Wed Aug 18 22:33:56 2010

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: 168524 $")
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    AST_LIST_HEAD_NOLOCK(srv_entries, srv_entry) entries;
00068 };
00069 
00070 static int parse_srv(unsigned char *answer, int len, unsigned char *msg, struct srv_entry **result)
00071 {
00072    struct srv {
00073       unsigned short priority;
00074       unsigned short weight;
00075       unsigned short port;
00076    } __attribute__((__packed__)) *srv = (struct srv *) answer;
00077 
00078    int res = 0;
00079    char repl[256] = "";
00080    struct srv_entry *entry;
00081 
00082    if (len < sizeof(*srv))
00083       return -1;
00084 
00085    answer += sizeof(*srv);
00086    len -= sizeof(*srv);
00087 
00088    if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) <= 0) {
00089       ast_log(LOG_WARNING, "Failed to expand hostname\n");
00090       return -1;
00091    }
00092 
00093    /* the magic value "." for the target domain means that this service
00094       is *NOT* available at the domain we searched */
00095    if (!strcmp(repl, "."))
00096       return -1;
00097 
00098    if (!(entry = ast_calloc(1, sizeof(*entry) + strlen(repl))))
00099       return -1;
00100    
00101    entry->priority = ntohs(srv->priority);
00102    entry->weight = ntohs(srv->weight);
00103    entry->port = ntohs(srv->port);
00104    strcpy(entry->host, repl);
00105 
00106    *result = entry;
00107    
00108    return 0;
00109 }
00110 
00111 static int srv_callback(void *context, unsigned char *answer, int len, unsigned char *fullanswer)
00112 {
00113    struct srv_context *c = (struct srv_context *) context;
00114    struct srv_entry *entry = NULL;
00115    struct srv_entry *current;
00116 
00117    if (parse_srv(answer, len, fullanswer, &entry))
00118       return -1;
00119 
00120    if (entry->weight)
00121       c->have_weights = 1;
00122 
00123    AST_LIST_TRAVERSE_SAFE_BEGIN(&c->entries, current, list) {
00124       /* insert this entry just before the first existing
00125          entry with a higher priority */
00126       if (current->priority <= entry->priority)
00127          continue;
00128 
00129       AST_LIST_INSERT_BEFORE_CURRENT(entry, list);
00130       entry = NULL;
00131       break;
00132    }
00133    AST_LIST_TRAVERSE_SAFE_END;
00134 
00135    /* if we didn't find a place to insert the entry before an existing
00136       entry, then just add it to the end */
00137    if (entry)
00138       AST_LIST_INSERT_TAIL(&c->entries, entry, list);
00139 
00140    return 0;
00141 }
00142 
00143 /* Do the bizarre SRV record weight-handling algorithm
00144    involving sorting and random number generation...
00145 
00146    See RFC 2782 if you want know why this code does this
00147 */
00148 static void process_weights(struct srv_context *context)
00149 {
00150    struct srv_entry *current;
00151    struct srv_entries newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
00152 
00153    while (AST_LIST_FIRST(&context->entries)) {
00154       unsigned int random_weight;
00155       unsigned int weight_sum;
00156       unsigned short cur_priority = AST_LIST_FIRST(&context->entries)->priority;
00157       struct srv_entries temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
00158       weight_sum = 0;
00159 
00160       AST_LIST_TRAVERSE_SAFE_BEGIN(&context->entries, current, list) {
00161          if (current->priority != cur_priority)
00162             break;
00163 
00164          AST_LIST_MOVE_CURRENT(&temp_list, list);
00165       }
00166       AST_LIST_TRAVERSE_SAFE_END;
00167 
00168       while (AST_LIST_FIRST(&temp_list)) {
00169          weight_sum = 0;
00170          AST_LIST_TRAVERSE(&temp_list, current, list)
00171             current->weight_sum = weight_sum += current->weight;
00172 
00173          /* if all the remaining entries have weight == 0,
00174             then just append them to the result list and quit */
00175          if (weight_sum == 0) {
00176             AST_LIST_APPEND_LIST(&newlist, &temp_list, list);
00177             break;
00178          }
00179 
00180          random_weight = 1 + (unsigned int) ((float) weight_sum * (ast_random() / ((float) RAND_MAX + 1.0)));
00181 
00182          AST_LIST_TRAVERSE_SAFE_BEGIN(&temp_list, current, list) {
00183             if (current->weight < random_weight)
00184                continue;
00185 
00186             AST_LIST_MOVE_CURRENT(&newlist, list);
00187             break;
00188          }
00189          AST_LIST_TRAVERSE_SAFE_END;
00190       }
00191 
00192    }
00193 
00194    /* now that the new list has been ordered,
00195       put it in place */
00196 
00197    AST_LIST_APPEND_LIST(&context->entries, &newlist, list);
00198 }
00199 
00200 int ast_get_srv(struct ast_channel *chan, char *host, int hostlen, int *port, const char *service)
00201 {
00202    struct srv_context context = { .entries = AST_LIST_HEAD_NOLOCK_INIT_VALUE };
00203    struct srv_entry *current;
00204    int ret;
00205 
00206    if (chan && ast_autoservice_start(chan) < 0)
00207       return -1;
00208 
00209    ret = ast_search_dns(&context, service, C_IN, T_SRV, srv_callback);
00210 
00211    if (context.have_weights)
00212       process_weights(&context);
00213 
00214    if (chan)
00215       ret |= ast_autoservice_stop(chan);
00216 
00217    /* TODO: there could be a "." entry in the returned list of
00218       answers... if so, this requires special handling */
00219 
00220    /* the list of entries will be sorted in the proper selection order
00221       already, so we just need the first one (if any) */
00222 
00223    if ((ret > 0) && (current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
00224       ast_copy_string(host, current->host, hostlen);
00225       *port = current->port;
00226       ast_free(current);
00227       ast_verb(4, "ast_get_srv: SRV lookup for '%s' mapped to host %s, port %d\n",
00228                 service, host, *port);
00229    } else {
00230       host[0] = '\0';
00231       *port = -1;
00232    }
00233 
00234    while ((current = AST_LIST_REMOVE_HEAD(&context.entries, list)))
00235       ast_free(current);
00236 
00237    return ret;
00238 }

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