dns_srv.c

Go to the documentation of this file.

/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2015, Digium, Inc.
 *
 * Joshua Colp <jcolp@digium.com>
 *
 * 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 <jcolp@digium.com>
 */
 
/*** 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;
}