/* $Id: resolver.c 5534 2017-01-19 07:41:25Z nanang $ */
/* 
 * Copyright (C) 2008-2011 Teluu Inc. (http://www.teluu.com)
 * Copyright (C) 2003-2008 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 
 */
#include <pjlib-util/resolver.h>
#include <pjlib-util/errno.h>
#include <pj/compat/socket.h>
#include <pj/assert.h>
#include <pj/ctype.h>
#include <pj/except.h>
#include <pj/hash.h>
#include <pj/ioqueue.h>
#include <pj/log.h>
#include <pj/os.h>
#include <pj/pool.h>
#include <pj/pool_buf.h>
#include <pj/rand.h>
#include <pj/string.h>
#include <pj/sock.h>
#include <pj/timer.h>


#define THIS_FILE	    "resolver.c"


/* Check that maximum DNS nameservers is not too large. 
 * This has got todo with the datatype to index the nameserver in the query.
 */
#if PJ_DNS_RESOLVER_MAX_NS > 256
#   error "PJ_DNS_RESOLVER_MAX_NS is too large (max=256)"
#endif


#define RES_HASH_TABLE_SIZE 127		/**< Hash table size (must be 2^n-1 */
#define PORT		    53		/**< Default NS port.		    */
#define Q_HASH_TABLE_SIZE   127		/**< Query hash table size	    */
#define TIMER_SIZE	    127		/**< Initial number of timers.	    */
#define MAX_FD		    3		/**< Maximum internal sockets.	    */

#define RES_BUF_SZ	    PJ_DNS_RESOLVER_RES_BUF_SIZE
#define UDPSZ		    PJ_DNS_RESOLVER_MAX_UDP_SIZE
#define TMP_SZ		    PJ_DNS_RESOLVER_TMP_BUF_SIZE


/* Nameserver state */
enum ns_state
{
    STATE_PROBING,
    STATE_ACTIVE,
    STATE_BAD,
};

static const char *state_names[3] =
{
    "Probing",
    "Active",
    "Bad"
};


/* 
 * Each nameserver entry.
 * A name server is identified by its socket address (IP and port).
 * Each NS will have a flag to indicate whether it's properly functioning.
 */
struct nameserver
{
    pj_sockaddr     addr;		/**< Server address.		    */

    enum ns_state   state;		/**< Nameserver state.		    */
    pj_time_val	    state_expiry;	/**< Time set next state.	    */
    pj_time_val	    rt_delay;		/**< Response time.		    */
    

    /* For calculating rt_delay: */
    pj_uint16_t	    q_id;		/**< Query ID.			    */
    pj_time_val	    sent_time;		/**< Time this query is sent.	    */
};


/* Child query list head 
 * See comments on pj_dns_async_query below.
 */
struct query_head
{
    PJ_DECL_LIST_MEMBER(pj_dns_async_query);
};


/* Key to look for outstanding query and/or cached response */
struct res_key
{
    pj_uint16_t		     qtype;		    /**< Query type.	    */
    char		     name[PJ_MAX_HOSTNAME]; /**< Name being queried */
};


/* 
 * This represents each asynchronous query entry.
 * This entry will be put in two hash tables, the first one keyed on the DNS 
 * transaction ID to match response with the query, and the second one keyed
 * on "res_key" structure above to match a new request against outstanding 
 * requests.
 *
 * An asynchronous entry may have child entries; child entries are subsequent
 * queries to the same resource while there is pending query on the same
 * DNS resource name and type. When a query has child entries, once the
 * response is received (or error occurs), the response will trigger callback
 * invocations for all childs entries.
 *
 * Note: when application cancels the query, the callback member will be
 *       set to NULL, but for simplicity, the query will be let running.
 */
struct pj_dns_async_query
{
    PJ_DECL_LIST_MEMBER(pj_dns_async_query);	/**< List member.	    */

    pj_dns_resolver	*resolver;	/**< The resolver instance.	    */
    pj_uint16_t		 id;		/**< Transaction ID.		    */

    unsigned		 transmit_cnt;	/**< Number of transmissions.	    */

    struct res_key	 key;		/**< Key to index this query.	    */
    pj_hash_entry_buf	 hbufid;	/**< Hash buffer 1		    */
    pj_hash_entry_buf	 hbufkey;	/**< Hash buffer 2		    */
    pj_timer_entry	 timer_entry;	/**< Timer to manage timeouts	    */
    unsigned		 options;	/**< Query options.		    */
    void		*user_data;	/**< Application data.		    */
    pj_dns_callback	*cb;		/**< Callback to be called.	    */
    struct query_head	 child_head;	/**< Child queries list head.	    */
};


/* This structure is used to keep cached response entry.
 * The cache is a hash table keyed on "res_key" structure above.
 */
struct cached_res
{
    PJ_DECL_LIST_MEMBER(struct cached_res);

    pj_pool_t		    *pool;	    /**< Cache's pool.		    */
    struct res_key	     key;	    /**< Resource key.		    */
    pj_hash_entry_buf	     hbuf;	    /**< Hash buffer		    */
    pj_time_val		     expiry_time;   /**< Expiration time.	    */
    pj_dns_parsed_packet    *pkt;	    /**< The response packet.	    */
    unsigned		     ref_cnt;	    /**< Reference counter.	    */
};


/* Resolver entry */
struct pj_dns_resolver
{
    pj_str_t		 name;		/**< Resolver instance name for id. */

    /* Internals */
    pj_pool_t		*pool;		/**< Internal pool.		    */
    pj_mutex_t		*mutex;		/**< Mutex protection.		    */
    pj_bool_t		 own_timer;	/**< Do we own timer?		    */
    pj_timer_heap_t	*timer;		/**< Timer instance.		    */
    pj_bool_t		 own_ioqueue;	/**< Do we own ioqueue?		    */
    pj_ioqueue_t	*ioqueue;	/**< Ioqueue instance.		    */
    char		 tmp_pool[TMP_SZ];/**< Temporary pool buffer.	    */

    /* Socket */
    pj_sock_t		 udp_sock;	/**< UDP socket.		    */
    pj_ioqueue_key_t	*udp_key;	/**< UDP socket ioqueue key.	    */
    unsigned char	 udp_rx_pkt[UDPSZ];/**< UDP receive buffer.	    */
    unsigned char	 udp_tx_pkt[UDPSZ];/**< UDP transmit buffer.	    */
    pj_ioqueue_op_key_t	 udp_op_rx_key;	/**< UDP read operation key.	    */
    pj_ioqueue_op_key_t	 udp_op_tx_key;	/**< UDP write operation key.	    */
    pj_sockaddr		 udp_src_addr;	/**< Source address of packet	    */
    int			 udp_addr_len;	/**< Source address length.	    */

#if PJ_HAS_IPV6
    /* IPv6 socket */
    pj_sock_t		 udp6_sock;	/**< UDP socket.		    */
    pj_ioqueue_key_t	*udp6_key;	/**< UDP socket ioqueue key.	    */
    unsigned char	 udp6_rx_pkt[UDPSZ];/**< UDP receive buffer.	    */
    //unsigned char	 udp6_tx_pkt[UDPSZ];/**< UDP transmit buffer.	    */
    pj_ioqueue_op_key_t	 udp6_op_rx_key;/**< UDP read operation key.	    */
    pj_ioqueue_op_key_t	 udp6_op_tx_key;/**< UDP write operation key.	    */
    pj_sockaddr		 udp6_src_addr;	/**< Source address of packet	    */
    int			 udp6_addr_len;	/**< Source address length.	    */
#endif

    /* Settings */
    pj_dns_settings	 settings;	/**< Resolver settings.		    */

    /* Nameservers */
    unsigned		 ns_count;	/**< Number of name servers.	    */
    struct nameserver	 ns[PJ_DNS_RESOLVER_MAX_NS];	/**< Array of NS.   */

    /* Last DNS transaction ID used. */
    pj_uint16_t		 last_id;

    /* Hash table for cached response */
    pj_hash_table_t	*hrescache;	/**< Cached response in hash table  */

    /* Pending asynchronous query, hashed by transaction ID. */
    pj_hash_table_t	*hquerybyid;

    /* Pending asynchronous query, hashed by "res_key" */
    pj_hash_table_t	*hquerybyres;

    /* Query entries free list */
    struct query_head	 query_free_nodes;
};


/* Callback from ioqueue when packet is received */
static void on_read_complete(pj_ioqueue_key_t *key, 
                             pj_ioqueue_op_key_t *op_key, 
                             pj_ssize_t bytes_read);

/* Callback to be called when query has timed out */
static void on_timeout( pj_timer_heap_t *timer_heap,
			struct pj_timer_entry *entry);

/* Select which nameserver to use */
static pj_status_t select_nameservers(pj_dns_resolver *resolver,
				      unsigned *count,
				      unsigned servers[]);


/* Close UDP socket */
static void close_sock(pj_dns_resolver *resv)
{
    /* Close existing socket */
    if (resv->udp_key != NULL) {
	pj_ioqueue_unregister(resv->udp_key);
	resv->udp_key = NULL;
	resv->udp_sock = PJ_INVALID_SOCKET;
    } else if (resv->udp_sock != PJ_INVALID_SOCKET) {
	pj_sock_close(resv->udp_sock);
	resv->udp_sock = PJ_INVALID_SOCKET;
    }

#if PJ_HAS_IPV6
    if (resv->udp6_key != NULL) {
	pj_ioqueue_unregister(resv->udp6_key);
	resv->udp6_key = NULL;
	resv->udp6_sock = PJ_INVALID_SOCKET;
    } else if (resv->udp6_sock != PJ_INVALID_SOCKET) {
	pj_sock_close(resv->udp6_sock);
	resv->udp6_sock = PJ_INVALID_SOCKET;
    }
#endif
}


/* Initialize UDP socket */
static pj_status_t init_sock(pj_dns_resolver *resv)
{
    pj_ioqueue_callback socket_cb;
    pj_sockaddr bound_addr;
    pj_ssize_t rx_pkt_size;
    pj_status_t status;

    /* Create the UDP socket */
    status = pj_sock_socket(pj_AF_INET(), pj_SOCK_DGRAM(), 0, &resv->udp_sock);
    if (status != PJ_SUCCESS)
	return status;

    /* Bind to any address/port */
    status = pj_sock_bind_in(resv->udp_sock, 0, 0);
    if (status != PJ_SUCCESS)
	return status;

    /* Register to ioqueue */
    pj_bzero(&socket_cb, sizeof(socket_cb));
    socket_cb.on_read_complete = &on_read_complete;
    status = pj_ioqueue_register_sock(resv->pool, resv->ioqueue,
				      resv->udp_sock, resv, &socket_cb,
				      &resv->udp_key);
    if (status != PJ_SUCCESS)
	return status;

    pj_ioqueue_op_key_init(&resv->udp_op_rx_key, sizeof(resv->udp_op_rx_key));
    pj_ioqueue_op_key_init(&resv->udp_op_tx_key, sizeof(resv->udp_op_tx_key));

    /* Start asynchronous read to the UDP socket */
    rx_pkt_size = sizeof(resv->udp_rx_pkt);
    resv->udp_addr_len = sizeof(resv->udp_src_addr);
    status = pj_ioqueue_recvfrom(resv->udp_key, &resv->udp_op_rx_key,
				 resv->udp_rx_pkt, &rx_pkt_size,
				 PJ_IOQUEUE_ALWAYS_ASYNC,
				 &resv->udp_src_addr, &resv->udp_addr_len);
    if (status != PJ_EPENDING)
	return status;


#if PJ_HAS_IPV6
    /* Also setup IPv6 socket */

    /* Create the UDP socket */
    status = pj_sock_socket(pj_AF_INET6(), pj_SOCK_DGRAM(), 0,
			    &resv->udp6_sock);
    if (status != PJ_SUCCESS) {
	/* Skip IPv6 socket on system without IPv6 (see ticket #1953) */
	if (status == PJ_STATUS_FROM_OS(OSERR_EAFNOSUPPORT)) {
	    PJ_LOG(3,(resv->name.ptr,
		      "System does not support IPv6, resolver will "
		      "ignore any IPv6 nameservers"));
	    return PJ_SUCCESS;
	}
	return status;
    }

    /* Bind to any address/port */
    pj_sockaddr_init(pj_AF_INET6(), &bound_addr, NULL, 0);
    status = pj_sock_bind(resv->udp6_sock, &bound_addr,
			  pj_sockaddr_get_len(&bound_addr));
    if (status != PJ_SUCCESS)
	return status;

    /* Register to ioqueue */
    pj_bzero(&socket_cb, sizeof(socket_cb));
    socket_cb.on_read_complete = &on_read_complete;
    status = pj_ioqueue_register_sock(resv->pool, resv->ioqueue,
				      resv->udp6_sock, resv, &socket_cb,
				      &resv->udp6_key);
    if (status != PJ_SUCCESS)
	return status;

    pj_ioqueue_op_key_init(&resv->udp6_op_rx_key,
			   sizeof(resv->udp6_op_rx_key));
    pj_ioqueue_op_key_init(&resv->udp6_op_tx_key,
			   sizeof(resv->udp6_op_tx_key));

    /* Start asynchronous read to the UDP socket */
    rx_pkt_size = sizeof(resv->udp6_rx_pkt);
    resv->udp6_addr_len = sizeof(resv->udp6_src_addr);
    status = pj_ioqueue_recvfrom(resv->udp6_key, &resv->udp6_op_rx_key,
				 resv->udp6_rx_pkt, &rx_pkt_size,
				 PJ_IOQUEUE_ALWAYS_ASYNC,
				 &resv->udp6_src_addr, &resv->udp6_addr_len);
    if (status != PJ_EPENDING)
	return status;
#else
    PJ_UNUSED_ARG(bound_addr);
#endif

    return PJ_SUCCESS;
}


/* Initialize DNS settings with default values */
PJ_DEF(void) pj_dns_settings_default(pj_dns_settings *s)
{
    pj_bzero(s, sizeof(pj_dns_settings));
    s->qretr_delay = PJ_DNS_RESOLVER_QUERY_RETRANSMIT_DELAY;
    s->qretr_count = PJ_DNS_RESOLVER_QUERY_RETRANSMIT_COUNT;
    s->cache_max_ttl = PJ_DNS_RESOLVER_MAX_TTL;
    s->good_ns_ttl = PJ_DNS_RESOLVER_GOOD_NS_TTL;
    s->bad_ns_ttl = PJ_DNS_RESOLVER_BAD_NS_TTL;
}


/*
 * Create the resolver.
 */
PJ_DEF(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)
{
    pj_pool_t *pool;
    pj_dns_resolver *resv;
    pj_status_t status;

    /* Sanity check */
    PJ_ASSERT_RETURN(pf && p_resolver, PJ_EINVAL);

    if (name == NULL)
	name = THIS_FILE;

    /* Create and initialize resolver instance */
    pool = pj_pool_create(pf, name, 4000, 4000, NULL);
    if (!pool)
	return PJ_ENOMEM;

    /* Create pool and name */
    resv = PJ_POOL_ZALLOC_T(pool, struct pj_dns_resolver);
    resv->pool = pool;
    resv->udp_sock = PJ_INVALID_SOCKET;
    pj_strdup2_with_null(pool, &resv->name, name);
    
    /* Create the mutex */
    status = pj_mutex_create_recursive(pool, name, &resv->mutex);
    if (status != PJ_SUCCESS)
	goto on_error;

    /* Timer, ioqueue, and settings */
    resv->timer = timer;
    resv->ioqueue = ioqueue;
    resv->last_id = 1;

    pj_dns_settings_default(&resv->settings);
    resv->settings.options = options;

    /* Create the timer heap if one is not specified */
    if (resv->timer == NULL) {
	status = pj_timer_heap_create(pool, TIMER_SIZE, &resv->timer);
	if (status != PJ_SUCCESS)
	    goto on_error;
    }

    /* Create the ioqueue if one is not specified */
    if (resv->ioqueue == NULL) {
	status = pj_ioqueue_create(pool, MAX_FD, &resv->ioqueue);
	if (status != PJ_SUCCESS)
	    goto on_error;
    }

    /* Response cache hash table */
    resv->hrescache = pj_hash_create(pool, RES_HASH_TABLE_SIZE);

    /* Query hash table and free list. */
    resv->hquerybyid = pj_hash_create(pool, Q_HASH_TABLE_SIZE);
    resv->hquerybyres = pj_hash_create(pool, Q_HASH_TABLE_SIZE);
    pj_list_init(&resv->query_free_nodes);

    /* Initialize the UDP socket */
    status = init_sock(resv);
    if (status != PJ_SUCCESS)
	goto on_error;

    /* Looks like everything is okay */
    *p_resolver = resv;
    return PJ_SUCCESS;

on_error:
    pj_dns_resolver_destroy(resv, PJ_FALSE);
    return status;
}


/*
 * Destroy DNS resolver instance.
 */
PJ_DEF(pj_status_t) pj_dns_resolver_destroy( pj_dns_resolver *resolver,
					     pj_bool_t notify)
{
    pj_hash_iterator_t it_buf, *it;
    PJ_ASSERT_RETURN(resolver, PJ_EINVAL);

    if (notify) {
	/*
	 * Notify pending queries if requested.
	 */
	it = pj_hash_first(resolver->hquerybyid, &it_buf);
	while (it) {
	    pj_dns_async_query *q = (pj_dns_async_query *)
	    			    pj_hash_this(resolver->hquerybyid, it);
	    pj_dns_async_query *cq;
	    if (q->cb)
		(*q->cb)(q->user_data, PJ_ECANCELLED, NULL);

	    cq = q->child_head.next;
	    while (cq != (pj_dns_async_query*)&q->child_head) {
		if (cq->cb)
		    (*cq->cb)(cq->user_data, PJ_ECANCELLED, NULL);
		cq = cq->next;
	    }
	    it = pj_hash_next(resolver->hquerybyid, it);
	}
    }

    /* Destroy cached entries */
    it = pj_hash_first(resolver->hrescache, &it_buf);
    while (it) {
	struct cached_res *cache;

	cache = (struct cached_res*) pj_hash_this(resolver->hrescache, it);
	pj_hash_set(NULL, resolver->hrescache, &cache->key, 
		    sizeof(cache->key), 0, NULL);
	pj_pool_release(cache->pool);

	it = pj_hash_first(resolver->hrescache, &it_buf);
    }

    if (resolver->own_timer && resolver->timer) {
	pj_timer_heap_destroy(resolver->timer);
	resolver->timer = NULL;
    }

    close_sock(resolver);

    if (resolver->own_ioqueue && resolver->ioqueue) {
	pj_ioqueue_destroy(resolver->ioqueue);
	resolver->ioqueue = NULL;
    }

    if (resolver->mutex) {
	pj_mutex_destroy(resolver->mutex);
	resolver->mutex = NULL;
    }

    pj_pool_safe_release(&resolver->pool);

    return PJ_SUCCESS;
}



/*
 * Configure name servers for the DNS resolver. 
 */
PJ_DEF(pj_status_t) pj_dns_resolver_set_ns( pj_dns_resolver *resolver,
					    unsigned count,
					    const pj_str_t servers[],
					    const pj_uint16_t ports[])
{
    unsigned i;
    pj_time_val now;
    pj_status_t status;

    PJ_ASSERT_RETURN(resolver && count && servers, PJ_EINVAL);
    PJ_ASSERT_RETURN(count < PJ_DNS_RESOLVER_MAX_NS, PJ_EINVAL);

    pj_mutex_lock(resolver->mutex);

    if (count > PJ_DNS_RESOLVER_MAX_NS)
	count = PJ_DNS_RESOLVER_MAX_NS;

    resolver->ns_count = 0;
    pj_bzero(resolver->ns, sizeof(resolver->ns));

    pj_gettimeofday(&now);

    for (i=0; i<count; ++i) {
	struct nameserver *ns = &resolver->ns[i];

	status = pj_sockaddr_init(pj_AF_INET(), &ns->addr, &servers[i], 
				  (pj_uint16_t)(ports ? ports[i] : PORT));
	if (status != PJ_SUCCESS)
	    status = pj_sockaddr_init(pj_AF_INET6(), &ns->addr, &servers[i], 
				      (pj_uint16_t)(ports ? ports[i] : PORT));
	if (status != PJ_SUCCESS) {
	    pj_mutex_unlock(resolver->mutex);
	    return PJLIB_UTIL_EDNSINNSADDR;
	}

	ns->state = STATE_ACTIVE;
	ns->state_expiry = now;
	ns->rt_delay.sec = 10;
    }
    
    resolver->ns_count = count;

    pj_mutex_unlock(resolver->mutex);
    return PJ_SUCCESS;
}



/*
 * Modify the resolver settings.
 */
PJ_DEF(pj_status_t) pj_dns_resolver_set_settings(pj_dns_resolver *resolver,
						 const pj_dns_settings *st)
{
    PJ_ASSERT_RETURN(resolver && st, PJ_EINVAL);

    pj_mutex_lock(resolver->mutex);
    pj_memcpy(&resolver->settings, st, sizeof(*st));
    pj_mutex_unlock(resolver->mutex);
    return PJ_SUCCESS;
}


/*
 * Get the resolver current settings.
 */
PJ_DEF(pj_status_t) pj_dns_resolver_get_settings( pj_dns_resolver *resolver,
						  pj_dns_settings *st)
{
    PJ_ASSERT_RETURN(resolver && st, PJ_EINVAL);

    pj_mutex_lock(resolver->mutex);
    pj_memcpy(st, &resolver->settings, sizeof(*st));
    pj_mutex_unlock(resolver->mutex);
    return PJ_SUCCESS;
}


/*
 * Poll for events from the resolver. 
 */
PJ_DEF(void) pj_dns_resolver_handle_events(pj_dns_resolver *resolver,
					   const pj_time_val *timeout)
{
    PJ_ASSERT_ON_FAIL(resolver, return);

    pj_mutex_lock(resolver->mutex);
    pj_timer_heap_poll(resolver->timer, NULL);
    pj_mutex_unlock(resolver->mutex);

    pj_ioqueue_poll(resolver->ioqueue, timeout);
}


/* Get one query node from the free node, if any, or allocate 
 * a new one.
 */
static pj_dns_async_query *alloc_qnode(pj_dns_resolver *resolver,
				       unsigned options,
				       void *user_data,
				       pj_dns_callback *cb)
{
    pj_dns_async_query *q;

    /* Merge query options with resolver options */
    options |= resolver->settings.options;

    if (!pj_list_empty(&resolver->query_free_nodes)) {
	q = resolver->query_free_nodes.next;
	pj_list_erase(q);
	pj_bzero(q, sizeof(*q));
    } else {
	q = PJ_POOL_ZALLOC_T(resolver->pool, pj_dns_async_query);
    }

    /* Init query */
    q->resolver = resolver;
    q->options = options;
    q->user_data = user_data;
    q->cb = cb;
    pj_list_init(&q->child_head);

    return q;
}


/*
 * Transmit query.
 */
static pj_status_t transmit_query(pj_dns_resolver *resolver,
				  pj_dns_async_query *q)
{
    unsigned pkt_size;
    unsigned i, server_cnt, send_cnt;
    unsigned servers[PJ_DNS_RESOLVER_MAX_NS];
    pj_time_val now;
    pj_str_t name;
    pj_time_val delay;
    pj_status_t status;

    /* Select which nameserver(s) to send requests to. */
    server_cnt = PJ_ARRAY_SIZE(servers);
    status = select_nameservers(resolver, &server_cnt, servers);
    if (status != PJ_SUCCESS) {
	return status;
    }

    if (server_cnt == 0) {
	return PJLIB_UTIL_EDNSNOWORKINGNS;
    }

    /* Start retransmit/timeout timer for the query */
    pj_assert(q->timer_entry.id == 0);
    q->timer_entry.id = 1;
    q->timer_entry.user_data = q;
    q->timer_entry.cb = &on_timeout;

    delay.sec = 0;
    delay.msec = resolver->settings.qretr_delay;
    pj_time_val_normalize(&delay);
    status = pj_timer_heap_schedule(resolver->timer, &q->timer_entry, &delay);
    if (status != PJ_SUCCESS) {
	return status;
    }

    /* Check if the socket is available for sending */
    if (pj_ioqueue_is_pending(resolver->udp_key, &resolver->udp_op_tx_key)
#if PJ_HAS_IPV6
	|| (resolver->udp6_key &&
	    pj_ioqueue_is_pending(resolver->udp6_key,
				  &resolver->udp6_op_tx_key))
#endif
	)
    {
	++q->transmit_cnt;
	PJ_LOG(4,(resolver->name.ptr,
		  "Socket busy in transmitting DNS %s query for %s%s",
		  pj_dns_get_type_name(q->key.qtype),
		  q->key.name,
		  (q->transmit_cnt < resolver->settings.qretr_count?
		   ", will try again later":"")));
	return PJ_SUCCESS;
    }

    /* Create DNS query packet */
    pkt_size = sizeof(resolver->udp_tx_pkt);
    name = pj_str(q->key.name);
    status = pj_dns_make_query(resolver->udp_tx_pkt, &pkt_size,
			       q->id, q->key.qtype, &name);
    if (status != PJ_SUCCESS) {
	pj_timer_heap_cancel(resolver->timer, &q->timer_entry);
	return status;
    }

    /* Get current time. */
    pj_gettimeofday(&now);

    /* Send the packet to name servers */
    send_cnt = 0;
    for (i=0; i<server_cnt; ++i) {
        char addr[PJ_INET6_ADDRSTRLEN];
	pj_ssize_t sent  = (pj_ssize_t) pkt_size;
	struct nameserver *ns = &resolver->ns[servers[i]];

	if (ns->addr.addr.sa_family == pj_AF_INET()) {
	    status = pj_ioqueue_sendto(resolver->udp_key,
				       &resolver->udp_op_tx_key,
				       resolver->udp_tx_pkt, &sent, 0,
				       &ns->addr,
				       pj_sockaddr_get_len(&ns->addr));
	    if (status == PJ_SUCCESS || status == PJ_EPENDING)
		send_cnt++;
	}
#if PJ_HAS_IPV6
	else if (resolver->udp6_key) {
	    status = pj_ioqueue_sendto(resolver->udp6_key,
				       &resolver->udp6_op_tx_key,
				       resolver->udp_tx_pkt, &sent, 0,
				       &ns->addr,
				       pj_sockaddr_get_len(&ns->addr));
	    if (status == PJ_SUCCESS || status == PJ_EPENDING)
		send_cnt++;
	}
#endif
	else {
	    continue;
	}

	PJ_PERROR(4,(resolver->name.ptr, status,
		  "%s %d bytes to NS %d (%s:%d): DNS %s query for %s",
		  (q->transmit_cnt==0? "Transmitting":"Re-transmitting"),
		  (int)pkt_size, servers[i],
		  pj_sockaddr_print(&ns->addr, addr, sizeof(addr), 2),
		  pj_sockaddr_get_port(&ns->addr),
		  pj_dns_get_type_name(q->key.qtype), 
		  q->key.name));

	if (ns->q_id == 0) {
	    ns->q_id = q->id;
	    ns->sent_time = now;
	}
    }

    if (send_cnt == 0)
	return PJLIB_UTIL_EDNSNOWORKINGNS;

    ++q->transmit_cnt;

    return PJ_SUCCESS;
}


/*
 * Initialize resource key for hash table lookup.
 */
static void init_res_key(struct res_key *key, int type, const pj_str_t *name)
{
    unsigned i;
    pj_size_t len;
    char *dst = key->name;
    const char *src = name->ptr;

    pj_bzero(key, sizeof(struct res_key));
    key->qtype = (pj_uint16_t)type;

    len = name->slen;
    if (len > PJ_MAX_HOSTNAME) len = PJ_MAX_HOSTNAME;

    /* Copy key, in lowercase */
    for (i=0; i<len; ++i) {
	*dst++ = (char)pj_tolower(*src++);
    }
}


/* Allocate new cache entry */
static struct cached_res *alloc_entry(pj_dns_resolver *resolver)
{
    pj_pool_t *pool;
    struct cached_res *cache;

    pool = pj_pool_create(resolver->pool->factory, "dnscache",
			  RES_BUF_SZ, 256, NULL);
    cache = PJ_POOL_ZALLOC_T(pool, struct cached_res);
    cache->pool = pool;
    cache->ref_cnt = 1;

    return cache;
}

/* Re-allocate cache entry, to free cached packet */
static void reset_entry(struct cached_res **p_cached)
{
    pj_pool_t *pool;
    struct cached_res *cache = *p_cached;
    unsigned ref_cnt;

    pool = cache->pool;
    ref_cnt = cache->ref_cnt;

    pj_pool_reset(pool);

    cache = PJ_POOL_ZALLOC_T(pool, struct cached_res);
    cache->pool = pool;
    cache->ref_cnt = ref_cnt;
    *p_cached = cache;
}

/* Put unused/expired cached entry to the free list */
static void free_entry(pj_dns_resolver *resolver, struct cached_res *cache)
{
    PJ_UNUSED_ARG(resolver);
    pj_pool_release(cache->pool);
}


/*
 * Create and start asynchronous DNS query for a single resource.
 */
PJ_DEF(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)
{
    pj_time_val now;
    struct res_key key;
    struct cached_res *cache;
    pj_dns_async_query *q, *p_q = NULL;
    pj_uint32_t hval;
    pj_status_t status = PJ_SUCCESS;

    /* Validate arguments */
    PJ_ASSERT_RETURN(resolver && name && type, PJ_EINVAL);

    /* Check name is not too long. */
    PJ_ASSERT_RETURN(name->slen>0 && name->slen < PJ_MAX_HOSTNAME,
		     PJ_ENAMETOOLONG);

    /* Check type */
    PJ_ASSERT_RETURN(type > 0 && type < 0xFFFF, PJ_EINVAL);

    /* Build resource key for looking up hash tables */
    init_res_key(&key, type, name);

    /* Start working with the resolver */
    pj_mutex_lock(resolver->mutex);

    /* Get current time. */
    pj_gettimeofday(&now);

    /* First, check if we have cached response for the specified name/type,
     * and the cached entry has not expired.
     */
    hval = 0;
    cache = (struct cached_res *) pj_hash_get(resolver->hrescache, &key, 
    					      sizeof(key), &hval);
    if (cache) {
	/* We've found a cached entry. */

	/* Check for expiration */
	if (PJ_TIME_VAL_GT(cache->expiry_time, now)) {

	    /* Log */
	    PJ_LOG(5,(resolver->name.ptr, 
		      "Picked up DNS %s record for %.*s from cache, ttl=%d",
		      pj_dns_get_type_name(type),
		      (int)name->slen, name->ptr,
		      (int)(cache->expiry_time.sec - now.sec)));

	    /* Map DNS Rcode in the response into PJLIB status name space */
	    status = PJ_DNS_GET_RCODE(cache->pkt->hdr.flags);
	    status = PJ_STATUS_FROM_DNS_RCODE(status);

	    /* Workaround for deadlock problem. Need to increment the cache's
	     * ref counter first before releasing mutex, so the cache won't be
	     * destroyed by other thread while in callback.
	     */
	    cache->ref_cnt++;
	    pj_mutex_unlock(resolver->mutex);

	    /* This cached response is still valid. Just return this
	     * response to caller.
	     */
	    if (cb) {
		(*cb)(user_data, status, cache->pkt);
	    }

	    /* Done. No host resolution is necessary */
	    pj_mutex_lock(resolver->mutex);

	    /* Decrement the ref counter. Also check if it is time to free
	     * the cache (as it has been expired).
	     */
	    cache->ref_cnt--;
	    if (cache->ref_cnt <= 0)
		free_entry(resolver, cache);

	    /* Must return PJ_SUCCESS */
	    status = PJ_SUCCESS;

	    /*
	     * We cannot write to *p_query after calling cb because what
	     * p_query points to may have been freed by cb.
             * Refer to ticket #1974.
	     */
	    pj_mutex_unlock(resolver->mutex);
	    return status;
	}

	/* At this point, we have a cached entry, but this entry has expired.
	 * Remove this entry from the cached list.
	 */
	pj_hash_set(NULL, resolver->hrescache, &key, sizeof(key), 0, NULL);

	/* Also free the cache, if it is not being used (by callback). */
	cache->ref_cnt--;
	if (cache->ref_cnt <= 0)
	    free_entry(resolver, cache);

	/* Must continue with creating a query now */
    }

    /* Next, check if we have pending query on the same resource */
    q = (pj_dns_async_query *) pj_hash_get(resolver->hquerybyres, &key, 
    					   sizeof(key), NULL);
    if (q) {
	/* Yes, there's another pending query to the same key.
	 * Just create a new child query and add this query to
	 * pending query's child queries.
	 */
	pj_dns_async_query *nq;

	nq = alloc_qnode(resolver, options, user_data, cb);
	pj_list_push_back(&q->child_head, nq);

	/* Done. This child query will be notified once the "parent"
	 * query completes.
	 */
	p_q = nq;
	status = PJ_SUCCESS;
	goto on_return;
    } 

    /* There's no pending query to the same key, initiate a new one. */
    q = alloc_qnode(resolver, options, user_data, cb);

    /* Save the ID and key */
    /* TODO: dnsext-forgery-resilient: randomize id for security */
    q->id = resolver->last_id++;
    if (resolver->last_id == 0)
	resolver->last_id = 1;
    pj_memcpy(&q->key, &key, sizeof(struct res_key));

    /* Send the query */
    status = transmit_query(resolver, q);
    if (status != PJ_SUCCESS) {
	pj_list_push_back(&resolver->query_free_nodes, q);
	goto on_return;
    }

    /* Add query entry to the hash tables */
    pj_hash_set_np(resolver->hquerybyid, &q->id, sizeof(q->id), 
		   0, q->hbufid, q);
    pj_hash_set_np(resolver->hquerybyres, &q->key, sizeof(q->key),
		   0, q->hbufkey, q);

    p_q = q;

on_return:
    if (p_query)
	*p_query = p_q;

    pj_mutex_unlock(resolver->mutex);
    return status;
}


/*
 * Cancel a pending query.
 */
PJ_DEF(pj_status_t) pj_dns_resolver_cancel_query(pj_dns_async_query *query,
						 pj_bool_t notify)
{
    pj_dns_callback *cb;

    PJ_ASSERT_RETURN(query, PJ_EINVAL);

    pj_mutex_lock(query->resolver->mutex);

    cb = query->cb;
    query->cb = NULL;

    if (notify)
	(*cb)(query->user_data, PJ_ECANCELLED, NULL);

    pj_mutex_unlock(query->resolver->mutex);
    return PJ_SUCCESS;
}


/* 
 * DNS response containing A packet. 
 */
PJ_DEF(pj_status_t) pj_dns_parse_a_response(const pj_dns_parsed_packet *pkt,
					    pj_dns_a_record *rec)
{
    enum { MAX_SEARCH = 20 };
    pj_str_t hostname, alias = {NULL, 0}, *resname;
    pj_size_t bufstart = 0;
    pj_size_t bufleft = sizeof(rec->buf_);
    unsigned i, ansidx, search_cnt=0;

    PJ_ASSERT_RETURN(pkt && rec, PJ_EINVAL);

    /* Init the record */
    pj_bzero(rec, sizeof(pj_dns_a_record));

    /* Return error if there's error in the packet. */
    if (PJ_DNS_GET_RCODE(pkt->hdr.flags))
	return PJ_STATUS_FROM_DNS_RCODE(PJ_DNS_GET_RCODE(pkt->hdr.flags));

    /* Return error if there's no query section */
    if (pkt->hdr.qdcount == 0)
	return PJLIB_UTIL_EDNSINANSWER;

    /* Return error if there's no answer */
    if (pkt->hdr.anscount == 0)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    /* Get the hostname from the query. */
    hostname = pkt->q[0].name;

    /* Copy hostname to the record */
    if (hostname.slen > (int)bufleft) {
	return PJ_ENAMETOOLONG;
    }

    pj_memcpy(&rec->buf_[bufstart], hostname.ptr, hostname.slen);
    rec->name.ptr = &rec->buf_[bufstart];
    rec->name.slen = hostname.slen;

    bufstart += hostname.slen;
    bufleft -= hostname.slen;

    /* Find the first RR which name matches the hostname */
    for (ansidx=0; ansidx < pkt->hdr.anscount; ++ansidx) {
	if (pj_stricmp(&pkt->ans[ansidx].name, &hostname)==0)
	    break;
    }

    if (ansidx == pkt->hdr.anscount)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    resname = &hostname;

    /* Keep following CNAME records. */
    while (pkt->ans[ansidx].type == PJ_DNS_TYPE_CNAME &&
	   search_cnt++ < MAX_SEARCH)
    {
	resname = &pkt->ans[ansidx].rdata.cname.name;

	if (!alias.slen)
	    alias = *resname;

	for (i=0; i < pkt->hdr.anscount; ++i) {
	    if (pj_stricmp(resname, &pkt->ans[i].name)==0) {
		break;
	    }
	}

	if (i==pkt->hdr.anscount)
	    return PJLIB_UTIL_EDNSNOANSWERREC;

	ansidx = i;
    }

    if (search_cnt >= MAX_SEARCH)
	return PJLIB_UTIL_EDNSINANSWER;

    if (pkt->ans[ansidx].type != PJ_DNS_TYPE_A)
	return PJLIB_UTIL_EDNSINANSWER;

    /* Copy alias to the record, if present. */
    if (alias.slen) {
	if (alias.slen > (int)bufleft)
	    return PJ_ENAMETOOLONG;

	pj_memcpy(&rec->buf_[bufstart], alias.ptr, alias.slen);
	rec->alias.ptr = &rec->buf_[bufstart];
	rec->alias.slen = alias.slen;

	bufstart += alias.slen;
	bufleft -= alias.slen;
    }

    /* Get the IP addresses. */
    for (i=0; i < pkt->hdr.anscount; ++i) {
	if (pkt->ans[i].type == PJ_DNS_TYPE_A &&
	    pj_stricmp(&pkt->ans[i].name, resname)==0 &&
	    rec->addr_count < PJ_DNS_MAX_IP_IN_A_REC)
	{
	    rec->addr[rec->addr_count++].s_addr =
		pkt->ans[i].rdata.a.ip_addr.s_addr;
	}
    }

    if (rec->addr_count == 0)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    return PJ_SUCCESS;
}


/* 
 * DNS response containing A and/or AAAA packet. 
 */
PJ_DEF(pj_status_t) pj_dns_parse_addr_response(
					    const pj_dns_parsed_packet *pkt,
					    pj_dns_addr_record *rec)
{
    enum { MAX_SEARCH = 20 };
    pj_str_t hostname, alias = {NULL, 0}, *resname;
    pj_size_t bufstart = 0;
    pj_size_t bufleft;
    unsigned i, ansidx, cnt=0;

    PJ_ASSERT_RETURN(pkt && rec, PJ_EINVAL);

    /* Init the record */
    pj_bzero(rec, sizeof(*rec));

    bufleft = sizeof(rec->buf_);

    /* Return error if there's error in the packet. */
    if (PJ_DNS_GET_RCODE(pkt->hdr.flags))
	return PJ_STATUS_FROM_DNS_RCODE(PJ_DNS_GET_RCODE(pkt->hdr.flags));

    /* Return error if there's no query section */
    if (pkt->hdr.qdcount == 0)
	return PJLIB_UTIL_EDNSINANSWER;

    /* Return error if there's no answer */
    if (pkt->hdr.anscount == 0)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    /* Get the hostname from the query. */
    hostname = pkt->q[0].name;

    /* Copy hostname to the record */
    if (hostname.slen > (int)bufleft) {
	return PJ_ENAMETOOLONG;
    }

    pj_memcpy(&rec->buf_[bufstart], hostname.ptr, hostname.slen);
    rec->name.ptr = &rec->buf_[bufstart];
    rec->name.slen = hostname.slen;

    bufstart += hostname.slen;
    bufleft -= hostname.slen;

    /* Find the first RR which name matches the hostname. */
    for (ansidx=0; ansidx < pkt->hdr.anscount; ++ansidx) {
	if (pj_stricmp(&pkt->ans[ansidx].name, &hostname)==0)
	    break;
    }

    if (ansidx == pkt->hdr.anscount)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    resname = &hostname;

    /* Keep following CNAME records. */
    while (pkt->ans[ansidx].type == PJ_DNS_TYPE_CNAME &&
	   cnt++ < MAX_SEARCH)
    {
	resname = &pkt->ans[ansidx].rdata.cname.name;

	if (!alias.slen)
	    alias = *resname;

	for (i=0; i < pkt->hdr.anscount; ++i) {
	    if (pj_stricmp(resname, &pkt->ans[i].name)==0)
		break;
	}

	if (i==pkt->hdr.anscount)
	    return PJLIB_UTIL_EDNSNOANSWERREC;

	ansidx = i;
    }

    if (cnt >= MAX_SEARCH)
	return PJLIB_UTIL_EDNSINANSWER;

    if (pkt->ans[ansidx].type != PJ_DNS_TYPE_A &&
	pkt->ans[ansidx].type != PJ_DNS_TYPE_AAAA)
    {
	return PJLIB_UTIL_EDNSINANSWER;
    }

    /* Copy alias to the record, if present. */
    if (alias.slen) {
	if (alias.slen > (int)bufleft)
	    return PJ_ENAMETOOLONG;

	pj_memcpy(&rec->buf_[bufstart], alias.ptr, alias.slen);
	rec->alias.ptr = &rec->buf_[bufstart];
	rec->alias.slen = alias.slen;

	bufstart += alias.slen;
	bufleft -= alias.slen;
    }

    /* Get the IP addresses. */
    cnt = 0;
    for (i=0; i < pkt->hdr.anscount && cnt < PJ_DNS_MAX_IP_IN_A_REC ; ++i) {
	if ((pkt->ans[i].type == PJ_DNS_TYPE_A ||
	     pkt->ans[i].type == PJ_DNS_TYPE_AAAA) &&
	    pj_stricmp(&pkt->ans[i].name, resname)==0)
	{
	    if (pkt->ans[i].type == PJ_DNS_TYPE_A) {
		rec->addr[cnt].af = pj_AF_INET();
		rec->addr[cnt].ip.v4 = pkt->ans[i].rdata.a.ip_addr;
	    } else {
		rec->addr[cnt].af = pj_AF_INET6();
		rec->addr[cnt].ip.v6 = pkt->ans[i].rdata.aaaa.ip_addr;
	    }
	    ++cnt;
	}
    }
    rec->addr_count = cnt;

    if (cnt == 0)
	return PJLIB_UTIL_EDNSNOANSWERREC;

    return PJ_SUCCESS;
}


/* Set nameserver state */
static void set_nameserver_state(pj_dns_resolver *resolver,
				 unsigned index,
				 enum ns_state state,
				 const pj_time_val *now)
{
    struct nameserver *ns = &resolver->ns[index];
    enum ns_state old_state = ns->state;
    char addr[PJ_INET6_ADDRSTRLEN];

    ns->state = state;
    ns->state_expiry = *now;

    if (state == STATE_PROBING)
	ns->state_expiry.sec += ((resolver->settings.qretr_count + 2) *
				 resolver->settings.qretr_delay) / 1000;
    else if (state == STATE_ACTIVE)
	ns->state_expiry.sec += resolver->settings.good_ns_ttl;
    else
	ns->state_expiry.sec += resolver->settings.bad_ns_ttl;

    PJ_LOG(5, (resolver->name.ptr, "Nameserver %s:%d state changed %s --> %s",
	       pj_sockaddr_print(&ns->addr, addr, sizeof(addr), 2),
	       pj_sockaddr_get_port(&ns->addr),
	       state_names[old_state], state_names[state]));
}


/* Select which nameserver(s) to use. Note this may return multiple
 * name servers. The algorithm to select which nameservers to be
 * sent the request to is as follows:
 *  - select the first nameserver that is known to be good for the
 *    last PJ_DNS_RESOLVER_GOOD_NS_TTL interval.
 *  - for all NSes, if last_known_good >= PJ_DNS_RESOLVER_GOOD_NS_TTL, 
 *    include the NS to re-check again that the server is still good,
 *    unless the NS is known to be bad in the last PJ_DNS_RESOLVER_BAD_NS_TTL
 *    interval.
 *  - for all NSes, if last_known_bad >= PJ_DNS_RESOLVER_BAD_NS_TTL, 
 *    also include the NS to re-check again that the server is still bad.
 */
static pj_status_t select_nameservers(pj_dns_resolver *resolver,
				      unsigned *count,
				      unsigned servers[])
{
    unsigned i, max_count=*count;
    int min;
    pj_time_val now;

    pj_assert(max_count > 0);

    *count = 0;
    servers[0] = 0xFFFF;

    /* Check that nameservers are configured. */
    if (resolver->ns_count == 0)
	return PJLIB_UTIL_EDNSNONS;

    pj_gettimeofday(&now);

    /* Select one Active nameserver with best response time. */
    for (min=-1, i=0; i<resolver->ns_count; ++i) {
	struct nameserver *ns = &resolver->ns[i];

	if (ns->state != STATE_ACTIVE)
	    continue;

	if (min == -1)
	    min = i;
	else if (PJ_TIME_VAL_LT(ns->rt_delay, resolver->ns[min].rt_delay))
	    min = i;
    }
    if (min != -1) {
	servers[0] = min;
	++(*count);
    }

    /* Scan nameservers. */
    for (i=0; i<resolver->ns_count && *count < max_count; ++i) {
	struct nameserver *ns = &resolver->ns[i];

	if (PJ_TIME_VAL_LTE(ns->state_expiry, now)) {
	    if (ns->state == STATE_PROBING) {
		set_nameserver_state(resolver, i, STATE_BAD, &now);
	    } else {
		set_nameserver_state(resolver, i, STATE_PROBING, &now);
		if ((int)i != min) {
		    servers[*count] = i;
		    ++(*count);
		}
	    }
	} else if (ns->state == STATE_PROBING && (int)i != min) {
	    servers[*count] = i;
	    ++(*count);
	}
    }

    return PJ_SUCCESS;
}


/* Update name server status */
static void report_nameserver_status(pj_dns_resolver *resolver,
				     const pj_sockaddr *ns_addr,
				     const pj_dns_parsed_packet *pkt)
{
    unsigned i;
    int rcode;
    pj_uint32_t q_id;
    pj_time_val now;
    pj_bool_t is_good;

    /* Only mark nameserver as "bad" if it returned non-parseable response or
     * it returned the following status codes
     */
    if (pkt) {
	rcode = PJ_DNS_GET_RCODE(pkt->hdr.flags);
	q_id = pkt->hdr.id;
    } else {
	rcode = 0;
	q_id = (pj_uint32_t)-1;
    }

    /* Some nameserver is reported to respond with PJ_DNS_RCODE_SERVFAIL for
     * missing AAAA record, and the standard doesn't seem to specify that
     * SERVFAIL should prevent the server to be contacted again for other
     * queries. So let's not mark nameserver as bad for SERVFAIL response.
     */
    if (!pkt || /* rcode == PJ_DNS_RCODE_SERVFAIL || */
	        rcode == PJ_DNS_RCODE_REFUSED ||
	        rcode == PJ_DNS_RCODE_NOTAUTH) 
    {
	is_good = PJ_FALSE;
    } else {
	is_good = PJ_TRUE;
    }


    /* Mark time */
    pj_gettimeofday(&now);

    /* Recheck all nameservers. */
    for (i=0; i<resolver->ns_count; ++i) {
	struct nameserver *ns = &resolver->ns[i];

	if (pj_sockaddr_cmp(&ns->addr, ns_addr) == 0) {
	    if (q_id == ns->q_id) {
		/* Calculate response time */
		pj_time_val rt = now;
		PJ_TIME_VAL_SUB(rt, ns->sent_time);
		ns->rt_delay = rt;
		ns->q_id = 0;
	    }
	    set_nameserver_state(resolver, i, 
				 (is_good ? STATE_ACTIVE : STATE_BAD), &now);
	    break;
	}
    }
}


/* Update response cache */
static void update_res_cache(pj_dns_resolver *resolver,
			     const struct res_key *key,
			     pj_status_t status,
			     pj_bool_t set_expiry,
			     const pj_dns_parsed_packet *pkt)
{
    struct cached_res *cache;
    pj_uint32_t hval=0, ttl;

    /* If status is unsuccessful, clear the same entry from the cache */
    if (status != PJ_SUCCESS) {
	cache = (struct cached_res *) pj_hash_get(resolver->hrescache, key, 
						  sizeof(*key), &hval);
	/* Remove the entry before releasing its pool (see ticket #1710) */
	pj_hash_set(NULL, resolver->hrescache, key, sizeof(*key), hval, NULL);
	
	/* Free the entry */
	if (cache && --cache->ref_cnt <= 0)
	    free_entry(resolver, cache);
    }


    /* Calculate expiration time. */
    if (set_expiry) {
	if (pkt->hdr.anscount == 0 || status != PJ_SUCCESS) {
	    /* If we don't have answers for the name, then give a different
	     * ttl value (note: PJ_DNS_RESOLVER_INVALID_TTL may be zero, 
	     * which means that invalid names won't be kept in the cache)
	     */
	    ttl = PJ_DNS_RESOLVER_INVALID_TTL;

	} else {
	    /* Otherwise get the minimum TTL from the answers */
	    unsigned i;
	    ttl = 0xFFFFFFFF;
	    for (i=0; i<pkt->hdr.anscount; ++i) {
		if (pkt->ans[i].ttl < ttl)
		    ttl = pkt->ans[i].ttl;
	    }
	}
    } else {
	ttl = 0xFFFFFFFF;
    }

    /* Apply maximum TTL */
    if (ttl > resolver->settings.cache_max_ttl)
	ttl = resolver->settings.cache_max_ttl;

    /* Get a cache response entry */
    cache = (struct cached_res *) pj_hash_get(resolver->hrescache, key,
    					      sizeof(*key), &hval);

    /* If TTL is zero, clear the same entry in the hash table */
    if (ttl == 0) {
	/* Remove the entry before releasing its pool (see ticket #1710) */
	pj_hash_set(NULL, resolver->hrescache, key, sizeof(*key), hval, NULL);

	/* Free the entry */
	if (cache && --cache->ref_cnt <= 0)
	    free_entry(resolver, cache);
	return;
    }

    if (cache == NULL) {
	cache = alloc_entry(resolver);
    } else {
	/* Remove the entry before resetting its pool (see ticket #1710) */
	pj_hash_set(NULL, resolver->hrescache, key, sizeof(*key), hval, NULL);

	if (cache->ref_cnt > 1) {
	    /* When cache entry is being used by callback (to app),
	     * just decrement ref_cnt so it will be freed after
	     * the callback returns and allocate new entry.
	     */
	    cache->ref_cnt--;
	    cache = alloc_entry(resolver);
	} else {
	    /* Reset cache to avoid bloated cache pool */
	    reset_entry(&cache);
	}
    }

    /* Duplicate the packet.
     * We don't need to keep the NS and AR sections from the packet,
     * so exclude from duplication. We do need to keep the Query
     * section since DNS A parser needs the query section to know
     * the name being requested.
     */
    pj_dns_packet_dup(cache->pool, pkt, 
		      PJ_DNS_NO_NS | PJ_DNS_NO_AR,
		      &cache->pkt);

    /* Calculate expiration time */
    if (set_expiry) {
	pj_gettimeofday(&cache->expiry_time);
	cache->expiry_time.sec += ttl;
    } else {
	cache->expiry_time.sec = 0x7FFFFFFFL;
	cache->expiry_time.msec = 0;
    }

    /* Copy key to the cached response */
    pj_memcpy(&cache->key, key, sizeof(*key));

    /* Update the hash table */
    pj_hash_set_np(resolver->hrescache, &cache->key, sizeof(*key), hval,
		   cache->hbuf, cache);

}


/* Callback to be called when query has timed out */
static void on_timeout( pj_timer_heap_t *timer_heap,
			struct pj_timer_entry *entry)
{
    pj_dns_resolver *resolver;
    pj_dns_async_query *q, *cq;
    pj_status_t status;

    PJ_UNUSED_ARG(timer_heap);

    q = (pj_dns_async_query *) entry->user_data;
    resolver = q->resolver;

    pj_mutex_lock(resolver->mutex);

    /* Recheck that this query is still pending, since there is a slight
     * possibility of race condition (timer elapsed while at the same time
     * response arrives)
     */
    if (pj_hash_get(resolver->hquerybyid, &q->id, sizeof(q->id), NULL)==NULL) {
	/* Yeah, this query is done. */
	pj_mutex_unlock(resolver->mutex);
	return;
    }

    /* Invalidate id. */
    q->timer_entry.id = 0;

    /* Check to see if we should retransmit instead of time out */
    if (q->transmit_cnt < resolver->settings.qretr_count) {
	status = transmit_query(resolver, q);
	if (status == PJ_SUCCESS) {
	    pj_mutex_unlock(resolver->mutex);
	    return;
	} else {
	    /* Error occurs */
	    char errmsg[PJ_ERR_MSG_SIZE];

	    pj_strerror(status, errmsg, sizeof(errmsg));
	    PJ_LOG(4,(resolver->name.ptr,
		      "Error transmitting request: %s", errmsg));

	    /* Let it fallback to timeout section below */
	}
    }

    /* Clear hash table entries */
    pj_hash_set(NULL, resolver->hquerybyid, &q->id, sizeof(q->id), 0, NULL);
    pj_hash_set(NULL, resolver->hquerybyres, &q->key, sizeof(q->key), 0, NULL);

    /* Workaround for deadlock problem in #1565 (similar to #1108) */
    pj_mutex_unlock(resolver->mutex);

    /* Call application callback, if any. */
    if (q->cb)
	(*q->cb)(q->user_data, PJ_ETIMEDOUT, NULL);

    /* Call application callback for child queries. */
    cq = q->child_head.next;
    while (cq != (void*)&q->child_head) {
	if (cq->cb)
	    (*cq->cb)(cq->user_data, PJ_ETIMEDOUT, NULL);
	cq = cq->next;
    }

    /* Workaround for deadlock problem in #1565 (similar to #1108) */
    pj_mutex_lock(resolver->mutex);

    /* Clear data */
    q->timer_entry.id = 0;
    q->user_data = NULL;

    /* Put child entries into recycle list */
    cq = q->child_head.next;
    while (cq != (void*)&q->child_head) {
	pj_dns_async_query *next = cq->next;
	pj_list_push_back(&resolver->query_free_nodes, cq);
	cq = next;
    }

    /* Put query entry into recycle list */
    pj_list_push_back(&resolver->query_free_nodes, q);

    pj_mutex_unlock(resolver->mutex);
}


/* Callback from ioqueue when packet is received */
static void on_read_complete(pj_ioqueue_key_t *key, 
                             pj_ioqueue_op_key_t *op_key, 
                             pj_ssize_t bytes_read)
{
    pj_dns_resolver *resolver;
    pj_pool_t *pool = NULL;
    pj_dns_parsed_packet *dns_pkt;
    pj_dns_async_query *q;
    char addr[PJ_INET6_ADDRSTRLEN];
    pj_sockaddr *src_addr;
    int *src_addr_len;
    unsigned char *rx_pkt;
    pj_ssize_t rx_pkt_size;
    pj_status_t status;
    PJ_USE_EXCEPTION;


    resolver = (pj_dns_resolver *) pj_ioqueue_get_user_data(key);
    pj_assert(resolver);

#if PJ_HAS_IPV6
    if (key == resolver->udp6_key) {
	src_addr = &resolver->udp6_src_addr;
	src_addr_len = &resolver->udp6_addr_len;
	rx_pkt = resolver->udp6_rx_pkt;
	rx_pkt_size = sizeof(resolver->udp6_rx_pkt);
    } else 
#endif
    {
	src_addr = &resolver->udp_src_addr;
	src_addr_len = &resolver->udp_addr_len;
	rx_pkt = resolver->udp_rx_pkt;
	rx_pkt_size = sizeof(resolver->udp_rx_pkt);
    }

    pj_mutex_lock(resolver->mutex);


    /* Check for errors */
    if (bytes_read < 0) {
	char errmsg[PJ_ERR_MSG_SIZE];

	status = (pj_status_t)-bytes_read;
	pj_strerror(status, errmsg, sizeof(errmsg));
	PJ_LOG(4,(resolver->name.ptr, "DNS resolver read error: %s", errmsg));

	goto read_next_packet;
    }

    PJ_LOG(5,(resolver->name.ptr, 
	      "Received %d bytes DNS response from %s:%d",
	      (int)bytes_read, 
	      pj_sockaddr_print(src_addr, addr, sizeof(addr), 2),
	      pj_sockaddr_get_port(src_addr)));


    /* Check for zero packet */
    if (bytes_read == 0)
	goto read_next_packet;

    /* Create temporary pool from a fixed buffer */
    pool = pj_pool_create_on_buf("restmp", resolver->tmp_pool, 
				 sizeof(resolver->tmp_pool));

    /* Parse DNS response */
    status = -1;
    dns_pkt = NULL;
    PJ_TRY {
	status = pj_dns_parse_packet(pool, rx_pkt, 
				     (unsigned)bytes_read, &dns_pkt);
    }
    PJ_CATCH_ANY {
	status = PJ_ENOMEM;
    }
    PJ_END;

    /* Update nameserver status */
    report_nameserver_status(resolver, src_addr, dns_pkt);

    /* Handle parse error */
    if (status != PJ_SUCCESS) {
	char errmsg[PJ_ERR_MSG_SIZE];

	pj_strerror(status, errmsg, sizeof(errmsg));
	PJ_LOG(3,(resolver->name.ptr, 
		  "Error parsing DNS response from %s:%d: %s", 
		  pj_sockaddr_print(src_addr, addr, sizeof(addr), 2),
		  pj_sockaddr_get_port(src_addr),
		  errmsg));
	goto read_next_packet;
    }

    /* Find the query based on the transaction ID */
    q = (pj_dns_async_query*) 
        pj_hash_get(resolver->hquerybyid, &dns_pkt->hdr.id,
		    sizeof(dns_pkt->hdr.id), NULL);
    if (!q) {
	PJ_LOG(5,(resolver->name.ptr, 
		  "DNS response from %s:%d id=%d discarded",
		  pj_sockaddr_print(src_addr, addr, sizeof(addr), 2),
		  pj_sockaddr_get_port(src_addr),
		  (unsigned)dns_pkt->hdr.id));
	goto read_next_packet;
    }

    /* Map DNS Rcode in the response into PJLIB status name space */
    status = PJ_STATUS_FROM_DNS_RCODE(PJ_DNS_GET_RCODE(dns_pkt->hdr.flags));

    /* Cancel query timeout timer. */
    pj_assert(q->timer_entry.id != 0);
    pj_timer_heap_cancel(resolver->timer, &q->timer_entry);
    q->timer_entry.id = 0;

    /* Clear hash table entries */
    pj_hash_set(NULL, resolver->hquerybyid, &q->id, sizeof(q->id), 0, NULL);
    pj_hash_set(NULL, resolver->hquerybyres, &q->key, sizeof(q->key), 0, NULL);

    /* Workaround for deadlock problem in #1108 */
    pj_mutex_unlock(resolver->mutex);

    /* Notify applications first, to allow application to modify the 
     * record before it is saved to the hash table.
     */
    if (q->cb)
	(*q->cb)(q->user_data, status, dns_pkt);

    /* If query has subqueries, notify subqueries's application callback */
    if (!pj_list_empty(&q->child_head)) {
	pj_dns_async_query *child_q;

	child_q = q->child_head.next;
	while (child_q != (pj_dns_async_query*)&q->child_head) {
	    if (child_q->cb)
		(*child_q->cb)(child_q->user_data, status, dns_pkt);
	    child_q = child_q->next;
	}
    }

    /* Workaround for deadlock problem in #1108 */
    pj_mutex_lock(resolver->mutex);

    /* Save/update response cache. */
    update_res_cache(resolver, &q->key, status, PJ_TRUE, dns_pkt);
    
    /* Recycle query objects, starting with the child queries */
    if (!pj_list_empty(&q->child_head)) {
	pj_dns_async_query *child_q;

	child_q = q->child_head.next;
	while (child_q != (pj_dns_async_query*)&q->child_head) {
	    pj_dns_async_query *next = child_q->next;
	    pj_list_erase(child_q);
	    pj_list_push_back(&resolver->query_free_nodes, child_q);
	    child_q = next;
	}
    }
    pj_list_push_back(&resolver->query_free_nodes, q);

read_next_packet:
    if (pool) {
	/* needed just in case PJ_HAS_POOL_ALT_API is set */
	pj_pool_release(pool);
    }

    status = pj_ioqueue_recvfrom(key, op_key, rx_pkt, &rx_pkt_size,
				 PJ_IOQUEUE_ALWAYS_ASYNC,
				 src_addr, src_addr_len);

    if (status != PJ_EPENDING) {
	char errmsg[PJ_ERR_MSG_SIZE];

	pj_strerror(status, errmsg, sizeof(errmsg));	
	PJ_LOG(4,(resolver->name.ptr, "DNS resolver ioqueue read error: %s",
		  errmsg));

	pj_assert(!"Unhandled error");
    }

    pj_mutex_unlock(resolver->mutex);
}


/*
 * 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.
 */
PJ_DEF(pj_status_t) pj_dns_resolver_add_entry( pj_dns_resolver *resolver,
					       const pj_dns_parsed_packet *pkt,
					       pj_bool_t set_ttl)
{
    struct res_key key;

    /* Sanity check */
    PJ_ASSERT_RETURN(resolver && pkt, PJ_EINVAL);

    /* Packet must be a DNS response */
    PJ_ASSERT_RETURN(PJ_DNS_GET_QR(pkt->hdr.flags) & 1, PJ_EINVAL);

    /* Make sure there are answers in the packet */
    PJ_ASSERT_RETURN((pkt->hdr.anscount && pkt->ans) ||
		      (pkt->hdr.qdcount && pkt->q),
		     PJLIB_UTIL_EDNSNOANSWERREC);

    pj_mutex_lock(resolver->mutex);

    /* Build resource key for looking up hash tables */
    pj_bzero(&key, sizeof(struct res_key));
    if (pkt->hdr.anscount) {
	/* Make sure name is not too long. */
	PJ_ASSERT_RETURN(pkt->ans[0].name.slen < PJ_MAX_HOSTNAME, 
			 PJ_ENAMETOOLONG);

	init_res_key(&key, pkt->ans[0].type, &pkt->ans[0].name);

    } else {
	/* Make sure name is not too long. */
	PJ_ASSERT_RETURN(pkt->q[0].name.slen < PJ_MAX_HOSTNAME, 
			 PJ_ENAMETOOLONG);

	init_res_key(&key, pkt->q[0].type, &pkt->q[0].name);
    }

    /* Insert entry. */
    update_res_cache(resolver, &key, PJ_SUCCESS, set_ttl, pkt);

    pj_mutex_unlock(resolver->mutex);

    return PJ_SUCCESS;
}


/*
 * Get the total number of response in the response cache.
 */
PJ_DEF(unsigned) pj_dns_resolver_get_cached_count(pj_dns_resolver *resolver)
{
    unsigned count;

    PJ_ASSERT_RETURN(resolver, 0);

    pj_mutex_lock(resolver->mutex);
    count = pj_hash_count(resolver->hrescache);
    pj_mutex_unlock(resolver->mutex);

    return count;
}


/*
 * Dump resolver state to the log.
 */
PJ_DEF(void) pj_dns_resolver_dump(pj_dns_resolver *resolver,
				  pj_bool_t detail)
{
#if PJ_LOG_MAX_LEVEL >= 3
    unsigned i;
    pj_time_val now;

    pj_mutex_lock(resolver->mutex);

    pj_gettimeofday(&now);

    PJ_LOG(3,(resolver->name.ptr, " Dumping resolver state:"));

    PJ_LOG(3,(resolver->name.ptr, "  Name servers:"));
    for (i=0; i<resolver->ns_count; ++i) {
	char addr[PJ_INET6_ADDRSTRLEN];
	struct nameserver *ns = &resolver->ns[i];

	PJ_LOG(3,(resolver->name.ptr,
		  "   NS %d: %s:%d (state=%s until %ds, rtt=%d ms)",
		  i,
		  pj_sockaddr_print(&ns->addr, addr, sizeof(addr), 2),
		  pj_sockaddr_get_port(&ns->addr),
		  state_names[ns->state],
		  ns->state_expiry.sec - now.sec,
		  PJ_TIME_VAL_MSEC(ns->rt_delay)));
    }

    PJ_LOG(3,(resolver->name.ptr, "  Nb. of cached responses: %u",
	      pj_hash_count(resolver->hrescache)));
    if (detail) {
	pj_hash_iterator_t itbuf, *it;
	it = pj_hash_first(resolver->hrescache, &itbuf);
	while (it) {
	    struct cached_res *cache;
	    cache = (struct cached_res*)pj_hash_this(resolver->hrescache, it);
	    PJ_LOG(3,(resolver->name.ptr, 
		      "   Type %s: %s",
		      pj_dns_get_type_name(cache->key.qtype), 
		      cache->key.name));
	    it = pj_hash_next(resolver->hrescache, it);
	}
    }
    PJ_LOG(3,(resolver->name.ptr, "  Nb. of pending queries: %u (%u)",
	      pj_hash_count(resolver->hquerybyid),
	      pj_hash_count(resolver->hquerybyres)));
    if (detail) {
	pj_hash_iterator_t itbuf, *it;
	it = pj_hash_first(resolver->hquerybyid, &itbuf);
	while (it) {
	    struct pj_dns_async_query *q;
	    q = (pj_dns_async_query*) pj_hash_this(resolver->hquerybyid, it);
	    PJ_LOG(3,(resolver->name.ptr, 
		      "   Type %s: %s",
		      pj_dns_get_type_name(q->key.qtype), 
		      q->key.name));
	    it = pj_hash_next(resolver->hquerybyid, it);
	}
    }
    PJ_LOG(3,(resolver->name.ptr, "  Nb. of pending query free nodes: %u",
	      pj_list_size(&resolver->query_free_nodes)));
    PJ_LOG(3,(resolver->name.ptr, "  Nb. of timer entries: %u",
	      pj_timer_heap_count(resolver->timer)));
    PJ_LOG(3,(resolver->name.ptr, "  Pool capacity: %d, used size: %d",
	      pj_pool_get_capacity(resolver->pool),
	      pj_pool_get_used_size(resolver->pool)));

    pj_mutex_unlock(resolver->mutex);
#endif
}

