dns_srv.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2015, Digium, Inc.
 *
 * Joshua Colp <[email protected]>
 *
 * 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 Support
 *
 * \author Joshua Colp <[email protected]>
 */

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

#include "asterisk.h"

#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

#include "asterisk/dns_core.h"
#include "asterisk/dns_srv.h"
#include "asterisk/linkedlists.h"
#include "asterisk/dns_internal.h"
#include "asterisk/utils.h"

struct ast_dns_record *dns_srv_alloc(struct ast_dns_query *query, const char *data, const size_t size)
{
 uint16_t priority;
 uint16_t weight;
 uint16_t port;
 const char *ptr;
 const char *end_of_record;
 struct ast_dns_srv_record *srv;
 int host_size;
 char host[NI_MAXHOST] = "";
 size_t host_len;

 ptr = dns_find_record(data, size, query->result->answer, query->result->answer_size);
 ast_assert(ptr != NULL);

 end_of_record = ptr + size;

 /* PRIORITY */
 ptr += dns_parse_short((unsigned char *) ptr, &priority);
 if (ptr >= end_of_record) {
 return NULL;
 }

 /* WEIGHT */
 ptr += dns_parse_short((unsigned char *) ptr, &weight);
 if (ptr >= end_of_record) {
 return NULL;
 }

 /* PORT */
 ptr += dns_parse_short((unsigned char *) ptr, &port);
 if (ptr >= end_of_record) {
 return NULL;
 }

 /*
 * The return value from dn_expand represents the size of the replacement
 * in the buffer which MAY be compressed. Since the expanded replacement
 * is NULL terminated, you can use strlen() to get the expanded size.
 */
 host_size = dn_expand((unsigned char *)query->result->answer,
 (unsigned char *) end_of_record, (unsigned char *) ptr, host, sizeof(host) - 1);
 if (host_size < 0) {
 ast_log(LOG_ERROR, "Failed to expand domain name: %s\n", strerror(errno));
 return NULL;
 }

 if (!strcmp(host, ".")) {
 return NULL;
 }

 host_len = strlen(host) + 1;
 srv = ast_calloc(1, sizeof(*srv) + size + host_len);
 if (!srv) {
 return NULL;
 }

 srv->priority = priority;
 srv->weight = weight;
 srv->port = port;

 srv->host = srv->data + size;
 ast_copy_string((char *)srv->host, host, host_len); /* SAFE */
 srv->generic.data_ptr = srv->data;

 return (struct ast_dns_record *)srv;
}

/* This implementation was taken from the existing srv.c which, after reading the RFC, implements it
 * as it should.
 */
void dns_srv_sort(struct ast_dns_result *result)
{
 struct ast_dns_record *current;
 struct dns_records newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;

 while (AST_LIST_FIRST(&result->records)) {
 unsigned short cur_priority = ((struct ast_dns_srv_record *)(AST_LIST_FIRST(&result->records)))->priority;
 struct dns_records temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;

 /* Find the lowest current priority to work on, but if the priority is already zero there is no lower priority */
 if (cur_priority) {
 AST_LIST_TRAVERSE(&result->records, current, list) {
 if (((struct ast_dns_srv_record *)current)->priority < cur_priority) {
 cur_priority = ((struct ast_dns_srv_record *)current)->priority;
 }
 }
 }

 /* Find all records which match this priority */
 AST_LIST_TRAVERSE_SAFE_BEGIN(&result->records, current, list) {
 if (((struct ast_dns_srv_record *)current)->priority != cur_priority) {
 continue;
 }

 AST_LIST_REMOVE_CURRENT(list);

 /* Records with a weight of zero must always be at the head */
 if (((struct ast_dns_srv_record *)current)->weight == 0) {
 AST_LIST_INSERT_HEAD(&temp_list, current, list);
 } else {
 AST_LIST_INSERT_TAIL(&temp_list, current, list);
 }
 }
 AST_LIST_TRAVERSE_SAFE_END;

 /* Apply weighting - as each record is passed the sum of all previous weights (plus its own) is stored away, and then a random weight
 * is calculated. The first record with a weight sum greater than the random weight is put in the new list and the whole thing starts
 * once again.
 */
 while (AST_LIST_FIRST(&temp_list)) {
 unsigned int weight_sum = 0;
 unsigned int random_weight;

 AST_LIST_TRAVERSE(&temp_list, current, list) {
 ((struct ast_dns_srv_record *)current)->weight_sum = weight_sum += ((struct ast_dns_srv_record *)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 (((struct ast_dns_srv_record *)current)->weight_sum < 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(&result->records, &newlist, list);
}

const char *ast_dns_srv_get_host(const struct ast_dns_record *record)
{
 struct ast_dns_srv_record *srv = (struct ast_dns_srv_record *) record;

 ast_assert(ast_dns_record_get_rr_type(record) == T_SRV);
 return srv->host;
}

unsigned short ast_dns_srv_get_priority(const struct ast_dns_record *record)
{
 struct ast_dns_srv_record *srv = (struct ast_dns_srv_record *) record;

 ast_assert(ast_dns_record_get_rr_type(record) == T_SRV);
 return srv->priority;
}

unsigned short ast_dns_srv_get_weight(const struct ast_dns_record *record)
{
 struct ast_dns_srv_record *srv = (struct ast_dns_srv_record *) record;

 ast_assert(ast_dns_record_get_rr_type(record) == T_SRV);
 return srv->weight;
}

unsigned short ast_dns_srv_get_port(const struct ast_dns_record *record)
{
 struct ast_dns_srv_record *srv = (struct ast_dns_srv_record *) record;

 ast_assert(ast_dns_record_get_rr_type(record) == T_SRV);
 return srv->port;
}