srv_resolver.h

一个开源的sip源代码

/* $Id: srv_resolver.h 1102 2007-03-24 17:37:25Z bennylp $ */
/* 
 * Copyright (C) 2003-2007 Benny Prijono <[e-mail]>
 *
 * 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_SRV_RESOLVER_H__
#define __PJLIB_UTIL_SRV_RESOLVER_H__

/**
 * @file srv_resolver.h
 * @brief DNS SRV resolver
 */
#include <pjlib-util/resolver.h>

PJ_BEGIN_DECL

/**
 * @defgroup PJ_DNS_SRV_RESOLVER DNS SRV Resolution Helper
 * @ingroup PJ_DNS
 * @{
 *
 * \subsection PJ_DNS_SRV_RESOLVER_INTRO DNS SRV Resolution Helper
 *
 * This module provides an even higher layer of abstraction for the DNS
 * resolution framework, to resolve DNS SRV names.
 *
 * The #pj_dns_srv_resolve() function will asynchronously resolve the server
 * name into IP address(es) with a single function call. If the SRV name
 * contains multiple names, then each will be resolved with individual
 * DNS A resolution to get the IP addresses. Upon successful completion, 
 * application callback will be called with each IP address of the
 * target selected based on the load-balancing and fail-over criteria
 * below.
 *
 * When the resolver fails to resolve the name using DNS SRV resolution
 * (for example when the DNS SRV record is not present in the DNS server),
 * the resolver will fallback to using DNS A record resolution to resolve
 * the name.
 *
 * \subsection PJ_DNS_SRV_RESOLVER_FAILOVER_LOADBALANCE Load-Balancing and Fail-Over
 *
 * When multiple targets are returned in the DNS SRV response, server entries
 * are selected based on the following rule (which is described in RFC 2782):
 *  - targets will be sorted based on the priority first.
 *  - for targets with the same priority, #pj_dns_srv_resolve() will select
 *    only one target according to its weight. To select this one target,
 *    the function associates running-sum for all targets, and generates 
 *    a random number between zero and the total running-sum (inclusive).
 *    The target selected is the first target with running-sum greater than
 *    or equal to this random number.
 *
 * The above procedure will select one target for each priority, allowing
 * application to fail-over to the next target when the previous target fails.
 * These targets are returned in the #pj_dns_srv_record structure 
 * argument of the callback. 
 *
 * \section PJ_DNS_SRV_RESOLVER_REFERENCE Reference
 *
 * Reference:
 *  - <A HREF="http://www.ietf.org/rfc/rfc2782.txt">RFC 2782</A>: 
 *	A DNS RR for specifying the location of services (DNS SRV)
 */

/**
 * Maximum server address entries per one SRV record
 */
#ifndef PJ_DNS_SRV_MAX_ADDR
#   define PJ_DNS_SRV_MAX_ADDR	    8
#endif


/**
 * The server addresses returned by the resolver.
 */
typedef struct pj_dns_srv_record
{
    /** Number of address records. */
    unsigned	count;

    /** Address records. */
    struct
    {
	/** Server priority (the lower the higher the priority). */
	unsigned		priority;

	/** Server weight (the higher the more load it can handle). */
	unsigned		weight;

	/** The server's address. */
	pj_sockaddr		addr;

	/** Address length. */
	int			addr_len;

    } entry[PJ_DNS_SRV_MAX_ADDR];

} pj_dns_srv_record;


/**
 * Type of callback function to receive notification from the resolver
 * when the resolution process completes.
 */
typedef void pj_dns_srv_resolver_cb(void *user_data,
				    pj_status_t status,
				    const pj_dns_srv_record *rec);


/**
 * Start DNS SRV resolution for the specified name. The full name of the
 * entry will be concatenated from \a res_name and \a domain_name fragments.
 *
 * @param domain_name	The domain name part of the name.
 * @param res_name	The full service name, including the transport name
 *			and with all the leading underscore characters and
 *			ending dot (e.g. "_sip._udp.", "_stun._udp.").
 * @param def_port	The port number to be assigned to the resolved address
 *			when the DNS SRV resolution fails and the name is 
 *			resolved with DNS A resolution.
 * @param pool		Memory pool used to allocate memory for the query.
 * @param resolver	The resolver instance.
 * @param fallback_a	Specify if the resolver should fallback with DNS A
 *			resolution when the SRV resolution fails.
 * @param token		Arbitrary data to be associated with this query when
 *			the calback is called.
 * @param cb		Pointer to callback function to receive the
 *			notification when the resolution process completes.
 *
 * @return		PJ_SUCCESS on success, or the appropriate error code.
 */
PJ_DECL(pj_status_t) pj_dns_srv_resolve(const pj_str_t *domain_name,
					const pj_str_t *res_name,
					unsigned def_port,
					pj_pool_t *pool,
					pj_dns_resolver *resolver,
					pj_bool_t fallback_a,
					void *token,
					pj_dns_srv_resolver_cb *cb);


/**
 * @}
 */

PJ_END_DECL


#endif	/* __PJLIB_UTIL_SRV_RESOLVER_H__ */