curl/lib/hostip.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2018, 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.haxx.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.
 *
 ***************************************************************************/

#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

#ifdef HAVE_SETJMP_H
#include <setjmp.h>
#endif
#ifdef HAVE_SIGNAL_H
#include <signal.h>
#endif

#ifdef HAVE_PROCESS_H
#include <process.h>
#endif

#include "urldata.h"
#include "sendf.h"
#include "hostip.h"
#include "hash.h"
#include "rand.h"
#include "share.h"
#include "strerror.h"
#include "url.h"
#include "inet_ntop.h"
#include "multiif.h"
#include "warnless.h"
/* The last 3 #include files should be in this order */
#include "curl_printf.h"
#include "curl_memory.h"
#include "memdebug.h"

#if defined(CURLRES_SYNCH) && \
    defined(HAVE_ALARM) && defined(SIGALRM) && defined(HAVE_SIGSETJMP)
/* alarm-based timeouts can only be used with all the dependencies satisfied */
#define USE_ALARM_TIMEOUT
#endif

/*
 * hostip.c explained
 * ==================
 *
 * The main COMPILE-TIME DEFINES to keep in mind when reading the host*.c
 * source file are these:
 *
 * CURLRES_IPV6 - this host has getaddrinfo() and family, and thus we use
 * that. The host may not be able to resolve IPv6, but we don't really have to
 * take that into account. Hosts that aren't IPv6-enabled have CURLRES_IPV4
 * defined.
 *
 * CURLRES_ARES - is defined if libcurl is built to use c-ares for
 * asynchronous name resolves. This can be Windows or *nix.
 *
 * CURLRES_THREADED - is defined if libcurl is built to run under (native)
 * Windows, and then the name resolve will be done in a new thread, and the
 * supported API will be the same as for ares-builds.
 *
 * If any of the two previous are defined, CURLRES_ASYNCH is defined too. If
 * libcurl is not built to use an asynchronous resolver, CURLRES_SYNCH is
 * defined.
 *
 * The host*.c sources files are split up like this:
 *
 * hostip.c   - method-independent resolver functions and utility functions
 * hostasyn.c - functions for asynchronous name resolves
 * hostsyn.c  - functions for synchronous name resolves
 * hostip4.c  - IPv4 specific functions
 * hostip6.c  - IPv6 specific functions
 *
 * The two asynchronous name resolver backends are implemented in:
 * asyn-ares.c   - functions for ares-using name resolves
 * asyn-thread.c - functions for threaded name resolves

 * The hostip.h is the united header file for all this. It defines the
 * CURLRES_* defines based on the config*.h and curl_setup.h defines.
 */

/* These two symbols are for the global DNS cache */
static struct curl_hash hostname_cache;
static int host_cache_initialized;

static void freednsentry(void *freethis);

/*
 * Curl_global_host_cache_init() initializes and sets up a global DNS cache.
 * Global DNS cache is general badness. Do not use. This will be removed in
 * a future version. Use the share interface instead!
 *
 * Returns a struct curl_hash pointer on success, NULL on failure.
 */
struct curl_hash *Curl_global_host_cache_init(void)
{
  int rc = 0;
  if(!host_cache_initialized) {
    rc = Curl_hash_init(&hostname_cache, 7, Curl_hash_str,
                        Curl_str_key_compare, freednsentry);
    if(!rc)
      host_cache_initialized = 1;
  }
  return rc?NULL:&hostname_cache;
}

/*
 * Destroy and cleanup the global DNS cache
 */
void Curl_global_host_cache_dtor(void)
{
  if(host_cache_initialized) {
    Curl_hash_destroy(&hostname_cache);
    host_cache_initialized = 0;
  }
}

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

/*
 * Curl_printable_address() returns a printable version of the 1st address
 * given in the 'ai' argument. The result will be stored in the buf that is
 * bufsize bytes big.
 *
 * If the conversion fails, it returns NULL.
 */
const char *
Curl_printable_address(const Curl_addrinfo *ai, char *buf, size_t bufsize)
{
  const struct sockaddr_in *sa4;
  const struct in_addr *ipaddr4;
#ifdef ENABLE_IPV6
  const struct sockaddr_in6 *sa6;
  const struct in6_addr *ipaddr6;
#endif

  switch(ai->ai_family) {
    case AF_INET:
      sa4 = (const void *)ai->ai_addr;
      ipaddr4 = &sa4->sin_addr;
      return Curl_inet_ntop(ai->ai_family, (const void *)ipaddr4, buf,
                            bufsize);
#ifdef ENABLE_IPV6
    case AF_INET6:
      sa6 = (const void *)ai->ai_addr;
      ipaddr6 = &sa6->sin6_addr;
      return Curl_inet_ntop(ai->ai_family, (const void *)ipaddr6, buf,
                            bufsize);
#endif
    default:
      break;
  }
  return NULL;
}

/*
 * Return a hostcache id string for the provided host + port, to be used by
 * the DNS caching.
 */
static char *
create_hostcache_id(const char *name, int port)
{
  /* create and return the new allocated entry */
  char *id = aprintf("%s:%d", name, port);
  char *ptr = id;
  if(ptr) {
    /* lower case the name part */
    while(*ptr && (*ptr != ':')) {
      *ptr = (char)TOLOWER(*ptr);
      ptr++;
    }
  }
  return id;
}

struct hostcache_prune_data {
  long cache_timeout;
  time_t now;
};

/*
 * 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
hostcache_timestamp_remove(void *datap, void *hc)
{
  struct hostcache_prune_data *data =
    (struct hostcache_prune_data *) datap;
  struct Curl_dns_entry *c = (struct Curl_dns_entry *) hc;

  return (0 != c->timestamp)
    && (data->now - c->timestamp >= data->cache_timeout);
}

/*
 * Prune the DNS cache. This assumes that a lock has already been taken.
 */
static void
hostcache_prune(struct curl_hash *hostcache, long cache_timeout, time_t now)
{
  struct hostcache_prune_data user;

  user.cache_timeout = cache_timeout;
  user.now = now;

  Curl_hash_clean_with_criterium(hostcache,
                                 (void *) &user,
                                 hostcache_timestamp_remove);
}

/*
 * Library-wide function for pruning the DNS cache. This function takes and
 * returns the appropriate locks.
 */
void Curl_hostcache_prune(struct Curl_easy *data)
{
  time_t now;

  if((data->set.dns_cache_timeout == -1) || !data->dns.hostcache)
    /* cache forever means never prune, and NULL hostcache means
       we can't do it */
    return;

  if(data->share)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

  time(&now);

  /* Remove outdated and unused entries from the hostcache */
  hostcache_prune(data->dns.hostcache,
                  data->set.dns_cache_timeout,
                  now);

  if(data->share)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
}

#ifdef HAVE_SIGSETJMP
/* Beware this is a global and unique instance. This is used to store the
   return address that we can jump back to from inside a signal handler. This
   is not thread-safe stuff. */
sigjmp_buf curl_jmpenv;
#endif

/* lookup address, returns entry if found and not stale */
static struct Curl_dns_entry *
fetch_addr(struct connectdata *conn,
                const char *hostname,
                int port)
{
  char *entry_id = NULL;
  struct Curl_dns_entry *dns = NULL;
  size_t entry_len;
  struct Curl_easy *data = conn->data;

  /* Create an entry id, based upon the hostname and port */
  entry_id = create_hostcache_id(hostname, port);
  /* If we can't create the entry id, fail */
  if(!entry_id)
    return dns;

  entry_len = strlen(entry_id);

  /* See if its already in our dns cache */
  dns = Curl_hash_pick(data->dns.hostcache, entry_id, entry_len + 1);

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

    time(&user.now);
    user.cache_timeout = data->set.dns_cache_timeout;

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

  /* free the allocated entry_id again */
  free(entry_id);

  return dns;
}

/*
 * Curl_fetch_addr() 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 "unlocked" with Curl_resolv_unlock() after
 * use, or we'll leak memory!
 */
struct Curl_dns_entry *
Curl_fetch_addr(struct connectdata *conn,
                const char *hostname,
                int port)
{
  struct Curl_easy *data = conn->data;
  struct Curl_dns_entry *dns = NULL;

  if(data->share)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

  dns = fetch_addr(conn, hostname, port);

  if(dns)
    dns->inuse++; /* we use it! */

  if(data->share)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);

  return dns;
}

/*
 * Curl_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
 */
CURLcode Curl_shuffle_addr(struct Curl_easy *data, Curl_addrinfo **addr)
{
  CURLcode result = CURLE_OK;
  const int num_addrs = Curl_num_addresses(*addr);

  if(num_addrs > 1) {
    Curl_addrinfo **nodes;
    infof(data, "Shuffling %i addresses", num_addrs);

    nodes = 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 = malloc(rnd_size);
      if(rnd) {
        /* Fisher-Yates shuffle */
        if(Curl_rand(data, (unsigned char *)rnd, rnd_size) == CURLE_OK) {
          Curl_addrinfo *swap_tmp;
          for(i = num_addrs - 1; i > 0; i--) {
            swap_tmp = nodes[rnd[i] % (i + 1)];
            nodes[rnd[i] % (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];
        }
        free(rnd);
      }
      else
        result = CURLE_OUT_OF_MEMORY;
      free(nodes);
    }
    else
      result = CURLE_OUT_OF_MEMORY;
  }
  return result;
}

/*
 * Curl_cache_addr() stores a 'Curl_addrinfo' struct in the DNS cache.
 *
 * When calling Curl_resolv() has resulted in a response with a returned
 * address, we call this function to store the information in the dns
 * cache etc
 *
 * Returns the Curl_dns_entry entry pointer or NULL if the storage failed.
 */
struct Curl_dns_entry *
Curl_cache_addr(struct Curl_easy *data,
                Curl_addrinfo *addr,
                const char *hostname,
                int port)
{
  char *entry_id;
  size_t entry_len;
  struct Curl_dns_entry *dns;
  struct Curl_dns_entry *dns2;

  /* shuffle addresses if requested */
  if(data->set.dns_shuffle_addresses) {
    CURLcode result = Curl_shuffle_addr(data, &addr);
    if(!result)
      return NULL;
  }

  /* Create an entry id, based upon the hostname and port */
  entry_id = create_hostcache_id(hostname, port);
  /* If we can't create the entry id, fail */
  if(!entry_id)
    return NULL;
  entry_len = strlen(entry_id);

  /* Create a new cache entry */
  dns = calloc(1, sizeof(struct Curl_dns_entry));
  if(!dns) {
    free(entry_id);
    return NULL;
  }

  dns->inuse = 1;   /* the cache has the first reference */
  dns->addr = addr; /* this is the address(es) */
  time(&dns->timestamp);
  if(dns->timestamp == 0)
    dns->timestamp = 1;   /* zero indicates CURLOPT_RESOLVE entry */

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

  dns = dns2;
  dns->inuse++;         /* mark entry as in-use */

  /* free the allocated entry_id */
  free(entry_id);

  return dns;
}

/*
 * Curl_resolv() is the main name resolve function within libcurl. It resolves
 * a name and returns a pointer to the entry in the 'entry' argument (if one
 * is provided). This function might return immediately if we're using asynch
 * resolves. See the return codes.
 *
 * The cache entry we return will get its 'inuse' counter increased when this
 * function is used. You MUST call Curl_resolv_unlock() later (when you're
 * done using this struct) to decrease the counter again.
 *
 * In debug mode, we specifically test for an interface name "LocalHost"
 * and resolve "localhost" instead as a means to permit test cases
 * to connect to a local test server with any host name.
 *
 * Return codes:
 *
 * CURLRESOLV_ERROR   (-1) = error, no pointer
 * CURLRESOLV_RESOLVED (0) = OK, pointer provided
 * CURLRESOLV_PENDING  (1) = waiting for response, no pointer
 */

int Curl_resolv(struct connectdata *conn,
                const char *hostname,
                int port,
                struct Curl_dns_entry **entry)
{
  struct Curl_dns_entry *dns = NULL;
  struct Curl_easy *data = conn->data;
  CURLcode result;
  int rc = CURLRESOLV_ERROR; /* default to failure */

  *entry = NULL;

  if(data->share)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

  dns = fetch_addr(conn, hostname, port);

  if(dns) {
    infof(data, "Hostname %s was found in DNS cache\n", hostname);
    dns->inuse++; /* we use it! */
    rc = CURLRESOLV_RESOLVED;
  }

  if(data->share)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);

  if(!dns) {
    /* The entry was not in the cache. Resolve it to IP address */

    Curl_addrinfo *addr;
    int respwait;

    /* Check what IP specifics the app has requested and if we can provide it.
     * If not, bail out. */
    if(!Curl_ipvalid(conn))
      return CURLRESOLV_ERROR;

    /* notify the resolver start callback */
    if(data->set.resolver_start) {
      int st;
      Curl_set_in_callback(data, true);
      st = data->set.resolver_start(data->state.resolver, NULL,
                                    data->set.resolver_start_client);
      Curl_set_in_callback(data, false);
      if(st)
        return CURLRESOLV_ERROR;
    }

    /* If Curl_getaddrinfo() returns NULL, 'respwait' might be set to a
       non-zero value indicating that we need to wait for the response to the
       resolve call */
    addr = Curl_getaddrinfo(conn,
#ifdef DEBUGBUILD
                            (data->set.str[STRING_DEVICE]
                             && !strcmp(data->set.str[STRING_DEVICE],
                                        "LocalHost"))?"localhost":
#endif
                            hostname, port, &respwait);

    if(!addr) {
      if(respwait) {
        /* the response to our resolve call will come asynchronously at
           a later time, good or bad */
        /* First, check that we haven't received the info by now */
        result = Curl_resolver_is_resolved(conn, &dns);
        if(result) /* error detected */
          return CURLRESOLV_ERROR;
        if(dns)
          rc = CURLRESOLV_RESOLVED; /* pointer provided */
        else
          rc = CURLRESOLV_PENDING; /* no info yet */
      }
    }
    else {
      if(data->share)
        Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

      /* we got a response, store it in the cache */
      dns = Curl_cache_addr(data, addr, hostname, port);

      if(data->share)
        Curl_share_unlock(data, CURL_LOCK_DATA_DNS);

      if(!dns)
        /* returned failure, bail out nicely */
        Curl_freeaddrinfo(addr);
      else
        rc = CURLRESOLV_RESOLVED;
    }
  }

  *entry = dns;

  return rc;
}

#ifdef USE_ALARM_TIMEOUT
/*
 * This signal handler jumps back into the main libcurl code and continues
 * execution.  This effectively causes the remainder of the application to run
 * within a signal handler which is nonportable and could lead to problems.
 */
static
RETSIGTYPE alarmfunc(int sig)
{
  /* this is for "-ansi -Wall -pedantic" to stop complaining!   (rabe) */
  (void)sig;
  siglongjmp(curl_jmpenv, 1);
}
#endif /* USE_ALARM_TIMEOUT */

/*
 * Curl_resolv_timeout() is the same as Curl_resolv() but specifies a
 * timeout.  This function might return immediately if we're using asynch
 * resolves. See the return codes.
 *
 * The cache entry we return will get its 'inuse' counter increased when this
 * function is used. You MUST call Curl_resolv_unlock() later (when you're
 * done using this struct) to decrease the counter again.
 *
 * If built with a synchronous resolver and use of signals is not
 * disabled by the application, then a nonzero timeout will cause a
 * timeout after the specified number of milliseconds. Otherwise, timeout
 * is ignored.
 *
 * Return codes:
 *
 * CURLRESOLV_TIMEDOUT(-2) = warning, time too short or previous alarm expired
 * CURLRESOLV_ERROR   (-1) = error, no pointer
 * CURLRESOLV_RESOLVED (0) = OK, pointer provided
 * CURLRESOLV_PENDING  (1) = waiting for response, no pointer
 */

int Curl_resolv_timeout(struct connectdata *conn,
                        const char *hostname,
                        int port,
                        struct Curl_dns_entry **entry,
                        time_t timeoutms)
{
#ifdef USE_ALARM_TIMEOUT
#ifdef HAVE_SIGACTION
  struct sigaction keep_sigact;   /* store the old struct here */
  volatile bool keep_copysig = FALSE; /* whether old sigact has been saved */
  struct sigaction sigact;
#else
#ifdef HAVE_SIGNAL
  void (*keep_sigact)(int);       /* store the old handler here */
#endif /* HAVE_SIGNAL */
#endif /* HAVE_SIGACTION */
  volatile long timeout;
  volatile unsigned int prev_alarm = 0;
  struct Curl_easy *data = conn->data;
#endif /* USE_ALARM_TIMEOUT */
  int rc;

  *entry = NULL;

  if(timeoutms < 0)
    /* got an already expired timeout */
    return CURLRESOLV_TIMEDOUT;

#ifdef USE_ALARM_TIMEOUT
  if(data->set.no_signal)
    /* Ignore the timeout when signals are disabled */
    timeout = 0;
  else
    timeout = (timeoutms > LONG_MAX) ? LONG_MAX : (long)timeoutms;

  if(!timeout)
    /* USE_ALARM_TIMEOUT defined, but no timeout actually requested */
    return Curl_resolv(conn, hostname, port, entry);

  if(timeout < 1000) {
    /* The alarm() function only provides integer second resolution, so if
       we want to wait less than one second we must bail out already now. */
    failf(data,
        "remaining timeout of %ld too small to resolve via SIGALRM method",
        timeout);
    return CURLRESOLV_TIMEDOUT;
  }
  /* This allows us to time-out from the name resolver, as the timeout
     will generate a signal and we will siglongjmp() from that here.
     This technique has problems (see alarmfunc).
     This should be the last thing we do before calling Curl_resolv(),
     as otherwise we'd have to worry about variables that get modified
     before we invoke Curl_resolv() (and thus use "volatile"). */
  if(sigsetjmp(curl_jmpenv, 1)) {
    /* this is coming from a siglongjmp() after an alarm signal */
    failf(data, "name lookup timed out");
    rc = CURLRESOLV_ERROR;
    goto clean_up;
  }
  else {
    /*************************************************************
     * Set signal handler to catch SIGALRM
     * Store the old value to be able to set it back later!
     *************************************************************/
#ifdef HAVE_SIGACTION
    sigaction(SIGALRM, NULL, &sigact);
    keep_sigact = sigact;
    keep_copysig = TRUE; /* yes, we have a copy */
    sigact.sa_handler = alarmfunc;
#ifdef SA_RESTART
    /* HPUX doesn't have SA_RESTART but defaults to that behaviour! */
    sigact.sa_flags &= ~SA_RESTART;
#endif
    /* now set the new struct */
    sigaction(SIGALRM, &sigact, NULL);
#else /* HAVE_SIGACTION */
    /* no sigaction(), revert to the much lamer signal() */
#ifdef HAVE_SIGNAL
    keep_sigact = signal(SIGALRM, alarmfunc);
#endif
#endif /* HAVE_SIGACTION */

    /* alarm() makes a signal get sent when the timeout fires off, and that
       will abort system calls */
    prev_alarm = alarm(curlx_sltoui(timeout/1000L));
  }

#else
#ifndef CURLRES_ASYNCH
  if(timeoutms)
    infof(conn->data, "timeout on name lookup is not supported\n");
#else
  (void)timeoutms; /* timeoutms not used with an async resolver */
#endif
#endif /* USE_ALARM_TIMEOUT */

  /* Perform the actual name resolution. This might be interrupted by an
   * alarm if it takes too long.
   */
  rc = Curl_resolv(conn, hostname, port, entry);

#ifdef USE_ALARM_TIMEOUT
clean_up:

  if(!prev_alarm)
    /* deactivate a possibly active alarm before uninstalling the handler */
    alarm(0);

#ifdef HAVE_SIGACTION
  if(keep_copysig) {
    /* we got a struct as it looked before, now put that one back nice
       and clean */
    sigaction(SIGALRM, &keep_sigact, NULL); /* put it back */
  }
#else
#ifdef HAVE_SIGNAL
  /* restore the previous SIGALRM handler */
  signal(SIGALRM, keep_sigact);
#endif
#endif /* HAVE_SIGACTION */

  /* switch back the alarm() to either zero or to what it was before minus
     the time we spent until now! */
  if(prev_alarm) {
    /* there was an alarm() set before us, now put it back */
    timediff_t elapsed_secs = Curl_timediff(Curl_now(),
                                            conn->created) / 1000;

    /* the alarm period is counted in even number of seconds */
    unsigned long alarm_set = prev_alarm - elapsed_secs;

    if(!alarm_set ||
       ((alarm_set >= 0x80000000) && (prev_alarm < 0x80000000)) ) {
      /* if the alarm time-left reached zero or turned "negative" (counted
         with unsigned values), we should fire off a SIGALRM here, but we
         won't, and zero would be to switch it off so we never set it to
         less than 1! */
      alarm(1);
      rc = CURLRESOLV_TIMEDOUT;
      failf(data, "Previous alarm fired off!");
    }
    else
      alarm((unsigned int)alarm_set);
  }
#endif /* USE_ALARM_TIMEOUT */

  return rc;
}

/*
 * Curl_resolv_unlock() unlocks the given cached DNS entry. When this has been
 * made, the struct may be destroyed due to pruning. It is important that only
 * one unlock is made for each Curl_resolv() call.
 *
 * May be called with 'data' == NULL for global cache.
 */
void Curl_resolv_unlock(struct Curl_easy *data, struct Curl_dns_entry *dns)
{
  if(data && data->share)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

  freednsentry(dns);

  if(data && data->share)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
}

/*
 * File-internal: release cache dns entry reference, free if inuse drops to 0
 */
static void freednsentry(void *freethis)
{
  struct Curl_dns_entry *dns = (struct Curl_dns_entry *) freethis;
  DEBUGASSERT(dns && (dns->inuse>0));

  dns->inuse--;
  if(dns->inuse == 0) {
    Curl_freeaddrinfo(dns->addr);
    free(dns);
  }
}

/*
 * Curl_mk_dnscache() inits a new DNS cache and returns success/failure.
 */
int Curl_mk_dnscache(struct curl_hash *hash)
{
  return Curl_hash_init(hash, 7, Curl_hash_str, Curl_str_key_compare,
                        freednsentry);
}

/*
 * Curl_hostcache_clean()
 *
 * This _can_ be called with 'data' == NULL but then of course no locking
 * can be done!
 */

void Curl_hostcache_clean(struct Curl_easy *data,
                          struct curl_hash *hash)
{
  if(data && data->share)
    Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

  Curl_hash_clean(hash);

  if(data && data->share)
    Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
}


CURLcode Curl_loadhostpairs(struct Curl_easy *data)
{
  struct curl_slist *hostp;
  char hostname[256];
  int port = 0;

  for(hostp = data->change.resolve; hostp; hostp = hostp->next) {
    if(!hostp->data)
      continue;
    if(hostp->data[0] == '-') {
      char *entry_id;
      size_t entry_len;

      if(2 != sscanf(hostp->data + 1, "%255[^:]:%d", hostname, &port)) {
        infof(data, "Couldn't parse CURLOPT_RESOLVE removal entry '%s'!\n",
              hostp->data);
        continue;
      }

      /* Create an entry id, based upon the hostname and port */
      entry_id = create_hostcache_id(hostname, port);
      /* If we can't create the entry id, fail */
      if(!entry_id) {
        return CURLE_OUT_OF_MEMORY;
      }

      entry_len = strlen(entry_id);

      if(data->share)
        Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

      /* delete entry, ignore if it didn't exist */
      Curl_hash_delete(data->dns.hostcache, entry_id, entry_len + 1);

      if(data->share)
        Curl_share_unlock(data, CURL_LOCK_DATA_DNS);

      /* free the allocated entry_id again */
      free(entry_id);
    }
    else {
      struct Curl_dns_entry *dns;
      Curl_addrinfo *head = NULL, *tail = NULL;
      char *entry_id;
      size_t entry_len;
      char address[64];
#if !defined(CURL_DISABLE_VERBOSE_STRINGS)
      char *addresses = NULL;
#endif
      char *addr_begin;
      char *addr_end;
      char *port_ptr;
      char *end_ptr;
      char *host_end;
      unsigned long tmp_port;
      bool error = true;

      host_end = strchr(hostp->data, ':');
      if(!host_end ||
         ((host_end - hostp->data) >= (ptrdiff_t)sizeof(hostname)))
        goto err;

      memcpy(hostname, hostp->data, host_end - hostp->data);
      hostname[host_end - hostp->data] = '\0';

      port_ptr = host_end + 1;
      tmp_port = strtoul(port_ptr, &end_ptr, 10);
      if(tmp_port > USHRT_MAX || end_ptr == port_ptr || *end_ptr != ':')
        goto err;

      port = (int)tmp_port;
#if !defined(CURL_DISABLE_VERBOSE_STRINGS)
      addresses = end_ptr + 1;
#endif

      while(*end_ptr) {
        size_t alen;
        Curl_addrinfo *ai;

        addr_begin = end_ptr + 1;
        addr_end = strchr(addr_begin, ',');
        if(!addr_end)
          addr_end = addr_begin + strlen(addr_begin);
        end_ptr = addr_end;

        /* allow IP(v6) address within [brackets] */
        if(*addr_begin == '[') {
          if(addr_end == addr_begin || *(addr_end - 1) != ']')
            goto err;
          ++addr_begin;
          --addr_end;
        }

        alen = addr_end - addr_begin;
        if(!alen)
          continue;

        if(alen >= sizeof(address))
          goto err;

        memcpy(address, addr_begin, alen);
        address[alen] = '\0';

#ifndef ENABLE_IPV6
        if(strchr(address, ':')) {
          infof(data, "Ignoring resolve address '%s', missing IPv6 support.\n",
                address);
          continue;
        }
#endif

        ai = Curl_str2addr(address, port);
        if(!ai) {
          infof(data, "Resolve address '%s' found illegal!\n", address);
          goto err;
        }

        if(tail) {
          tail->ai_next = ai;
          tail = tail->ai_next;
        }
        else {
          head = tail = ai;
        }
      }

      if(!head)
        goto err;

      error = false;
   err:
      if(error) {
        infof(data, "Couldn't parse CURLOPT_RESOLVE entry '%s'!\n",
              hostp->data);
        Curl_freeaddrinfo(head);
        continue;
      }

      /* Create an entry id, based upon the hostname and port */
      entry_id = create_hostcache_id(hostname, port);
      /* If we can't create the entry id, fail */
      if(!entry_id) {
        Curl_freeaddrinfo(head);
        return CURLE_OUT_OF_MEMORY;
      }
      entry_len = strlen(entry_id);

      if(data->share)
        Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);

      /* See if its already in our dns cache */
      dns = Curl_hash_pick(data->dns.hostcache, entry_id, entry_len + 1);

      if(dns) {
        infof(data, "RESOLVE %s:%d is - old addresses discarded!\n",
                hostname, port);
        /* delete old entry 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 added by CURLOPT_RESOLVE. */

        Curl_hash_delete(data->dns.hostcache, entry_id, entry_len + 1);
      }
      /* free the allocated entry_id again */
      free(entry_id);

      /* put this new host in the cache */
      dns = Curl_cache_addr(data, head, hostname, port);
      if(dns) {
        dns->timestamp = 0; /* mark as added by CURLOPT_RESOLVE */
        /* release the returned reference; the cache itself will keep the
         * entry alive: */
            dns->inuse--;
      }

      if(data->share)
        Curl_share_unlock(data, CURL_LOCK_DATA_DNS);

      if(!dns) {
        Curl_freeaddrinfo(head);
        return CURLE_OUT_OF_MEMORY;
      }
      infof(data, "Added %s:%d:%s to DNS cache\n",
            hostname, port, addresses);
    }
  }
  data->change.resolve = NULL; /* dealt with now */

  return CURLE_OK;
}