srv.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2005, Digium, Inc.
 *
 * Mark Spencer <[email protected]>
 *
 * Funding provided by nic.at
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*! \file
 *
 * \brief DNS SRV Record Lookup Support for Asterisk
 *
 * \author Mark Spencer <[email protected]>
 *
 * \arg See also \ref AstENUM
 *
 * \note Funding provided by nic.at
 */

/*** MODULEINFO
   <support_level>core</support_level>
 ***/

#include "asterisk.h"

#include <netinet/in.h>
#include <arpa/nameser.h>
#ifdef __APPLE__
#include <arpa/nameser_compat.h>
#endif
#include <resolv.h>

#include "asterisk/channel.h"
#include "asterisk/srv.h"
#include "asterisk/dns.h"
#include "asterisk/utils.h"
#include "asterisk/linkedlists.h"

#ifdef __APPLE__
#undef T_SRV
#define T_SRV 33
#endif

struct srv_entry {
   unsigned short priority;
   unsigned short weight;
   unsigned short port;
   unsigned int weight_sum;
   AST_LIST_ENTRY(srv_entry) list;
   char host[1];
};

struct srv_context {
   unsigned int have_weights:1;
   struct srv_entry *prev;
   unsigned int num_records;
   AST_LIST_HEAD_NOLOCK(srv_entries, srv_entry) entries;
};

static int parse_srv(unsigned char *answer, int len, unsigned char *msg, struct srv_entry **result)
{
   struct srv {
      unsigned short priority;
      unsigned short weight;
      unsigned short port;
   } __attribute__((__packed__)) *srv = (struct srv *) answer;

   int res = 0;
   char repl[256] = "";
   struct srv_entry *entry;

   if (len < sizeof(*srv))
      return -1;

   answer += sizeof(*srv);
   len -= sizeof(*srv);

   if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) <= 0) {
      ast_log(LOG_WARNING, "Failed to expand hostname\n");
      return -1;
   }

   /* the magic value "." for the target domain means that this service
      is *NOT* available at the domain we searched */
   if (!strcmp(repl, "."))
      return -1;

   if (!(entry = ast_calloc(1, sizeof(*entry) + strlen(repl))))
      return -1;

   entry->priority = ntohs(srv->priority);
   entry->weight = ntohs(srv->weight);
   entry->port = ntohs(srv->port);
   strcpy(entry->host, repl);

   *result = entry;

   return 0;
}

static int srv_callback(void *context, unsigned char *answer, int len, unsigned char *fullanswer)
{
   struct srv_context *c = (struct srv_context *) context;
   struct srv_entry *entry = NULL;
   struct srv_entry *current;

   if (parse_srv(answer, len, fullanswer, &entry))
      return -1;

   if (entry->weight)
      c->have_weights = 1;

   AST_LIST_TRAVERSE_SAFE_BEGIN(&c->entries, current, list) {
      /* insert this entry just before the first existing
         entry with a higher priority */
      if (current->priority <= entry->priority)
         continue;

      AST_LIST_INSERT_BEFORE_CURRENT(entry, list);
      entry = NULL;
      break;
   }
   AST_LIST_TRAVERSE_SAFE_END;

   /* if we didn't find a place to insert the entry before an existing
      entry, then just add it to the end */
   if (entry)
      AST_LIST_INSERT_TAIL(&c->entries, entry, list);

   return 0;
}

/* Do the bizarre SRV record weight-handling algorithm
   involving sorting and random number generation...

   See RFC 2782 if you want know why this code does this
*/
static void process_weights(struct srv_context *context)
{
   struct srv_entry *current;
   struct srv_entries newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;

   while (AST_LIST_FIRST(&context->entries)) {
      unsigned int random_weight;
      unsigned int weight_sum;
      unsigned short cur_priority = AST_LIST_FIRST(&context->entries)->priority;
      struct srv_entries temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
      weight_sum = 0;

      AST_LIST_TRAVERSE_SAFE_BEGIN(&context->entries, current, list) {
         if (current->priority != cur_priority)
            break;

         AST_LIST_MOVE_CURRENT(&temp_list, list);
      }
      AST_LIST_TRAVERSE_SAFE_END;

      while (AST_LIST_FIRST(&temp_list)) {
         weight_sum = 0;
         AST_LIST_TRAVERSE(&temp_list, current, list)
            current->weight_sum = weight_sum += current->weight;

         /* if all the remaining entries have weight == 0,
            then just append them to the result list and quit */
         if (weight_sum == 0) {
            AST_LIST_APPEND_LIST(&newlist, &temp_list, list);
            break;
         }

         random_weight = 1 + (unsigned int) ((float) weight_sum * (ast_random() / ((float) RAND_MAX + 1.0)));

         AST_LIST_TRAVERSE_SAFE_BEGIN(&temp_list, current, list) {
            if (current->weight < random_weight)
               continue;

            AST_LIST_MOVE_CURRENT(&newlist, list);
            break;
         }
         AST_LIST_TRAVERSE_SAFE_END;
      }

   }

   /* now that the new list has been ordered,
      put it in place */

   AST_LIST_APPEND_LIST(&context->entries, &newlist, list);
}

int ast_srv_lookup(struct srv_context **context, const char *service, const char **host, unsigned short *port)
{
   struct srv_entry *cur;

   if (*context == NULL) {
      if (!(*context = ast_calloc(1, sizeof(struct srv_context)))) {
         return -1;
      }
      AST_LIST_HEAD_INIT_NOLOCK(&(*context)->entries);

      if (((ast_search_dns(*context, service, C_IN, T_SRV, srv_callback)) < 1) ||
         AST_LIST_EMPTY(&(*context)->entries)) {
         ast_free(*context);
         *context = NULL;
         return -1;
      }

      if ((*context)->have_weights) {
         process_weights(*context);
      }

      (*context)->prev = AST_LIST_FIRST(&(*context)->entries);
      *host = (*context)->prev->host;
      *port = (*context)->prev->port;
      AST_LIST_TRAVERSE(&(*context)->entries, cur, list) {
         ++((*context)->num_records);
      }
      return 0;
   }

   if (((*context)->prev = AST_LIST_NEXT((*context)->prev, list))) {
      /* Retrieve next item in result */
      *host = (*context)->prev->host;
      *port = (*context)->prev->port;
      return 0;
   } else {
      /* No more results */
      while ((cur = AST_LIST_REMOVE_HEAD(&(*context)->entries, list))) {
         ast_free(cur);
      }
      ast_free(*context);
      *context = NULL;
      return 1;
   }
}

void ast_srv_cleanup(struct srv_context **context)
{
   const char *host;
   unsigned short port;

   if (*context) {
      /* We have a context to clean up. */
      while (!(ast_srv_lookup(context, NULL, &host, &port))) {
      }
   }
}

int ast_get_srv(struct ast_channel *chan, char *host, int hostlen, int *port, const char *service)
{
   struct srv_context context = { .entries = AST_LIST_HEAD_NOLOCK_INIT_VALUE };
   struct srv_entry *current;
   int ret;

   if (chan && ast_autoservice_start(chan) < 0) {
      return -1;
   }

   ret = ast_search_dns(&context, service, C_IN, T_SRV, srv_callback);

   if (context.have_weights) {
      process_weights(&context);
   }

   if (chan) {
      ret |= ast_autoservice_stop(chan);
   }

   /* TODO: there could be a "." entry in the returned list of
      answers... if so, this requires special handling */

   /* the list of entries will be sorted in the proper selection order
      already, so we just need the first one (if any) */

   if ((ret > 0) && (current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
      ast_copy_string(host, current->host, hostlen);
      *port = current->port;
      ast_free(current);
      ast_debug(4, "ast_get_srv: SRV lookup for '%s' mapped to host %s, port %d\n",
                service, host, *port);
   } else {
      host[0] = '\0';
      *port = -1;
   }

   while ((current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
      ast_free(current);
   }

   return ret;
}

unsigned int ast_srv_get_record_count(struct srv_context *context)
{
   return context->num_records;
}

int ast_srv_get_nth_record(struct srv_context *context, int record_num, const char **host,
      unsigned short *port, unsigned short *priority, unsigned short *weight)
{
   int i = 1;
   int res = -1;
   struct srv_entry *entry;

   if (record_num < 1 || record_num > context->num_records) {
      return res;
   }

   AST_LIST_TRAVERSE(&context->entries, entry, list) {
      if (i == record_num) {
         *host = entry->host;
         *port = entry->port;
         *priority = entry->priority;
         *weight = entry->weight;
         res = 0;
         break;
      }
      ++i;
   }

   return res;
}