srv.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2005, Digium, Inc.
 *
 * Mark Spencer <markster@digium.com>
 *
 * 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 <markster@digium.com>
 *
 * \arg See also \ref ast_extension_states "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;
}