_NINIT
#else
static ast_mutex_t res_lock = AST_MUTEX_INITIALIZER;
#if 0
#warning "Warning, res_ninit is missing...  Could have reentrancy issues"
#endif
#endif

int ast_search_dns(void *context,
				   const char *dname, int class, int type,
				   int (*callback)(void *context, u_char *answer, int len, u_char *fullanswer))
{
#ifdef HAS_RES_NINIT
	struct __res_state dnsstate;
#endif
	char answer[MAX_SIZE];
	int res, ret = -1;

#ifdef HAS_RES_NINIT
	res_ninit(&dnsstate);
	res = res_nsearch(&dnsstate, dname, class, type, answer, sizeof(answer));
#else
	ast_mutex_lock(&res_lock);
	res_init();
	res = res_search(dname, class, type, answer, sizeof(answer));
#endif
	if (res > 0) {
		if ((res = dns_parse_answer(context, class, type, answer, res, callback)) < 0) {
			ast_log(LOG_WARNING, "Parse error\n");
			ret = -1;
		}
		else if (ret == 0) {
			ast_log(LOG_DEBUG, "No matches found\n");
			ret = 0;
		}
		else
			ret = 1;
	}
#ifdef HAS_RES_NINIT
	res_nclose(&dnsstate);
#else
#ifndef __APPLE__
	res_close();
#endif
	ast_mutex_unlock(&res_lock);
#endif
	return ret;
}