/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 * SPDX-License-Identifier: curl
 *
 ***************************************************************************/
#include "curl_setup.h"

#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#ifdef HAVE_NETINET_IN6_H
#include <netinet/in6.h>
#endif
#ifdef HAVE_NETDB_H
#include <netdb.h>
#endif
#ifdef HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif
#ifdef __VMS
#include <in.h>
#include <inet.h>
#endif

#include "urldata.h"
#include "curl_addrinfo.h"
#include "curl_share.h"
#include "curl_trc.h"
#include "dnscache.h"
#include "hash.h"
#include "hostip.h"
#include "httpsrr.h"
#include "progress.h"
#include "rand.h"
#include "strcase.h"
#include "curlx/inet_ntop.h"
#include "curlx/inet_pton.h"
#include "curlx/strcopy.h"
#include "curlx/strparse.h"

#define MAX_HOSTCACHE_LEN (255 + 7) /* max FQDN + colon + port number + zero */

#define MAX_DNS_CACHE_SIZE 29999

static void dnscache_entry_free(struct Curl_dns_entry *dns)
{
  Curl_freeaddrinfo(dns->addr);
#ifdef USE_HTTPSRR
  Curl_httpsrr_destroy(dns->hinfo);
#endif
  curlx_free(dns);
}

/*
 * Create a hostcache id string for the provided host + port, to be used by
 * the DNS caching. Without alloc. Return length of the id string.
 */
static size_t create_dnscache_id(char type, const char *name,
                                 size_t nlen, /* 0 or actual name length */
                                 uint16_t port,
                                 char *buf, size_t buflen)
{
  size_t len = nlen ? nlen : strlen(name);
  DEBUGASSERT(buflen >= MAX_HOSTCACHE_LEN);
  if(len > (buflen - 8))
    len = buflen - 8;
  /* store and lower case the name */
  buf[0] = type;
  Curl_strntolower(buf + 1, name, len);
  return curl_msnprintf(&buf[len + 1], 7, ":%u", port) + len + 1;
}

struct dnscache_prune_data {
  struct curltime now;
  timediff_t oldest_ms; /* oldest time in cache not pruned. */
  timediff_t max_age_ms;
};

/*
 * This function is set as a callback to be called for every entry in the DNS
 * cache when we want to prune old unused entries.
 *
 * Returning non-zero means remove the entry, return 0 to keep it in the
 * cache.
 */
static int dnscache_entry_is_stale(void *datap, void *hc)
{
  struct dnscache_prune_data *prune = (struct dnscache_prune_data *)datap;
  struct Curl_dns_entry *dns = (struct Curl_dns_entry *)hc;

  if(dns->timestamp.tv_sec || dns->timestamp.tv_usec) {
    /* get age in milliseconds */
    timediff_t age = curlx_ptimediff_ms(&prune->now, &dns->timestamp);
    if(!dns->addr)
      age *= 2; /* negative entries age twice as fast */
    if(age >= prune->max_age_ms)
      return TRUE;
    if(age > prune->oldest_ms)
      prune->oldest_ms = age;
  }
  return FALSE;
}

/*
 * Prune the DNS cache. This assumes that a lock has already been taken.
 * Returns the 'age' of the oldest still kept entry - in milliseconds.
 */
static timediff_t dnscache_prune(struct Curl_hash *hostcache,
                                 timediff_t cache_timeout_ms,
                                 struct curltime now)
{
  struct dnscache_prune_data user;

  user.max_age_ms = cache_timeout_ms;
  user.now = now;
  user.oldest_ms = 0;

  Curl_hash_clean_with_criterium(hostcache,
                                 (void *)&user,
                                 dnscache_entry_is_stale);

  return user.oldest_ms;
}

static struct Curl_dnscache *dnscache_get(struct Curl_easy *data)
{
  if(data->share && data->share->specifier & (1 << CURL_LOCK_DATA_DNS))
    return &data->share->dnscache;
  if(data->multi)
    return &data->multi->dnscache;
  return NULL;
}

static void dnscache_lock(struct Curl_easy *data,
                          struct Curl_dnscache *dnscache)
{
  if(data->share && dnscache == &data->share->dnscache)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);
}

static void dnscache_unlock(struct Curl_easy *data,
                            struct Curl_dnscache *dnscache)
{
  if(data->share && dnscache == &data->share->dnscache)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
}

/*
 * Library-wide function for pruning the DNS cache. This function takes and
 * returns the appropriate locks.
 */
void Curl_dnscache_prune(struct Curl_easy *data)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  /* the timeout may be set -1 (forever) */
  timediff_t timeout_ms = data->set.dns_cache_timeout_ms;

  if(!dnscache || (timeout_ms == -1))
    /* NULL hostcache means we cannot do it */
    return;

  dnscache_lock(data, dnscache);

  do {
    /* Remove outdated and unused entries from the hostcache */
    timediff_t oldest_ms =
      dnscache_prune(&dnscache->entries, timeout_ms, *Curl_pgrs_now(data));

    if(Curl_hash_count(&dnscache->entries) > MAX_DNS_CACHE_SIZE)
      /* prune the ones over half this age */
      timeout_ms = oldest_ms / 2;
    else
      break;

    /* if the cache size is still too big, use the oldest age as new prune
       limit */
  } while(timeout_ms);

  dnscache_unlock(data, dnscache);
}

void Curl_dnscache_clear(struct Curl_easy *data)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  if(dnscache) {
    dnscache_lock(data, dnscache);
    Curl_hash_clean(&dnscache->entries);
    dnscache_unlock(data, dnscache);
  }
}

/* lookup address, returns entry if found and not stale */
static CURLcode fetch_addr(struct Curl_easy *data,
                           struct Curl_dnscache *dnscache,
                           uint8_t dns_queries,
                           const char *hostname,
                           uint16_t port,
                           struct Curl_dns_entry **pdns)
{
  struct Curl_dns_entry *dns = NULL;
  char entry_id[MAX_HOSTCACHE_LEN];
  size_t entry_len;
  char entry_type = CURL_DNSQ_IS_ADDR(dns_queries) ?
                    CURL_DNST_ADDR : CURL_DNST_HTTPS;
  CURLcode result = CURLE_OK;

  *pdns = NULL;
  if(!dnscache)
    return CURLE_OK;

  /* Create an entry id, based upon the hostname and port */
  entry_len = create_dnscache_id(entry_type, hostname, 0, port,
                                 entry_id, sizeof(entry_id));

  /* See if it is already in our dns cache */
  dns = Curl_hash_pick(&dnscache->entries, entry_id, entry_len + 1);

  /* No entry found in cache, check if we might have a wildcard entry */
  if(!dns && data->state.wildcard_resolve && CURL_DNSQ_IS_ADDR(dns_queries)) {
    entry_len = create_dnscache_id(CURL_DNST_ADDR, "*", 1, port,
                                   entry_id, sizeof(entry_id));

    /* See if it is already in our dns cache */
    dns = Curl_hash_pick(&dnscache->entries, entry_id, entry_len + 1);
  }

  if(dns && (data->set.dns_cache_timeout_ms != -1)) {
    /* See whether the returned entry is stale. Done before we release lock */
    struct dnscache_prune_data user;

    user.now = *Curl_pgrs_now(data);
    user.max_age_ms = data->set.dns_cache_timeout_ms;
    user.oldest_ms = 0;

    if(dnscache_entry_is_stale(&user, dns)) {
      infof(data, "Hostname in DNS cache was stale, zapped");
      dns = NULL; /* the memory deallocation is being handled by the hash */
      Curl_hash_delete(&dnscache->entries, entry_id, entry_len + 1);
    }
  }

  /* We need to cache address information and HTTPS-RR separately. */
  if(dns && CURL_DNSQ_IS_ADDR(dns_queries)) {
    if((uint8_t)(dns->dns_queries & dns_queries) !=
       (uint8_t)(dns_queries & CURL_DNSQ_ADDR)) {
      /* The entry does not cover all wanted address queries, a miss. */
      dns = NULL;
    }
    else if(!(dns->dns_responses & dns_queries)) {
      /* The entry has no responses for the wanted DNS queries. */
      CURL_TRC_DNS(data, "cache entry does not have type=%s addresses",
                   Curl_resolv_query_str(dns_queries));
      dns = NULL;
      result = CURLE_COULDNT_RESOLVE_HOST;
    }
    else if(dns && !dns->addr) { /* negative entry */
      dns = NULL;
      result = CURLE_COULDNT_RESOLVE_HOST;
    }
  }

  *pdns = dns;
  return result;
}

/*
 * Curl_dnscache_get() fetches a 'Curl_dns_entry' already in the DNS cache.
 *
 * Curl_resolv() checks initially and multi_runsingle() checks each time
 * it discovers the handle in the state WAITRESOLVE whether the hostname
 * has already been resolved and the address has already been stored in
 * the DNS cache. This short circuits waiting for a lot of pending
 * lookups for the same hostname requested by different handles.
 *
 * Returns the Curl_dns_entry entry pointer or NULL if not in the cache.
 *
 * The returned data *MUST* be "released" with Curl_dns_entry_unlink() after
 * use, or we will leak memory!
 */
CURLcode Curl_dnscache_get(struct Curl_easy *data,
                           uint8_t dns_queries,
                           const char *hostname,
                           uint16_t port,
                           struct Curl_dns_entry **pentry)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  struct Curl_dns_entry *dns = NULL;
  CURLcode result = CURLE_OK;

  dnscache_lock(data, dnscache);
  result = fetch_addr(data, dnscache, dns_queries, hostname, port, &dns);
  if(!result && dns)
    dns->refcount++; /* we pass out a reference */
  else if(result) {
    DEBUGASSERT(!dns);
    dns = NULL;
  }
  dnscache_unlock(data, dnscache);

  CURL_TRC_DNS(data, "cache lookup %s:%u queries=%s -> %d %sfound",
               hostname, port, Curl_resolv_query_str(dns_queries),
               (int)result, dns ? "" : "not ");
  *pentry = dns;
  return result;
}

#ifndef CURL_DISABLE_SHUFFLE_DNS
/*
 * Return # of addresses in a Curl_addrinfo struct
 */
static int num_addresses(const struct Curl_addrinfo *addr)
{
  int i = 0;
  while(addr) {
    addr = addr->ai_next;
    i++;
  }
  return i;
}

/*
 * dns_shuffle_addr() shuffles the order of addresses in a 'Curl_addrinfo'
 * struct by re-linking its linked list.
 *
 * The addr argument should be the address of a pointer to the head node of a
 * `Curl_addrinfo` list and it will be modified to point to the new head after
 * shuffling.
 *
 * Not declared static only to make it easy to use in a unit test!
 *
 * @unittest 1608
 */
UNITTEST CURLcode dns_shuffle_addr(struct Curl_easy *data,
                                   struct Curl_addrinfo **addr);
UNITTEST CURLcode dns_shuffle_addr(struct Curl_easy *data,
                                   struct Curl_addrinfo **addr)
{
  CURLcode result = CURLE_OK;
  const int num_addrs = num_addresses(*addr);

  if(num_addrs > 1) {
    struct Curl_addrinfo **nodes;
    CURL_TRC_DNS(data, "Shuffling %d addresses", num_addrs);

    nodes = curlx_malloc(num_addrs * sizeof(*nodes));
    if(nodes) {
      int i;
      unsigned int *rnd;
      const size_t rnd_size = num_addrs * sizeof(*rnd);

      /* build a plain array of Curl_addrinfo pointers */
      nodes[0] = *addr;
      for(i = 1; i < num_addrs; i++) {
        nodes[i] = nodes[i - 1]->ai_next;
      }

      rnd = curlx_malloc(rnd_size);
      if(rnd) {
        /* Fisher-Yates shuffle */
        if(Curl_rand(data, (unsigned char *)rnd, rnd_size) == CURLE_OK) {
          struct Curl_addrinfo *swap_tmp;
          for(i = num_addrs - 1; i > 0; i--) {
            swap_tmp = nodes[rnd[i] % (unsigned int)(i + 1)];
            nodes[rnd[i] % (unsigned int)(i + 1)] = nodes[i];
            nodes[i] = swap_tmp;
          }

          /* relink list in the new order */
          for(i = 1; i < num_addrs; i++) {
            nodes[i - 1]->ai_next = nodes[i];
          }

          nodes[num_addrs - 1]->ai_next = NULL;
          *addr = nodes[0];
        }
        curlx_free(rnd);
      }
      else
        result = CURLE_OUT_OF_MEMORY;
      curlx_free(nodes);
    }
    else
      result = CURLE_OUT_OF_MEMORY;
  }
  return result;
}
#endif

static bool dnscache_ai_has_family(struct Curl_addrinfo *ai,
                                   int ai_family)
{
  for(; ai; ai = ai->ai_next) {
    if(ai->ai_family == ai_family)
      return TRUE;
  }
  return FALSE;
}

static struct Curl_dns_entry *dnsc_entry_create(
  struct Curl_easy *data,
  const char *hostname,
  size_t hostlen,
  uint16_t port,
  bool permanent)
{
  struct Curl_dns_entry *dns = NULL;

  /* Create a new cache entry, struct already has the hostname NUL */
  dns = curlx_calloc(1, sizeof(struct Curl_dns_entry) + hostlen);
  if(!dns)
    goto out;

  dns->refcount = 1; /* the cache has the first reference */
  dns->port = port;
  if(hostlen)
    memcpy(dns->hostname, hostname, hostlen);

  if(permanent) {
    dns->timestamp.tv_sec = 0; /* an entry that never goes stale */
    dns->timestamp.tv_usec = 0; /* an entry that never goes stale */
  }
  else {
    dns->timestamp = *Curl_pgrs_now(data);
  }

out:
  return dns;
}

static struct Curl_dns_entry *
dnsc_entry_assign_addr(
  struct Curl_easy *data,
  struct Curl_dns_entry *dns,
  uint8_t dns_queries,
  struct Curl_addrinfo **paddr1,
  struct Curl_addrinfo **paddr2)
{
  if(!dns)
    goto out;
  /* only do this when this is the only reference */
  DEBUGASSERT(dns->refcount == 1);
  DEBUGASSERT(dns->type == CURL_DNST_INIT);

  dns->type = CURL_DNST_ADDR;
  /* queries should only be about addresses */
  DEBUGASSERT(!(dns_queries & ~CURL_DNSQ_ADDR));
  dns->dns_queries = (dns_queries & CURL_DNSQ_ADDR);

  /* Take the given address lists into the entry */
  if(paddr1 && *paddr1) {
    dns->addr = *paddr1;
    *paddr1 = NULL;
  }
  if(paddr2 && *paddr2) {
    struct Curl_addrinfo **phead = &dns->addr;
    while(*phead)
      phead = &(*phead)->ai_next;
    *phead = *paddr2;
    *paddr2 = NULL;
  }

  if((dns_queries & CURL_DNSQ_A) &&
     dnscache_ai_has_family(dns->addr, PF_INET))
    dns->dns_responses |= CURL_DNSQ_A;

#ifdef USE_IPV6
  if((dns_queries & CURL_DNSQ_AAAA) &&
     dnscache_ai_has_family(dns->addr, PF_INET6))
    dns->dns_responses |= CURL_DNSQ_AAAA;
#endif /* USE_IPV6 */

#ifndef CURL_DISABLE_SHUFFLE_DNS
  /* shuffle addresses if requested */
  if(data->set.dns_shuffle_addresses && dns->addr) {
    CURLcode result = dns_shuffle_addr(data, &dns->addr);
    if(result) {
      /* free without lock, we are the sole owner */
      dnscache_entry_free(dns);
      dns = NULL;
      goto out;
    }
  }
#else
  (void)data;
#endif

out:
  if(paddr1 && *paddr1) {
    Curl_freeaddrinfo(*paddr1);
    *paddr1 = NULL;
  }
  if(paddr2 && *paddr2) {
    Curl_freeaddrinfo(*paddr2);
    *paddr2 = NULL;
  }
  return dns;
}

struct Curl_dns_entry *Curl_dnsc_mk_addr(struct Curl_easy *data,
                                              uint8_t dns_queries,
                                              struct Curl_addrinfo **paddr,
                                              const char *hostname,
                                              uint16_t port)
{
  struct Curl_dns_entry *dns = dnsc_entry_create(
    data, hostname, hostname ? strlen(hostname) : 0, port, FALSE);
  dns = dnsc_entry_assign_addr(data, dns, dns_queries, paddr, NULL);
  return dns;
}

struct Curl_dns_entry *Curl_dnsc_mk_addr2(struct Curl_easy *data,
                                               uint8_t dns_queries,
                                               struct Curl_addrinfo **paddr1,
                                               struct Curl_addrinfo **paddr2,
                                               const char *hostname,
                                               uint16_t port)
{
  struct Curl_dns_entry *dns = dnsc_entry_create(
    data, hostname, hostname ? strlen(hostname) : 0, port, FALSE);
  dns = dnsc_entry_assign_addr(data, dns, dns_queries, paddr1, paddr2);
  return dns;
}

#ifdef USE_HTTPSRR
static struct Curl_dns_entry *
dnsc_entry_assign_https(struct Curl_dns_entry *dns,
                        struct Curl_https_rrinfo **phinfo)
{
  if(!dns)
    goto out;
  /* only do this when this is the only reference */
  DEBUGASSERT(dns->refcount == 1);
  DEBUGASSERT(dns->type == CURL_DNST_INIT);

  if(dns->hinfo) {
    Curl_httpsrr_destroy(dns->hinfo);
    dns->hinfo = NULL;
  }
  dns->type = CURL_DNST_HTTPS;
  dns->dns_responses = dns->dns_queries = CURL_DNSQ_HTTPS;
  if(phinfo) {
    dns->hinfo = *phinfo;
    *phinfo = NULL;
  }
out:
  if(phinfo && *phinfo) {
    Curl_httpsrr_destroy(*phinfo);
    *phinfo = NULL;
  }
  return dns;
}

struct Curl_dns_entry *Curl_dnsc_mk_https(struct Curl_easy *data,
                                          struct Curl_https_rrinfo **phinfo,
                                          const char *hostname,
                                          uint16_t port)
{
  struct Curl_dns_entry *dns = dnsc_entry_create(
    data, hostname, hostname ? strlen(hostname) : 0, port, FALSE);
  dns = dnsc_entry_assign_https(dns, phinfo);
  return dns;
}

static struct Curl_dns_entry *dnsc_add_https(struct Curl_easy *data,
                                             struct Curl_dnscache *dnscache,
                                             struct Curl_https_rrinfo **phinfo,
                                             const char *hostname,
                                             size_t hlen,
                                             uint16_t port,
                                             bool permanent)
{
  char entry_id[MAX_HOSTCACHE_LEN];
  size_t entry_len;
  struct Curl_dns_entry *dns, *dns2;

  dns = dnsc_entry_create(data, hostname, hostname ? strlen(hostname) : 0,
                          port, permanent);
  dns = dnsc_entry_assign_https(dns, phinfo);
  if(!dns)
    return NULL;

  /* Create an entry id, based upon the hostname and port */
  entry_len = create_dnscache_id(CURL_DNST_HTTPS, hostname, hlen, port,
                                 entry_id, sizeof(entry_id));

  /* Store the resolved data in our DNS cache. */
  dns2 = Curl_hash_add(&dnscache->entries, entry_id, entry_len + 1,
                       (void *)dns);
  if(!dns2) {
    dnscache_entry_free(dns);
    return NULL;
  }

  dns = dns2;
  dns->refcount++;         /* mark entry as in-use */
  return dns;
}

#endif /* USE_HTTPSRR */

static struct Curl_dns_entry *dnsc_add_addr(struct Curl_easy *data,
                                            struct Curl_dnscache *dnscache,
                                            uint8_t dns_queries,
                                            struct Curl_addrinfo **paddr,
                                            const char *hostname,
                                            size_t hlen,
                                            uint16_t port,
                                            bool permanent)
{
  char entry_id[MAX_HOSTCACHE_LEN];
  size_t entry_len;
  struct Curl_dns_entry *dns;
  struct Curl_dns_entry *dns2;

  dns = dnsc_entry_create(data, hostname, hlen, port, permanent);
  dns = dnsc_entry_assign_addr(data, dns, dns_queries, paddr, NULL);
  if(!dns)
    return NULL;

  /* Create an entry id, based upon the hostname and port */
  entry_len = create_dnscache_id(CURL_DNST_ADDR, hostname, hlen, port,
                                 entry_id, sizeof(entry_id));

  /* Store the resolved data in our DNS cache. */
  dns2 = Curl_hash_add(&dnscache->entries, entry_id, entry_len + 1,
                       (void *)dns);
  if(!dns2) {
    dnscache_entry_free(dns);
    return NULL;
  }

  dns = dns2;
  dns->refcount++;         /* mark entry as in-use */
  return dns;
}

CURLcode Curl_dnscache_add(struct Curl_easy *data,
                           struct Curl_dns_entry *entry)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  char id[MAX_HOSTCACHE_LEN];
  size_t idlen;

  if(!dnscache)
    return CURLE_FAILED_INIT;
  if(!entry || (entry->type == CURL_DNST_INIT))
    return CURLE_BAD_FUNCTION_ARGUMENT;

  /* Create an entry id, based upon the hostname and port */
  idlen = create_dnscache_id(entry->type, entry->hostname, 0, entry->port,
                             id, sizeof(id));

  /* Store the resolved data in our DNS cache and up ref count */
  dnscache_lock(data, dnscache);
  if(!Curl_hash_add(&dnscache->entries, id, idlen + 1, (void *)entry)) {
    dnscache_unlock(data, dnscache);
    return CURLE_OUT_OF_MEMORY;
  }
  entry->refcount++;
  dnscache_unlock(data, dnscache);
  CURL_TRC_DNS(data, "cached entry for %s:%u queries=%s",
               entry->hostname, entry->port,
               Curl_resolv_query_str(entry->dns_queries));
  return CURLE_OK;
}

CURLcode Curl_dnscache_add_negative(struct Curl_easy *data,
                                    uint8_t dns_queries,
                                    const char *host,
                                    uint16_t port)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  struct Curl_dns_entry *dns = NULL;
  CURLcode result = CURLE_OK;

  DEBUGASSERT(dnscache);
  if(!dnscache)
    return CURLE_FAILED_INIT;

  dnscache_lock(data, dnscache);

  if(dns_queries & CURL_DNSQ_ADDR) {
    /* put this new host in the cache */
    dns = dnsc_add_addr(data, dnscache, dns_queries, NULL,
                            host, strlen(host), port, FALSE);
    if(!dns)
      result = CURLE_OUT_OF_MEMORY;
  }
#ifdef USE_HTTPSRR
  else if(dns_queries == CURL_DNSQ_HTTPS) {
    dns = dnsc_add_https(data, dnscache, NULL,
                            host, strlen(host), port, FALSE);
    if(!dns)
      result = CURLE_OUT_OF_MEMORY;
  }
#endif
  else {
    /* a query we do not know, just cache nothing */
    DEBUGASSERT(0);
  }

  if(dns) {
    /* release the returned reference; the cache itself will keep the
     * entry alive: */
    dns->refcount--;
    CURL_TRC_DNS(data, "cache negative name resolve for %s:%d type=%s",
                 host, port, Curl_resolv_query_str(dns_queries));
  }
  dnscache_unlock(data, dnscache);
  return result;
}

/*
 * Curl_dns_entry_unlink() releases a reference to the given cached DNS entry.
 * When the reference count reaches 0, the entry is destroyed. It is important
 * that only one unlink is made for each Curl_resolv() call.
 *
 * May be called with 'data' == NULL for global cache.
 */
void Curl_dns_entry_unlink(struct Curl_easy *data,
                           struct Curl_dns_entry **pdns)
{
  if(*pdns) {
    struct Curl_dnscache *dnscache = dnscache_get(data);
    struct Curl_dns_entry *dns = *pdns;
    *pdns = NULL;
    dnscache_lock(data, dnscache);
    dns->refcount--;
    if(dns->refcount == 0)
      dnscache_entry_free(dns);
    dnscache_unlock(data, dnscache);
  }
}

static void dnscache_entry_dtor(void *entry)
{
  struct Curl_dns_entry *dns = (struct Curl_dns_entry *)entry;
  DEBUGASSERT(dns && (dns->refcount > 0));
  dns->refcount--;
  if(dns->refcount == 0)
    dnscache_entry_free(dns);
}

/*
 * Curl_dnscache_init() inits a new DNS cache.
 */
void Curl_dnscache_init(struct Curl_dnscache *dns, size_t size)
{
  Curl_hash_init(&dns->entries, size, Curl_hash_str, curlx_str_key_compare,
                 dnscache_entry_dtor);
}

void Curl_dnscache_destroy(struct Curl_dnscache *dns)
{
  Curl_hash_destroy(&dns->entries);
}

CURLcode Curl_loadhostpairs(struct Curl_easy *data)
{
  struct Curl_dnscache *dnscache = dnscache_get(data);
  struct curl_slist *hostp;

  if(!dnscache)
    return CURLE_FAILED_INIT;

  /* Default is no wildcard found */
  data->state.wildcard_resolve = FALSE;

  for(hostp = data->state.resolve; hostp; hostp = hostp->next) {
    char entry_id[MAX_HOSTCACHE_LEN];
    const char *host = hostp->data;
    struct Curl_str source;
    if(!host)
      continue;
    if(*host == '-') {
      curl_off_t num = 0;
      size_t entry_len;
      host++;
      if(!curlx_str_single(&host, '[')) {
        if(curlx_str_until(&host, &source, MAX_IPADR_LEN, ']') ||
           curlx_str_single(&host, ']') ||
           curlx_str_single(&host, ':'))
          continue;
      }
      else {
        if(curlx_str_until(&host, &source, 4096, ':') ||
           curlx_str_single(&host, ':')) {
          continue;
        }
      }

      if(!curlx_str_number(&host, &num, 0xffff)) {
        /* Create an entry id, based upon the hostname and port */
        entry_len = create_dnscache_id(CURL_DNST_ADDR, curlx_str(&source),
                                       curlx_strlen(&source), (uint16_t)num,
                                       entry_id, sizeof(entry_id));
        dnscache_lock(data, dnscache);
        /* delete entry, ignore if it did not exist */
        Curl_hash_delete(&dnscache->entries, entry_id, entry_len + 1);
        dnscache_unlock(data, dnscache);
      }
    }
    else {
      struct Curl_dns_entry *dns;
      struct Curl_addrinfo *head = NULL, *tail = NULL;
      size_t entry_len;
      char address[64];
      curl_off_t tmpofft = 0;
      uint16_t port = 0;
      bool permanent = TRUE;
      bool error = TRUE;
      VERBOSE(const char *addresses = NULL);

      if(*host == '+') {
        host++;
        permanent = FALSE;
      }
      if(!curlx_str_single(&host, '[')) {
        if(curlx_str_until(&host, &source, MAX_IPADR_LEN, ']') ||
           curlx_str_single(&host, ']'))
          continue;
      }
      else {
        if(curlx_str_until(&host, &source, 4096, ':'))
          continue;
      }
      if(curlx_str_single(&host, ':') ||
         curlx_str_number(&host, &tmpofft, 0xffff) ||
         curlx_str_single(&host, ':'))
        goto err;
      port = (uint16_t)tmpofft;

      VERBOSE(addresses = host);

      /* start the address section */
      while(*host) {
        struct Curl_str target;
        struct Curl_addrinfo *ai;
        CURLcode result;

        if(!curlx_str_single(&host, '[')) {
          if(curlx_str_until(&host, &target, MAX_IPADR_LEN, ']') ||
             curlx_str_single(&host, ']'))
            goto err;
        }
        else {
          if(curlx_str_until(&host, &target, 4096, ',')) {
            if(curlx_str_single(&host, ','))
              goto err;
            /* survive nothing but a comma */
            continue;
          }
        }
#ifndef USE_IPV6
        if(memchr(curlx_str(&target), ':', curlx_strlen(&target))) {
          infof(data, "Ignoring resolve address '%.*s', missing IPv6 support.",
                (int)curlx_strlen(&target), curlx_str(&target));
          if(curlx_str_single(&host, ','))
            goto err;
          continue;
        }
#endif

        if(curlx_strlen(&target) >= sizeof(address))
          goto err;

        memcpy(address, curlx_str(&target), curlx_strlen(&target));
        address[curlx_strlen(&target)] = '\0';

        result = Curl_str2addr(address, port, &ai);
        if(result) {
          infof(data, "Resolve IP address '%s' found is illegal", address);
          goto err;
        }

        if(tail) {
          tail->ai_next = ai;
          tail = tail->ai_next;
        }
        else {
          head = tail = ai;
        }
        if(curlx_str_single(&host, ','))
          break;
      }

      if(!head)
        goto err;

      error = FALSE;
err:
      if(error) {
        failf(data, "Could not parse CURLOPT_RESOLVE entry '%s'", hostp->data);
        Curl_freeaddrinfo(head);
        return CURLE_SETOPT_OPTION_SYNTAX;
      }

      /* Create an entry id, based upon the hostname and port */
      entry_len = create_dnscache_id(CURL_DNST_ADDR,
                                     curlx_str(&source), curlx_strlen(&source),
                                     port, entry_id, sizeof(entry_id));

      dnscache_lock(data, dnscache);

      /* See if it is already in our dns cache */
      dns = Curl_hash_pick(&dnscache->entries, entry_id, entry_len + 1);

      if(dns) {
        infof(data, "RESOLVE %.*s:%u - old addresses discarded",
              (int)curlx_strlen(&source),
              curlx_str(&source), port);
        /* delete old entry, there are two reasons for this
         1. old entry may have different addresses.
         2. even if entry with correct addresses is already in the cache,
            but if it is close to expire, then by the time next http
            request is made, it can get expired and pruned because old
            entry is not necessarily marked as permanent.
         3. when adding a non-permanent entry, we want it to remove and
            replace an existing permanent entry.
         4. when adding a non-permanent entry, we want it to get a "fresh"
            timeout that starts _now_. */

        Curl_hash_delete(&dnscache->entries, entry_id, entry_len + 1);
      }

      /* put this new host in the cache, override all address queries */
      dns = dnsc_add_addr(data, dnscache, CURL_DNSQ_ADDR,
                          &head, curlx_str(&source),
                          curlx_strlen(&source), port, permanent);
      if(dns)
        /* release the returned reference; the cache itself will keep the
         * entry alive: */
        dns->refcount--;

      dnscache_unlock(data, dnscache);

      if(!dns)
        return CURLE_OUT_OF_MEMORY;

      infof(data, "[DNS] added %.*s:%u:%s to cache%s",
            (int)curlx_strlen(&source), curlx_str(&source), port, addresses,
            permanent ? "" : " (non-permanent)");

      /* Wildcard hostname */
      if(curlx_str_casecompare(&source, "*")) {
        infof(data, "RESOLVE *:%u using wildcard", port);
        data->state.wildcard_resolve = TRUE;
      }
    }
  }
  data->state.resolve = NULL; /* dealt with now */

  return CURLE_OK;
}
