Sat Mar 10 01:54:27 2012

Asterisk developer's documentation


srv.c

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00001 /*
00002  * Asterisk -- An open source telephony toolkit.
00003  *
00004  * Copyright (C) 1999 - 2005, Digium, Inc.
00005  *
00006  * Mark Spencer <markster@digium.com>
00007  *
00008  * Funding provided by nic.at
00009  *
00010  * See http://www.asterisk.org for more information about
00011  * the Asterisk project. Please do not directly contact
00012  * any of the maintainers of this project for assistance;
00013  * the project provides a web site, mailing lists and IRC
00014  * channels for your use.
00015  *
00016  * This program is free software, distributed under the terms of
00017  * the GNU General Public License Version 2. See the LICENSE file
00018  * at the top of the source tree.
00019  */
00020 
00021 /*! \file
00022  *
00023  * \brief DNS SRV Record Lookup Support for Asterisk
00024  *
00025  * \author Mark Spencer <markster@digium.com>
00026  * 
00027  * \arg See also \ref AstENUM
00028  *
00029  * \note Funding provided by nic.at
00030  */
00031 
00032 #include "asterisk.h"
00033 
00034 ASTERISK_FILE_VERSION(__FILE__, "$Revision: 321392 $")
00035 
00036 #include <netinet/in.h>
00037 #include <arpa/nameser.h>
00038 #ifdef __APPLE__
00039 #if __APPLE_CC__ >= 1495
00040 #include <arpa/nameser_compat.h>
00041 #endif
00042 #endif
00043 #include <resolv.h>
00044 
00045 #include "asterisk/channel.h"
00046 #include "asterisk/srv.h"
00047 #include "asterisk/dns.h"
00048 #include "asterisk/utils.h"
00049 #include "asterisk/linkedlists.h"
00050 
00051 #ifdef __APPLE__
00052 #undef T_SRV
00053 #define T_SRV 33
00054 #endif
00055 
00056 struct srv_entry {
00057    unsigned short priority;
00058    unsigned short weight;
00059    unsigned short port;
00060    unsigned int weight_sum;
00061    AST_LIST_ENTRY(srv_entry) list;
00062    char host[1];
00063 };
00064 
00065 struct srv_context {
00066    unsigned int have_weights:1;
00067    struct srv_entry *prev;
00068    unsigned int num_records;
00069    AST_LIST_HEAD_NOLOCK(srv_entries, srv_entry) entries;
00070 };
00071 
00072 static int parse_srv(unsigned char *answer, int len, unsigned char *msg, struct srv_entry **result)
00073 {
00074    struct srv {
00075       unsigned short priority;
00076       unsigned short weight;
00077       unsigned short port;
00078    } __attribute__((__packed__)) *srv = (struct srv *) answer;
00079 
00080    int res = 0;
00081    char repl[256] = "";
00082    struct srv_entry *entry;
00083 
00084    if (len < sizeof(*srv))
00085       return -1;
00086 
00087    answer += sizeof(*srv);
00088    len -= sizeof(*srv);
00089 
00090    if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) <= 0) {
00091       ast_log(LOG_WARNING, "Failed to expand hostname\n");
00092       return -1;
00093    }
00094 
00095    /* the magic value "." for the target domain means that this service
00096       is *NOT* available at the domain we searched */
00097    if (!strcmp(repl, "."))
00098       return -1;
00099 
00100    if (!(entry = ast_calloc(1, sizeof(*entry) + strlen(repl))))
00101       return -1;
00102    
00103    entry->priority = ntohs(srv->priority);
00104    entry->weight = ntohs(srv->weight);
00105    entry->port = ntohs(srv->port);
00106    strcpy(entry->host, repl);
00107 
00108    *result = entry;
00109    
00110    return 0;
00111 }
00112 
00113 static int srv_callback(void *context, unsigned char *answer, int len, unsigned char *fullanswer)
00114 {
00115    struct srv_context *c = (struct srv_context *) context;
00116    struct srv_entry *entry = NULL;
00117    struct srv_entry *current;
00118 
00119    if (parse_srv(answer, len, fullanswer, &entry))
00120       return -1;
00121 
00122    if (entry->weight)
00123       c->have_weights = 1;
00124 
00125    AST_LIST_TRAVERSE_SAFE_BEGIN(&c->entries, current, list) {
00126       /* insert this entry just before the first existing
00127          entry with a higher priority */
00128       if (current->priority <= entry->priority)
00129          continue;
00130 
00131       AST_LIST_INSERT_BEFORE_CURRENT(entry, list);
00132       entry = NULL;
00133       break;
00134    }
00135    AST_LIST_TRAVERSE_SAFE_END;
00136 
00137    /* if we didn't find a place to insert the entry before an existing
00138       entry, then just add it to the end */
00139    if (entry)
00140       AST_LIST_INSERT_TAIL(&c->entries, entry, list);
00141 
00142    return 0;
00143 }
00144 
00145 /* Do the bizarre SRV record weight-handling algorithm
00146    involving sorting and random number generation...
00147 
00148    See RFC 2782 if you want know why this code does this
00149 */
00150 static void process_weights(struct srv_context *context)
00151 {
00152    struct srv_entry *current;
00153    struct srv_entries newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
00154 
00155    while (AST_LIST_FIRST(&context->entries)) {
00156       unsigned int random_weight;
00157       unsigned int weight_sum;
00158       unsigned short cur_priority = AST_LIST_FIRST(&context->entries)->priority;
00159       struct srv_entries temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
00160       weight_sum = 0;
00161 
00162       AST_LIST_TRAVERSE_SAFE_BEGIN(&context->entries, current, list) {
00163          if (current->priority != cur_priority)
00164             break;
00165 
00166          AST_LIST_MOVE_CURRENT(&temp_list, list);
00167       }
00168       AST_LIST_TRAVERSE_SAFE_END;
00169 
00170       while (AST_LIST_FIRST(&temp_list)) {
00171          weight_sum = 0;
00172          AST_LIST_TRAVERSE(&temp_list, current, list)
00173             current->weight_sum = weight_sum += current->weight;
00174 
00175          /* if all the remaining entries have weight == 0,
00176             then just append them to the result list and quit */
00177          if (weight_sum == 0) {
00178             AST_LIST_APPEND_LIST(&newlist, &temp_list, list);
00179             break;
00180          }
00181 
00182          random_weight = 1 + (unsigned int) ((float) weight_sum * (ast_random() / ((float) RAND_MAX + 1.0)));
00183 
00184          AST_LIST_TRAVERSE_SAFE_BEGIN(&temp_list, current, list) {
00185             if (current->weight < random_weight)
00186                continue;
00187 
00188             AST_LIST_MOVE_CURRENT(&newlist, list);
00189             break;
00190          }
00191          AST_LIST_TRAVERSE_SAFE_END;
00192       }
00193 
00194    }
00195 
00196    /* now that the new list has been ordered,
00197       put it in place */
00198 
00199    AST_LIST_APPEND_LIST(&context->entries, &newlist, list);
00200 }
00201 
00202 int ast_srv_lookup(struct srv_context **context, const char *service, const char **host, unsigned short *port)
00203 {
00204    struct srv_entry *cur;
00205 
00206    if (*context == NULL) {
00207       if (!(*context = ast_calloc(1, sizeof(struct srv_context)))) {
00208          return -1;
00209       }
00210       AST_LIST_HEAD_INIT_NOLOCK(&(*context)->entries);
00211 
00212       if ((ast_search_dns(*context, service, C_IN, T_SRV, srv_callback)) < 0) {
00213          ast_free(*context);
00214          *context = NULL;
00215          return -1;
00216       }
00217 
00218       if ((*context)->have_weights) {
00219          process_weights(*context);
00220       }
00221 
00222       (*context)->prev = AST_LIST_FIRST(&(*context)->entries);
00223       *host = (*context)->prev->host;
00224       *port = (*context)->prev->port;
00225       AST_LIST_TRAVERSE(&(*context)->entries, cur, list) {
00226          ++((*context)->num_records);
00227       }
00228       return 0;
00229    }
00230 
00231    if (((*context)->prev = AST_LIST_NEXT((*context)->prev, list))) {
00232       /* Retrieve next item in result */
00233       *host = (*context)->prev->host;
00234       *port = (*context)->prev->port;
00235       return 0;
00236    } else {
00237       /* No more results */
00238       while ((cur = AST_LIST_REMOVE_HEAD(&(*context)->entries, list))) {
00239          ast_free(cur);
00240       }
00241       ast_free(*context);
00242       *context = NULL;
00243       return 1;
00244    }
00245 }
00246 
00247 void ast_srv_cleanup(struct srv_context **context)
00248 {
00249    const char *host;
00250    unsigned short port;
00251 
00252    if (*context) {
00253       /* We have a context to clean up. */
00254       while (!(ast_srv_lookup(context, NULL, &host, &port))) {
00255       }
00256    }
00257 }
00258 
00259 int ast_get_srv(struct ast_channel *chan, char *host, int hostlen, int *port, const char *service)
00260 {
00261    struct srv_context context = { .entries = AST_LIST_HEAD_NOLOCK_INIT_VALUE };
00262    struct srv_entry *current;
00263    int ret;
00264 
00265    if (chan && ast_autoservice_start(chan) < 0) {
00266       return -1;
00267    }
00268 
00269    ret = ast_search_dns(&context, service, C_IN, T_SRV, srv_callback);
00270 
00271    if (context.have_weights) {
00272       process_weights(&context);
00273    }
00274 
00275    if (chan) {
00276       ret |= ast_autoservice_stop(chan);
00277    }
00278 
00279    /* TODO: there could be a "." entry in the returned list of
00280       answers... if so, this requires special handling */
00281 
00282    /* the list of entries will be sorted in the proper selection order
00283       already, so we just need the first one (if any) */
00284 
00285    if ((ret > 0) && (current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
00286       ast_copy_string(host, current->host, hostlen);
00287       *port = current->port;
00288       ast_free(current);
00289       ast_verb(4, "ast_get_srv: SRV lookup for '%s' mapped to host %s, port %d\n",
00290                 service, host, *port);
00291    } else {
00292       host[0] = '\0';
00293       *port = -1;
00294    }
00295 
00296    while ((current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
00297       ast_free(current);
00298    }
00299 
00300    return ret;
00301 }
00302 
00303 unsigned int ast_srv_get_record_count(struct srv_context *context)
00304 {
00305    return context->num_records;
00306 }
00307 
00308 int ast_srv_get_nth_record(struct srv_context *context, int record_num, const char **host,
00309       unsigned short *port, unsigned short *priority, unsigned short *weight)
00310 {
00311    int i = 1;
00312    int res = -1;
00313    struct srv_entry *entry;
00314 
00315    if (record_num < 1 || record_num > context->num_records) {
00316       return res;
00317    }
00318 
00319    AST_LIST_TRAVERSE(&context->entries, entry, list) {
00320       if (i == record_num) {
00321          *host = entry->host;
00322          *port = entry->port;
00323          *priority = entry->priority;
00324          *weight = entry->weight;
00325          res = 0;
00326          break;
00327       }
00328       ++i;
00329    }
00330 
00331    return res;
00332 }

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