/* $Id$ */ /* * Copyright (C) 2008-2009 Teluu Inc. (http://www.teluu.com) * Copyright (C) 2003-2008 Benny Prijono * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #ifndef __PJLIB_UTIL_RESOLVER_H__ #define __PJLIB_UTIL_RESOLVER_H__ /** * @file resolver.h * @brief Asynchronous DNS resolver */ #include PJ_BEGIN_DECL /** * @defgroup PJ_DNS_RESOLVER DNS Asynchronous/Caching Resolution Engine * @ingroup PJ_DNS * @{ * * This module manages the host/server resolution by performing asynchronous * DNS queries and caching the results in the cache. It uses PJLIB-UTIL * low-level DNS parsing functions (see @ref PJ_DNS) and currently supports * several types of DNS resource records such as A record (typical query with * gethostbyname()) and SRV record. * * \section PJ_DNS_RESOLVER_FEATURES Features * * \subsection PJ_DNS_RESOLVER_FEATURES_ASYNC Asynchronous Query and Query Aggregation * * The DNS queries are performed asychronously, with timeout setting * configured on per resolver instance basis. Application can issue multiple * asynchronous queries simultaneously. Subsequent queries to the same resource * (name and DNS resource type) while existing query is still pending will be * merged into one query, so that only one DNS request packet is issued. * * \subsection PJ_DNS_RESOLVER_FEATURES_RETRANSMISSION Query Retransmission * * Asynchronous query will be retransmitted if no response is received * within the preconfigured time. Once maximum retransmission count is * exceeded and no response is received, the query will time out and the * callback will be called when error status. * * \subsection PJ_DNS_RESOLVER_FEATURES_CACHING Response Caching with TTL * * The resolver instance caches the results returned by nameservers, to * enhance the performance by minimizing the message round-trip to the server. * The TTL of the cached resposne is calculated from minimum TTL value found * across all resource record (RR) TTL in the response and further more it can * be limited to some preconfigured maximum TTL in the resolver. * * Response caching can be disabled by setting the maximum TTL value of the * resolver to zero. * * \subsection PJ_DNS_RESOLVER_FEATURES_PARALLEL Parallel and Backup Name Servers * * When the resolver is configured with multiple nameservers, initially the * queries will be issued to multiple name servers simultaneously to probe * which servers are not active. Once the probing stage is done, subsequent * queries will be directed to only one ACTIVE server which provides the best * response time. * * Name servers are probed periodically to see which nameservers are active * and which are down. This probing is done when a query is sent, thus no * timer is needed to maintain this. Also probing will be done in parallel * so that there would be no additional delay for the query. * * * \subsection PJ_DNS_RESOLVER_FEATURES_REC Supported Resource Records * * The low-level DNS parsing utility (see @ref PJ_DNS) supports parsing of * the following DNS resource records (RR): * - DNS A record * - DNS SRV record * - DNS PTR record * - DNS NS record * - DNS CNAME record * * For other types of record, application can parse the raw resource * record data (rdata) from the parsed DNS packet (#pj_dns_parsed_packet). * * * \section PJ_DNS_RESOLVER_USING Using the Resolver * * To use the resolver, application first creates the resolver instance by * calling #pj_dns_resolver_create(). If application already has its own * timer and ioqueue instances, it can instruct the resolver to use these * instances so that application does not need to poll the resolver * periodically to process events. If application does not specify the * timer and ioqueue instance for the resolver, an internal timer and * ioqueue will be created by the resolver. And since the resolver does not * create it's own thread, application MUST poll the resolver periodically * by calling #pj_dns_resolver_handle_events() to allow events (network and * timer) to be processed. * * Next, application MUST configure the nameservers to be used by the * resolver, by calling #pj_dns_resolver_set_ns(). * * Application performs asynchronous query by submitting the query with * #pj_dns_resolver_start_query(). Once the query completes (either * successfully or times out), the callback will be called. * * Application can cancel a pending query by calling #pj_dns_resolver_cancel_query(). * * Resolver must be destroyed by calling #pj_dns_resolver_destroy() to * release all resources back to the system. * * * \section PJ_DNS_RESOLVER_LIMITATIONS Resolver Limitations * * Current implementation mainly suffers from a growing memory problem, * which mainly is caused by the response caching. Although there is only * one cache entry per {query, name} combination, these cache entry will * never get deleted since there is no timer is created to invalidate these * entries. So the more unique names being queried by application, there more * enties will be created in the response cache. * * Note that a single response entry will occupy about 600-700 bytes of * pool memory (the PJ_DNS_RESOLVER_RES_BUF_SIZE value plus internal * structure). * * Application can work around this problem by doing one of these: * - disable caching by setting PJ_DNS_RESOLVER_MAX_TTL and * PJ_DNS_RESOLVER_INVALID_TTL to zero. * - periodically query #pj_dns_resolver_get_cached_count() and destroy- * recreate the resolver to recycle the memory used by the resolver. * * Note that future improvement may solve this problem by introducing * expiration timer to the cached entries. * * * \section PJ_DNS_RESOLVER_REFERENCE Reference * * The PJLIB-UTIL resolver was built from the information in the following * standards: * - * RFC 1035: "Domain names - implementation and specification" * - * RFC 2782: "A DNS RR for specifying the location of services (DNS SRV)" * */ /** * Opaque data type for DNS resolver object. */ typedef struct pj_dns_resolver pj_dns_resolver; /** * Opaque data type for asynchronous DNS query object. */ typedef struct pj_dns_async_query pj_dns_async_query; /** * Type of asynchronous callback which will be called when the asynchronous * query completes. * * @param user_data The user data set by application when creating the * asynchronous query. * @param status Status of the DNS resolution. * @param response The response packet received from the server. This * argument may be NULL when status is not PJ_SUCCESS. */ typedef void pj_dns_callback(void *user_data, pj_status_t status, pj_dns_parsed_packet *response); /** * This structure describes resolver settings. */ typedef struct pj_dns_settings { unsigned options; /**< Options flags. */ unsigned qretr_delay; /**< Query retransmit delay in msec. */ unsigned qretr_count; /**< Query maximum retransmission count. */ unsigned cache_max_ttl; /**< Maximum TTL for cached responses. If the value is zero, caching is disabled. */ unsigned good_ns_ttl; /**< See #PJ_DNS_RESOLVER_GOOD_NS_TTL */ unsigned bad_ns_ttl; /**< See #PJ_DNS_RESOLVER_BAD_NS_TTL */ } pj_dns_settings; /** * This structure represents DNS A record, as the result of parsing * DNS response packet using #pj_dns_parse_a_response(). */ typedef struct pj_dns_a_record { /** The target name being queried. */ pj_str_t name; /** If target name corresponds to a CNAME entry, the alias contains * the value of the CNAME entry, otherwise it will be empty. */ pj_str_t alias; /** Number of IP addresses. */ unsigned addr_count; /** IP addresses of the host found in the response */ pj_in_addr addr[PJ_DNS_MAX_IP_IN_A_REC]; /** Internal buffer for hostname and alias. */ char buf_[128]; } pj_dns_a_record; /** * Set default values to the DNS settings. * * @param s The DNS settings to be initialized. */ PJ_DECL(void) pj_dns_settings_default(pj_dns_settings *s); /** * Create DNS resolver instance. After the resolver is created, application * MUST configure the nameservers with #pj_dns_resolver_set_ns(). * * When creating the resolver, application may specify both timer heap * and ioqueue instance, so that it doesn't need to poll the resolver * periodically. * * @param pf Pool factory where the memory pool will be created from. * @param name Optional resolver name to identify the instance in * the log. * @param options Optional options, must be zero for now. * @param timer Optional timer heap instance to be used by the resolver. * If timer heap is not specified, an internal timer will be * created, and application would need to poll the resolver * periodically. * @param ioqueue Optional I/O Queue instance to be used by the resolver. * If ioqueue is not specified, an internal one will be * created, and application would need to poll the resolver * periodically. * @param p_resolver Pointer to receive the resolver instance. * * @return PJ_SUCCESS on success, or the appropriate error code, */ PJ_DECL(pj_status_t) pj_dns_resolver_create(pj_pool_factory *pf, const char *name, unsigned options, pj_timer_heap_t *timer, pj_ioqueue_t *ioqueue, pj_dns_resolver **p_resolver); /** * Update the name servers for the DNS resolver. The name servers MUST be * configured before any resolution can be done. The order of nameservers * specifies their priority; the first name server will be tried first * before the next in the list. * * @param resolver The resolver instance. * @param count Number of name servers in the array. * @param servers Array of name server IP addresses or hostnames. If * hostname is specified, the hostname must be resolvable * with pj_gethostbyname(). * @param ports Optional array of ports. If this argument is NULL, * the nameserver will use default port. * * @return PJ_SUCCESS on success, or the appropriate error code, */ PJ_DECL(pj_status_t) pj_dns_resolver_set_ns(pj_dns_resolver *resolver, unsigned count, const pj_str_t servers[], const pj_uint16_t ports[]); /** * Get the resolver current settings. * * @param resolver The resolver instance. * @param st Buffer to be filled up with resolver settings. * * @return The query timeout setting, in seconds. */ PJ_DECL(pj_status_t) pj_dns_resolver_get_settings(pj_dns_resolver *resolver, pj_dns_settings *st); /** * Modify the resolver settings. Application should initialize the settings * by retrieving current settings first before applying new settings, to * ensure that all fields are initialized properly. * * @param resolver The resolver instance. * @param st The resolver settings. * * @return PJ_SUCCESS on success, or the appropriate error code, */ PJ_DECL(pj_status_t) pj_dns_resolver_set_settings(pj_dns_resolver *resolver, const pj_dns_settings *st); /** * Poll for events from the resolver. This function MUST be called * periodically when the resolver is using it's own timer or ioqueue * (in other words, when NULL is specified as either \a timer or * \a ioqueue argument in #pj_dns_resolver_create()). * * @param resolver The resolver instance. * @param timeout Maximum time to wait for event occurence. If this * argument is NULL, this function will wait forever * until events occur. */ PJ_DECL(void) pj_dns_resolver_handle_events(pj_dns_resolver *resolver, const pj_time_val *timeout); /** * Destroy DNS resolver instance. * * @param resolver The resolver object to be destryed * @param notify If non-zero, all pending asynchronous queries will be * cancelled and its callback will be called. If FALSE, * then no callback will be called. * * @return PJ_SUCCESS on success, or the appropriate error code, */ PJ_DECL(pj_status_t) pj_dns_resolver_destroy(pj_dns_resolver *resolver, pj_bool_t notify); /** * Create and start asynchronous DNS query for a single resource. Depending * on whether response cache is available, this function will either start * an asynchronous DNS query or call the callback immediately. * * If response is not available in the cache, an asynchronous query will be * started, and callback will be called at some time later when the query * completes. If \a p_query argument is not NULL, it will be filled with * the asynchronous query object. * * If response is available in the cache, the callback will be called * immediately before this function returns. In this case, if \a p_query * argument is not NULL, the value will be set to NULL since no new query * is started. * * @param resolver The resolver object. * @param name The name to be resolved. * @param type The type of resource (see #pj_dns_type constants). * @param options Optional options, must be zero for now. * @param cb Callback to be called when the query completes, * either successfully or with failure. * @param user_data Arbitrary user data to be associated with the query, * and which will be given back in the callback. * @param p_query Optional pointer to receive the query object, if one * was started. If this pointer is specified, a NULL may * be returned if response cache is available immediately. * * @return PJ_SUCCESS if either an asynchronous query has been * started successfully or response cache is available and * the user callback has been called. */ PJ_DECL(pj_status_t) pj_dns_resolver_start_query(pj_dns_resolver *resolver, const pj_str_t *name, int type, unsigned options, pj_dns_callback *cb, void *user_data, pj_dns_async_query **p_query); /** * Cancel a pending query. * * @param query The pending asynchronous query to be cancelled. * @param notify If non-zero, the callback will be called with failure * status to notify that the query has been cancelled. * * @return PJ_SUCCESS on success, or the appropriate error code, */ PJ_DECL(pj_status_t) pj_dns_resolver_cancel_query(pj_dns_async_query *query, pj_bool_t notify); /** * A utility function to parse a DNS response containing A records into * DNS A record. * * @param pkt The DNS response packet. * @param rec The structure to be initialized with the parsed * DNS A record from the packet. * * @return PJ_SUCCESS if response can be parsed successfully. */ PJ_DECL(pj_status_t) pj_dns_parse_a_response(const pj_dns_parsed_packet *pkt, pj_dns_a_record *rec); /** * Put the specified DNS packet into DNS cache. This function is mainly used * for testing the resolver, however it can also be used to inject entries * into the resolver. * * The packet MUST contain either answer section or query section so that * it can be indexed. * * @param resolver The resolver instance. * @param pkt DNS packet to be added to the DNS cache. If the packet * matches existing entry, it will update the entry. * @param set_ttl If the value is PJ_FALSE, the entry will not expire * (so use with care). Otherwise cache expiration will be * calculated based on the TTL of the answeres. * * @return PJ_SUCCESS on success, or the appropriate error code. */ PJ_DECL(pj_status_t) pj_dns_resolver_add_entry(pj_dns_resolver *resolver, const pj_dns_parsed_packet *pkt, pj_bool_t set_ttl); /** * Get the total number of response in the response cache. * * @param resolver The resolver instance. * * @return Current number of entries being stored in the response * cache. */ PJ_DECL(unsigned) pj_dns_resolver_get_cached_count(pj_dns_resolver *resolver); /** * Dump resolver state to the log. * * @param resolver The resolver instance. * @param detail Will print detailed entries. */ PJ_DECL(void) pj_dns_resolver_dump(pj_dns_resolver *resolver, pj_bool_t detail); /** * @} */ PJ_END_DECL #endif /* __PJLIB_UTIL_RESOLVER_H__ */