dns.c

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/*
 * 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>
#include "asterisk/endian.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
AST_MUTEX_DEFINE_STATIC(res_lock);
#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;
}

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