Major addition to support DNS SRV resolution in PJSIP:

- added DNS asynchronous/caching resolver engine in
  PJLIB-UTIL (resolver.[hc])
- modified SIP resolver (sip_resolve.c) to properly
  perform DNS SRV/A resolution when DNS resolution is
  enabled.
- added dns_test.c in PJSIP-TEST for testing the SIP
  resolver.
- added nameserver configuration in PJSUA-LIB
- added "--nameserver" option in PJSUA.
- updated project/Makefiles and doxygen documentation.



git-svn-id: http://svn.pjsip.org/repos/pjproject/trunk@753 74dad513-b988-da41-8d7b-12977e46ad98

4 files changed, 756 insertions, 42 deletions

diff --git a/pjlib-util/include/pjlib-util.h b/pjlib-util/include/pjlib-util.h
index 741578fd..3ac8c50b 100644
--- a/pjlib-util/include/pjlib-util.h
+++ b/pjlib-util/include/pjlib-util.h
@@ -23,6 +23,7 @@
 #include <pjlib-util/errno.h>
 #include <pjlib-util/getopt.h>
 #include <pjlib-util/md5.h>
+#include <pjlib-util/resolver.h>
 #include <pjlib-util/scanner.h>
 #include <pjlib-util/stun.h>
 #include <pjlib-util/xml.h>
diff --git a/pjlib-util/include/pjlib-util/dns.h b/pjlib-util/include/pjlib-util/dns.h
index cb6a28f2..d431b0da 100644
--- a/pjlib-util/include/pjlib-util/dns.h
+++ b/pjlib-util/include/pjlib-util/dns.h
@@ -22,7 +22,7 @@
 
 /**
  * @file dns.h
- * @brief Asynchronous DNS Name Resolution/resolver
+ * @brief Low level DNS message parsing and packetization.
  */
 #include <pj/types.h>
 
@@ -31,16 +31,35 @@ PJ_BEGIN_DECL
 
 
 /**
- * @defgroup PJ_DNS_RESOLVER Asynchronous DNS Name Resolution/resolver
+ * @defgroup PJ_DNS Low-level DNS message parsing and packetization
  * @ingroup PJ
  * @{
  *
- * This module provides services to performing asynchronous DNS resolution.
+ * This module provides low-level services to parse and packetize DNS queries
+ * and responses. The functions support building a DNS query packet and parse
+ * the data in the DNS response.
+ *
+ * To create a DNS query packet, application should call #pj_dns_make_query()
+ * function, specifying the desired DNS query type, the name to be resolved,
+ * and the buffer where the DNS packet will be built into. 
+ *
+ * When incoming DNS query or response packet arrives, application can use
+ * #pj_dns_parse_packet() to parse the TCP/UDP payload into parsed DNS packet
+ * structure.
+ *
+ * This module does not provide any networking functionalities to send or
+ * receive DNS packets. This functionality should be provided by higher layer
+ * modules such as @ref PJ_DNS_RESOLVER.
  */
 
+enum
+{
+    PJ_DNS_CLASS_IN	= 1	/**< DNS class IN.			    */
+};
+
 /**
  * This enumeration describes standard DNS record types as described by
- * RFC 1035.
+ * RFC 1035, RFC 2782, and others.
  */
 typedef enum pj_dns_type
 {
@@ -158,31 +177,69 @@ typedef struct pj_dns_hdr
 #define PJ_DNS_GET_QR(val)	(((val) & PJ_DNS_SET_QR(1)) >> 15)
 
 
+/** 
+ * These constants describe DNS RCODEs. Application can fold these constants
+ * into PJLIB pj_status_t namespace by calling #PJ_STATUS_FROM_DNS_RCODE()
+ * macro.
+ */
+typedef enum pj_dns_rcode
+{
+    PJ_DNS_RCODE_FORMERR    = 1,    /**< Format error.			    */
+    PJ_DNS_RCODE_SERVFAIL   = 2,    /**< Server failure.		    */
+    PJ_DNS_RCODE_NXDOMAIN   = 3,    /**< Name Error.			    */
+    PJ_DNS_RCODE_NOTIMPL    = 4,    /**< Not Implemented.		    */
+    PJ_DNS_RCODE_REFUSED    = 5,    /**< Refused.			    */
+    PJ_DNS_RCODE_YXDOMAIN   = 6,    /**< The name exists.		    */
+    PJ_DNS_RCODE_YXRRSET    = 7,    /**< The RRset (name, type) exists.	    */
+    PJ_DNS_RCODE_NXRRSET    = 8,    /**< The RRset (name, type) doesn't exist*/
+    PJ_DNS_RCODE_NOTAUTH    = 9,    /**< Not authorized.		    */
+    PJ_DNS_RCODE_NOTZONE    = 10    /**< The zone specified is not a zone.  */
+
+} pj_dns_rcode;
+
 
 /**
- * This constants describes record types in the DNS packet.
+ * This constant specifies the maximum names to keep in the temporary name
+ * table when performing name compression scheme when duplicating DNS packet
+ * (the #pj_dns_packet_dup() function).
+ *
+ * Generally name compression is desired, since it saves some memory (see
+ * PJ_DNS_RESOLVER_RES_BUF_SIZE setting). However it comes at the expense of 
+ * a little processing overhead to perform name scanning and also a little
+ * bit more stack usage (8 bytes per entry on 32bit platform).
+ *
+ * Default: 16
  */
-typedef enum pj_dns_rec_type
+#ifndef PJ_DNS_MAX_NAMES_IN_NAMETABLE
+#   define PJ_DNS_MAX_NAMES_IN_NAMETABLE	16
+#endif
+
+
+/**
+ * This structure describes a DNS query record.
+ */
+typedef struct pj_dns_parsed_query
 {
-    DNS_QUERY_REC,  /**< The record is a query record	    */
-    DNS_RR_REC,	    /**< The record is resource record	    */
-    DNS_NS_REC,	    /**< The record is name server record   */
-    DNS_REC_AR,	    /**< The record is additional RR.	    */
-} pj_dns_rec_type;
+    pj_str_t	name;	    /**< The domain in the query.		    */
+    pj_uint16_t	type;	    /**< Type of the query (pj_dns_type)	    */
+    pj_uint16_t	dnsclass;   /**< Network class (PJ_DNS_CLASS_IN=1)	    */
+} pj_dns_parsed_query;
 
 
 /**
- * This structure describes a Resource Record parsed from the DNS response.
+ * This structure describes a Resource Record parsed from the DNS packet.
  * All integral values are in host byte order.
  */
 typedef struct pj_dns_parsed_rr
 {
     pj_str_t	 name;	    /**< The domain name which this rec pertains.   */
     pj_uint16_t	 type;	    /**< RR type code.				    */
-    pj_uint16_t	 class_;    /**< Class of data (normally 1, for IN).	    */
+    pj_uint16_t	 dnsclass;  /**< Class of data (PJ_DNS_CLASS_IN=1).	    */
     pj_uint32_t	 ttl;	    /**< Time to live.				    */
     pj_uint16_t	 rdlength;  /**< Resource data length.			    */
-    void	*data;	    /**< Pointer to the raw resource data.	    */
+    void	*data;	    /**< Pointer to the raw resource data, only
+				 when the type is not known. If it is known,
+				 the data will be put in rdata below.	    */
     
     /** For resource types that are recognized/supported by this library,
      *  the parsed resource data will be placed in this rdata union.
@@ -223,21 +280,29 @@ typedef struct pj_dns_parsed_rr
 
 
 /**
- * This structure describes the response parsed from the raw DNS response.
- * Note that all integral values in the parsed response are represented in
+ * This structure describes the parsed repersentation of the raw DNS packet.
+ * Note that all integral values in the parsed packet are represented in
  * host byte order.
  */
-typedef struct pj_dns_parsed_response
+typedef struct pj_dns_parsed_packet
 {
-    pj_dns_hdr	      hdr;  /**< Pointer to DNS hdr, in host byte order */
-    pj_dns_parsed_rr *ans;  /**< Array of DNS RR answer.		*/
-    pj_dns_parsed_rr *ns;   /**< Array of NS record in the answer.	*/
-    pj_dns_parsed_rr *arr;  /**< Array of additional RR answer.		*/
-} pj_dns_parsed_response;
+    pj_dns_hdr		 hdr;	/**< Pointer to DNS hdr, in host byte order */
+    pj_dns_parsed_query	*q;	/**< Array of DNS queries.		    */
+    pj_dns_parsed_rr	*ans;	/**< Array of DNS RR answer.		    */
+    pj_dns_parsed_rr	*ns;	/**< Array of NS record in the answer.	    */
+    pj_dns_parsed_rr	*arr;	/**< Array of additional RR answer.	    */
+} pj_dns_parsed_packet;
 
 
 /**
- * Create DNS query packet to resolve the specified names.
+ * Create DNS query packet to resolve the specified names. This function
+ * can be used to build any types of DNS query, such as A record or DNS SRV
+ * record.
+ *
+ * Application specifies the type of record and the name to be queried,
+ * and the function will build the DNS query packet into the buffer
+ * specified. Once the packet is successfully built, application can send
+ * the packet via TCP or UDP connection.
  *
  * @param packet	The buffer to put the DNS query packet.
  * @param size		On input, it specifies the size of the buffer.
@@ -245,7 +310,7 @@ typedef struct pj_dns_parsed_response
  *			the DNS query packet.
  * @param id		DNS query ID to associate DNS response with the
  *			query.
- * @param qtype		DNS type of record to be queried.
+ * @param qtype		DNS type of record to be queried (see #pj_dns_type).
  * @param name		Name to be queried from the DNS server.
  *
  * @return		PJ_SUCCESS on success, or the appropriate error code.
@@ -253,30 +318,55 @@ typedef struct pj_dns_parsed_response
 PJ_DECL(pj_status_t) pj_dns_make_query(void *packet,
 				       unsigned *size,
 				       pj_uint16_t id,
-				       pj_dns_type qtype,
+				       int qtype,
 				       const pj_str_t *name);
 
 /**
- * Parse raw DNS response packet into DNS response structure.
+ * Parse raw DNS packet into parsed DNS packet structure. This function is
+ * able to parse few DNS resource records such as A record, PTR record,
+ * CNAME record, NS record, and SRV record.
  *
- * @param pool		Pool to allocate memory for the parsed response.
- * @param packet
- * @param size
- * @param p_res
+ * @param pool		Pool to allocate memory for the parsed packet.
+ * @param packet	Pointer to the DNS packet (the TCP/UDP payload of 
+ *			the raw packet).
+ * @param size		The size of the DNS packet.
+ * @param p_res		Pointer to store the resulting parsed packet.
  *
  * @return		PJ_SUCCESS on success, or the appropriate error code.
  */
-PJ_DECL(pj_status_t) pj_dns_parse_response(pj_pool_t *pool,
-					   const void *packet,
-					   unsigned size,
-					   pj_dns_parsed_response **p_res);
+PJ_DECL(pj_status_t) pj_dns_parse_packet(pj_pool_t *pool,
+					 const void *packet,
+					 unsigned size,
+					 pj_dns_parsed_packet **p_res);
+
+/**
+ * Duplicate DNS packet.
+ *
+ * @param pool		The pool to allocate memory for the duplicated packet.
+ * @param p		The DNS packet to be cloned.
+ * @p_dst		Pointer to store the cloned DNS packet.
+ */
+PJ_DECL(void) pj_dns_packet_dup(pj_pool_t *pool,
+				const pj_dns_parsed_packet*p,
+				pj_dns_parsed_packet **p_dst);
+
+
+/**
+ * Utility function to get the type name string of the specified DNS type.
+ *
+ * @param type		DNS type (see #pj_dns_type).
+ *
+ * @return		String name of the type (e.g. "A", "SRV", etc.).
+ */
+PJ_DECL(const char *) pj_dns_get_type_name(int type);
+
 
 /**
- * Dump DNS response to standard log.
+ * Dump DNS packet to standard log.
  *
- * @param res		The DNS response.
+ * @param res		The DNS packet.
  */
-PJ_DECL(void) pj_dns_dump_response(const pj_dns_parsed_response *res);
+PJ_DECL(void) pj_dns_dump_packet(const pj_dns_parsed_packet *res);
 
 
 /**
diff --git a/pjlib-util/include/pjlib-util/errno.h b/pjlib-util/include/pjlib-util/errno.h
index 45e25f97..589fcb63 100644
--- a/pjlib-util/include/pjlib-util/errno.h
+++ b/pjlib-util/include/pjlib-util/errno.h
@@ -1,4 +1,4 @@
-/* $Id */
+/* $Id$ */
 /* 
  * Copyright (C)2003-2006 Benny Prijono <benny@prijono.org>
  *
@@ -107,34 +107,137 @@
  ***********************************************************/
 /**
  * @hideinitializer
- * Outgoing DNS query packet buffer is too small.
+ * DNS query packet buffer is too small.
  * This error occurs when the user supplied buffer for creating DNS
  * query (#pj_dns_make_query() function) is too small.
  */
 #define PJLIB_UTIL_EDNSQRYTOOSMALL  (PJLIB_UTIL_ERRNO_START+40)	/* 320040 */
 /**
  * @hideinitializer
- * Invalid packet length in DNS response.
+ * Invalid DNS packet length.
  * This error occurs when the received DNS response packet does not
  * match all the fields length.
  */
 #define PJLIB_UTIL_EDNSINSIZE	    (PJLIB_UTIL_ERRNO_START+41)	/* 320041 */
 /**
  * @hideinitializer
- * Invalid class in DNS response.
+ * Invalid DNS class.
  * This error occurs when the received DNS response contains network
  * class other than IN (Internet).
  */
 #define PJLIB_UTIL_EDNSINCLASS	    (PJLIB_UTIL_ERRNO_START+42)	/* 320042 */
 /**
  * @hideinitializer
- * Invalid name pointer in DNS response.
+ * Invalid DNS name pointer.
  * This error occurs when parsing the compressed names inside DNS
  * response packet, when the name pointer points to an invalid address
  * or the parsing has triggerred too much recursion.
  */
 #define PJLIB_UTIL_EDNSINNAMEPTR    (PJLIB_UTIL_ERRNO_START+43)	/* 320043 */
+/**
+ * @hideinitializer
+ * Invalid DNS nameserver address. If hostname was specified for nameserver
+ * address, this error means that the function was unable to resolve
+ * the nameserver hostname.
+ */
+#define PJLIB_UTIL_EDNSINNSADDR	    (PJLIB_UTIL_ERRNO_START+44)	/* 320044 */
+/**
+ * @hideinitializer
+ * No nameserver is in DNS resolver. No nameserver is configured in the 
+ * resolver.
+ */
+#define PJLIB_UTIL_EDNSNONS	    (PJLIB_UTIL_ERRNO_START+45)	/* 320045 */
+/**
+ * @hideinitializer
+ * No working DNS nameserver. All nameservers have been queried,
+ * but none was able to serve any DNS requests. These "bad" nameservers
+ * will be re-tested again for "goodness" after some period.
+ */
+#define PJLIB_UTIL_EDNSNOWORKINGNS  (PJLIB_UTIL_ERRNO_START+46)	/* 320046 */
+/**
+ * @hideinitializer
+ * No answer record in the DNS response.
+ */
+#define PJLIB_UTIL_EDNSNOANSWERREC  (PJLIB_UTIL_ERRNO_START+47)	/* 320047 */
+
 
+/* DNS ERRORS MAPPED FROM RCODE: */
+
+/**
+ * Start of error code mapped from DNS RCODE
+ */
+#define PJLIB_UTIL_DNS_RCODE_START  (PJLIB_UTIL_ERRNO_START+50)	/* 320050 */
+
+/**
+ * Map DNS RCODE status into pj_status_t.
+ */
+#define PJ_STATUS_FROM_DNS_RCODE(rcode)	(rcode==0 ? PJ_SUCCESS : \
+					 PJLIB_UTIL_DNS_RCODE_START+rcode)
+/**
+ * @hideinitializer
+ * Format error - The name server was unable to interpret the query.
+ * This corresponds to DNS RCODE 1.
+ */
+#define PJLIB_UTIL_EDNS_FORMERR	    PJ_STATUS_FROM_DNS_RCODE(1)	/* 320051 */
+/**
+ * @hideinitializer
+ * Server failure - The name server was unable to process this query due to a
+ * problem with the name server.
+ * This corresponds to DNS RCODE 2.
+ */
+#define PJLIB_UTIL_EDNS_SERVFAIL    PJ_STATUS_FROM_DNS_RCODE(2)	/* 320052 */
+/**
+ * @hideinitializer
+ * Name Error - Meaningful only for responses from an authoritative name
+ * server, this code signifies that the domain name referenced in the query 
+ * does not exist.
+ * This corresponds to DNS RCODE 3.
+ */
+#define PJLIB_UTIL_EDNS_NXDOMAIN    PJ_STATUS_FROM_DNS_RCODE(3)	/* 320053 */
+/**
+ * @hideinitializer
+ * Not Implemented - The name server does not support the requested kind of 
+ * query.
+ * This corresponds to DNS RCODE 4.
+ */
+#define PJLIB_UTIL_EDNS_NOTIMPL    PJ_STATUS_FROM_DNS_RCODE(4)	/* 320054 */
+/**
+ * @hideinitializer
+ * Refused - The name server refuses to perform the specified operation for
+ * policy reasons.
+ * This corresponds to DNS RCODE 5.
+ */
+#define PJLIB_UTIL_EDNS_REFUSED	    PJ_STATUS_FROM_DNS_RCODE(5)	/* 320055 */
+/**
+ * @hideinitializer
+ * The name exists.
+ * This corresponds to DNS RCODE 6.
+ */
+#define PJLIB_UTIL_EDNS_YXDOMAIN    PJ_STATUS_FROM_DNS_RCODE(6)	/* 320056 */
+/**
+ * @hideinitializer
+ * The RRset (name, type) exists.
+ * This corresponds to DNS RCODE 7.
+ */
+#define PJLIB_UTIL_EDNS_YXRRSET	    PJ_STATUS_FROM_DNS_RCODE(7)	/* 320057 */
+/**
+ * @hideinitializer
+ * The RRset (name, type) does not exist.
+ * This corresponds to DNS RCODE 8.
+ */
+#define PJLIB_UTIL_EDNS_NXRRSET	    PJ_STATUS_FROM_DNS_RCODE(8)	/* 320058 */
+/**
+ * @hideinitializer
+ * The requestor is not authorized to perform this operation.
+ * This corresponds to DNS RCODE 9.
+ */
+#define PJLIB_UTIL_EDNS_NOTAUTH	    PJ_STATUS_FROM_DNS_RCODE(9)	/* 320059 */
+/**
+ * @hideinitializer
+ * The zone specified is not a zone.
+ * This corresponds to DNS RCODE 10.
+ */
+#define PJLIB_UTIL_EDNS_NOTZONE	    PJ_STATUS_FROM_DNS_RCODE(10)/* 320060 */
 
 
 #endif	/* __PJLIB_UTIL_ERRNO_H__ */
diff --git a/pjlib-util/include/pjlib-util/resolver.h b/pjlib-util/include/pjlib-util/resolver.h
new file mode 100644
index 00000000..471b5717
--- /dev/null
+++ b/pjlib-util/include/pjlib-util/resolver.h
@@ -0,0 +1,520 @@
+/* $Id$ */
+/* 
+ * Copyright (C) 2003-2006 Benny Prijono <benny@prijono.org>
+ *
+ * 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 <pjlib-util/dns.h>
+
+
+PJ_BEGIN_DECL
+
+
+/**
+ * @defgroup PJ_DNS_RESOLVER Asynchronous DNS Server Resolution
+ * @ingroup PJLIB_UTIL
+ * @{
+ *
+ * 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. 
+ *
+ * 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)"
+ */
+
+
+/*
+ * CONFIGURATIONS
+ */
+
+/**
+ * Maximum numbers of DNS nameservers that can be configured in resolver.
+ */
+#ifndef PJ_DNS_RESOLVER_MAX_NS
+#   define PJ_DNS_RESOLVER_MAX_NS		    16
+#endif
+
+
+/**
+ * Default retransmission delay, in miliseconds. The combination of 
+ * retransmission delay and count determines the query timeout.
+ *
+ * Default: 2000 (2 seconds, according to RFC 1035)
+ */
+#ifndef PJ_DNS_RESOLVER_QUERY_RETRANSMIT_DELAY
+#   define PJ_DNS_RESOLVER_QUERY_RETRANSMIT_DELAY   2000
+#endif
+
+
+/**
+ * Maximum number of transmissions before timeout is declared for
+ * the query.
+ *
+ * Default: 2
+ */
+#ifndef PJ_DNS_RESOLVER_QUERY_RETRANSMIT_COUNT
+#   define PJ_DNS_RESOLVER_QUERY_RETRANSMIT_COUNT   5
+#endif
+
+
+/**
+ * Maximum life-time of DNS response in the resolver response cache, 
+ * in seconds. If the value is zero, then DNS response caching will be 
+ * disabled.
+ *
+ * Default is 300 seconds (5 minutes).
+ *
+ * @see PJ_DNS_RESOLVER_INVALID_TTL
+ */
+#ifndef PJ_DNS_RESOLVER_MAX_TTL
+#   define PJ_DNS_RESOLVER_MAX_TTL		    (5*60)
+#endif
+
+/**
+ * The life-time of invalid DNS response in the resolver response cache.
+ * An invalid DNS response is a response without an answer. These 
+ * responses can be put in the cache too to minimize message round-trip.
+ *
+ * Default: 0 (which means, invalid DNS response will not be cached).
+ *
+ * @see PJ_DNS_RESOLVER_MAX_TTL
+ */
+#ifndef PJ_DNS_RESOLVER_INVALID_TTL
+#   define PJ_DNS_RESOLVER_INVALID_TTL		    60
+#endif
+
+/**
+ * The interval on which nameservers which are known to be good to be 
+ * probed again to determine whether they are still good. Note that
+ * this applies to both active nameserver (the one currently being used)
+ * and idle nameservers (good nameservers that are not currently selected).
+ * The probing to query the "goodness" of nameservers involves sending
+ * the same query to multiple servers, so it's probably not a good idea
+ * to send this probing too often.
+ *
+ * Default: 600 (ten minutes)
+ *
+ * @see PJ_DNS_RESOLVER_BAD_NS_TTL
+ */
+#ifndef PJ_DNS_RESOLVER_GOOD_NS_TTL
+#   define PJ_DNS_RESOLVER_GOOD_NS_TTL		    (10*60)
+#endif
+
+/**
+ * The interval on which nameservers which known to be bad to be probed
+ * again to determine whether it is still bad.
+ *
+ * Default: 600 (ten minutes)
+ *
+ * @see PJ_DNS_RESOLVER_GOOD_NS_TTL
+ */
+#ifndef PJ_DNS_RESOLVER_BAD_NS_TTL
+#   define PJ_DNS_RESOLVER_BAD_NS_TTL		    (1*60)
+#endif
+
+
+/**
+ * Maximum size of UDP packet. RFC 1035 states that maximum size of
+ * DNS packet carried over UDP is 512 bytes.
+ *
+ * Default: 512 byes
+ */
+#ifndef PJ_DNS_RESOLVER_MAX_UDP_SIZE
+#   define PJ_DNS_RESOLVER_MAX_UDP_SIZE		    512
+#endif
+
+
+/**
+ * Size of memory pool allocated for each individual DNS response cache.
+ * This value here should be more or less the same as maximum UDP packet
+ * size (PJ_DNS_RESOLVER_MAX_UDP_SIZE), since the DNS replicator function
+ * (#pj_dns_packet_dup()) is also capable of performing name compressions.
+ *
+ * Default: 512 (as a broad guidance, 400 is good for 4 SRV entries).
+ */
+#ifndef PJ_DNS_RESOLVER_RES_BUF_SIZE
+#   define PJ_DNS_RESOLVER_RES_BUF_SIZE		    512
+#endif
+
+
+
+/**
+ * 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.    */
+} pj_dns_settings;
+
+
+/**
+ * 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);
+
+
+/**
+ * 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__ */
+