summaryrefslogtreecommitdiff
path: root/lib/asyn-ares.c
blob: 38ede999ce4f4ded6122d929a7894da92b14a1bc (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2017, 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"

/***********************************************************************
 * Only for ares-enabled builds
 * And only for functions that fulfill the asynch resolver backend API
 * as defined in asyn.h, nothing else belongs in this file!
 **********************************************************************/

#ifdef CURLRES_ARES

#ifdef HAVE_LIMITS_H
#include <limits.h>
#endif
#ifdef HAVE_NETINET_IN_H
#include <netinet/in.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_PROCESS_H
#include <process.h>
#endif

#if (defined(NETWARE) && defined(__NOVELL_LIBC__))
#undef in_addr_t
#define in_addr_t unsigned long
#endif

#include "urldata.h"
#include "sendf.h"
#include "hostip.h"
#include "hash.h"
#include "share.h"
#include "strerror.h"
#include "url.h"
#include "multiif.h"
#include "inet_pton.h"
#include "connect.h"
#include "select.h"
#include "progress.h"

#  if defined(CURL_STATICLIB) && !defined(CARES_STATICLIB) && \
     (defined(WIN32) || defined(_WIN32) || defined(__SYMBIAN32__))
#    define CARES_STATICLIB
#  endif
#  include <ares.h>
#  include <ares_version.h> /* really old c-ares didn't include this by
                               itself */

#if ARES_VERSION >= 0x010500
/* c-ares 1.5.0 or later, the callback proto is modified */
#define HAVE_CARES_CALLBACK_TIMEOUTS 1
#endif

/* The last 3 #include files should be in this order */
#include "curl_printf.h"
#include "curl_memory.h"
#include "memdebug.h"

struct ResolverResults {
  int num_pending; /* number of ares_gethostbyname() requests */
  Curl_addrinfo *temp_ai; /* intermediary result while fetching c-ares parts */
  int last_status;
};

/*
 * Curl_resolver_global_init() - the generic low-level asynchronous name
 * resolve API.  Called from curl_global_init() to initialize global resolver
 * environment.  Initializes ares library.
 */
int Curl_resolver_global_init(void)
{
#ifdef CARES_HAVE_ARES_LIBRARY_INIT
  if(ares_library_init(ARES_LIB_INIT_ALL)) {
    return CURLE_FAILED_INIT;
  }
#endif
  return CURLE_OK;
}

/*
 * Curl_resolver_global_cleanup()
 *
 * Called from curl_global_cleanup() to destroy global resolver environment.
 * Deinitializes ares library.
 */
void Curl_resolver_global_cleanup(void)
{
#ifdef CARES_HAVE_ARES_LIBRARY_CLEANUP
  ares_library_cleanup();
#endif
}

/*
 * Curl_resolver_init()
 *
 * Called from curl_easy_init() -> Curl_open() to initialize resolver
 * URL-state specific environment ('resolver' member of the UrlState
 * structure).  Fills the passed pointer by the initialized ares_channel.
 */
CURLcode Curl_resolver_init(void **resolver)
{
  int status = ares_init((ares_channel*)resolver);
  if(status != ARES_SUCCESS) {
    if(status == ARES_ENOMEM)
      return CURLE_OUT_OF_MEMORY;
    else
      return CURLE_FAILED_INIT;
  }
  return CURLE_OK;
  /* make sure that all other returns from this function should destroy the
     ares channel before returning error! */
}

/*
 * Curl_resolver_cleanup()
 *
 * Called from curl_easy_cleanup() -> Curl_close() to cleanup resolver
 * URL-state specific environment ('resolver' member of the UrlState
 * structure).  Destroys the ares channel.
 */
void Curl_resolver_cleanup(void *resolver)
{
  ares_destroy((ares_channel)resolver);
}

/*
 * Curl_resolver_duphandle()
 *
 * Called from curl_easy_duphandle() to duplicate resolver URL-state specific
 * environment ('resolver' member of the UrlState structure).  Duplicates the
 * 'from' ares channel and passes the resulting channel to the 'to' pointer.
 */
int Curl_resolver_duphandle(void **to, void *from)
{
  /* Clone the ares channel for the new handle */
  if(ARES_SUCCESS != ares_dup((ares_channel*)to, (ares_channel)from))
    return CURLE_FAILED_INIT;
  return CURLE_OK;
}

static void destroy_async_data(struct Curl_async *async);

/*
 * Cancel all possibly still on-going resolves for this connection.
 */
void Curl_resolver_cancel(struct connectdata *conn)
{
  if(conn->data && conn->data->state.resolver)
    ares_cancel((ares_channel)conn->data->state.resolver);
  destroy_async_data(&conn->async);
}

/*
 * destroy_async_data() cleans up async resolver data.
 */
static void destroy_async_data(struct Curl_async *async)
{
  free(async->hostname);

  if(async->os_specific) {
    struct ResolverResults *res = (struct ResolverResults *)async->os_specific;
    if(res) {
      if(res->temp_ai) {
        Curl_freeaddrinfo(res->temp_ai);
        res->temp_ai = NULL;
      }
      free(res);
    }
    async->os_specific = NULL;
  }

  async->hostname = NULL;
}

/*
 * Curl_resolver_getsock() is called when someone from the outside world
 * (using curl_multi_fdset()) wants to get our fd_set setup and we're talking
 * with ares. The caller must make sure that this function is only called when
 * we have a working ares channel.
 *
 * Returns: sockets-in-use-bitmap
 */

int Curl_resolver_getsock(struct connectdata *conn,
                          curl_socket_t *socks,
                          int numsocks)

{
  struct timeval maxtime;
  struct timeval timebuf;
  struct timeval *timeout;
  long milli;
  int max = ares_getsock((ares_channel)conn->data->state.resolver,
                         (ares_socket_t *)socks, numsocks);

  maxtime.tv_sec = CURL_TIMEOUT_RESOLVE;
  maxtime.tv_usec = 0;

  timeout = ares_timeout((ares_channel)conn->data->state.resolver, &maxtime,
                         &timebuf);
  milli = (timeout->tv_sec * 1000) + (timeout->tv_usec/1000);
  if(milli == 0)
    milli += 10;
  Curl_expire(conn->data, milli, EXPIRE_ASYNC_NAME);

  return max;
}

/*
 * waitperform()
 *
 * 1) Ask ares what sockets it currently plays with, then
 * 2) wait for the timeout period to check for action on ares' sockets.
 * 3) tell ares to act on all the sockets marked as "with action"
 *
 * return number of sockets it worked on
 */

static int waitperform(struct connectdata *conn, int timeout_ms)
{
  struct Curl_easy *data = conn->data;
  int nfds;
  int bitmask;
  ares_socket_t socks[ARES_GETSOCK_MAXNUM];
  struct pollfd pfd[ARES_GETSOCK_MAXNUM];
  int i;
  int num = 0;

  bitmask = ares_getsock((ares_channel)data->state.resolver, socks,
                         ARES_GETSOCK_MAXNUM);

  for(i = 0; i < ARES_GETSOCK_MAXNUM; i++) {
    pfd[i].events = 0;
    pfd[i].revents = 0;
    if(ARES_GETSOCK_READABLE(bitmask, i)) {
      pfd[i].fd = socks[i];
      pfd[i].events |= POLLRDNORM|POLLIN;
    }
    if(ARES_GETSOCK_WRITABLE(bitmask, i)) {
      pfd[i].fd = socks[i];
      pfd[i].events |= POLLWRNORM|POLLOUT;
    }
    if(pfd[i].events != 0)
      num++;
    else
      break;
  }

  if(num)
    nfds = Curl_poll(pfd, num, timeout_ms);
  else
    nfds = 0;

  if(!nfds)
    /* Call ares_process() unconditonally here, even if we simply timed out
       above, as otherwise the ares name resolve won't timeout! */
    ares_process_fd((ares_channel)data->state.resolver, ARES_SOCKET_BAD,
                    ARES_SOCKET_BAD);
  else {
    /* move through the descriptors and ask for processing on them */
    for(i = 0; i < num; i++)
      ares_process_fd((ares_channel)data->state.resolver,
                      pfd[i].revents & (POLLRDNORM|POLLIN)?
                      pfd[i].fd:ARES_SOCKET_BAD,
                      pfd[i].revents & (POLLWRNORM|POLLOUT)?
                      pfd[i].fd:ARES_SOCKET_BAD);
  }
  return nfds;
}

/*
 * Curl_resolver_is_resolved() is called repeatedly to check if a previous
 * name resolve request has completed. It should also make sure to time-out if
 * the operation seems to take too long.
 *
 * Returns normal CURLcode errors.
 */
CURLcode Curl_resolver_is_resolved(struct connectdata *conn,
                                   struct Curl_dns_entry **dns)
{
  struct Curl_easy *data = conn->data;
  struct ResolverResults *res = (struct ResolverResults *)
    conn->async.os_specific;
  CURLcode result = CURLE_OK;

  *dns = NULL;

  waitperform(conn, 0);

  if(res && !res->num_pending) {
    (void)Curl_addrinfo_callback(conn, res->last_status, res->temp_ai);
    /* temp_ai ownership is moved to the connection, so we need not free-up
       them */
    res->temp_ai = NULL;
    if(!conn->async.dns) {
      failf(data, "Could not resolve: %s (%s)",
            conn->async.hostname, ares_strerror(conn->async.status));
      result = conn->bits.proxy?CURLE_COULDNT_RESOLVE_PROXY:
        CURLE_COULDNT_RESOLVE_HOST;
    }
    else
      *dns = conn->async.dns;

    destroy_async_data(&conn->async);
  }

  return result;
}

/*
 * Curl_resolver_wait_resolv()
 *
 * waits for a resolve to finish. This function should be avoided since using
 * this risk getting the multi interface to "hang".
 *
 * If 'entry' is non-NULL, make it point to the resolved dns entry
 *
 * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved, and
 * CURLE_OPERATION_TIMEDOUT if a time-out occurred.
 */
CURLcode Curl_resolver_wait_resolv(struct connectdata *conn,
                                   struct Curl_dns_entry **entry)
{
  CURLcode result = CURLE_OK;
  struct Curl_easy *data = conn->data;
  timediff_t timeout;
  struct curltime now = Curl_now();
  struct Curl_dns_entry *temp_entry;

  if(entry)
    *entry = NULL; /* clear on entry */

  timeout = Curl_timeleft(data, &now, TRUE);
  if(timeout < 0) {
    /* already expired! */
    connclose(conn, "Timed out before name resolve started");
    return CURLE_OPERATION_TIMEDOUT;
  }
  if(!timeout)
    timeout = CURL_TIMEOUT_RESOLVE * 1000; /* default name resolve timeout */

  /* Wait for the name resolve query to complete. */
  while(!result) {
    struct timeval *tvp, tv, store;
    int itimeout;
    int timeout_ms;

    itimeout = (timeout > (long)INT_MAX) ? INT_MAX : (int)timeout;

    store.tv_sec = itimeout/1000;
    store.tv_usec = (itimeout%1000)*1000;

    tvp = ares_timeout((ares_channel)data->state.resolver, &store, &tv);

    /* use the timeout period ares returned to us above if less than one
       second is left, otherwise just use 1000ms to make sure the progress
       callback gets called frequent enough */
    if(!tvp->tv_sec)
      timeout_ms = (int)(tvp->tv_usec/1000);
    else
      timeout_ms = 1000;

    waitperform(conn, timeout_ms);
    result = Curl_resolver_is_resolved(conn, &temp_entry);

    if(result || conn->async.done)
      break;

    if(Curl_pgrsUpdate(conn))
      result = CURLE_ABORTED_BY_CALLBACK;
    else {
      struct curltime now2 = Curl_now();
      timediff_t timediff = Curl_timediff(now2, now); /* spent time */
      if(timediff <= 0)
        timeout -= 1; /* always deduct at least 1 */
      else if(timediff > timeout)
        timeout = -1;
      else
        timeout -= (long)timediff;
      now = now2; /* for next loop */
    }
    if(timeout < 0)
      result = CURLE_OPERATION_TIMEDOUT;
  }
  if(result)
    /* failure, so we cancel the ares operation */
    ares_cancel((ares_channel)data->state.resolver);

  /* Operation complete, if the lookup was successful we now have the entry
     in the cache. */
  if(entry)
    *entry = conn->async.dns;

  if(result)
    /* close the connection, since we can't return failure here without
       cleaning up this connection properly.
       TODO: remove this action from here, it is not a name resolver decision.
    */
    connclose(conn, "c-ares resolve failed");

  return result;
}

/* Connects results to the list */
static void compound_results(struct ResolverResults *res,
                             Curl_addrinfo *ai)
{
  Curl_addrinfo *ai_tail;
  if(!ai)
    return;
  ai_tail = ai;

  while(ai_tail->ai_next)
    ai_tail = ai_tail->ai_next;

  /* Add the new results to the list of old results. */
  ai_tail->ai_next = res->temp_ai;
  res->temp_ai = ai;
}

/*
 * ares_query_completed_cb() is the callback that ares will call when
 * the host query initiated by ares_gethostbyname() from Curl_getaddrinfo(),
 * when using ares, is completed either successfully or with failure.
 */
static void query_completed_cb(void *arg,  /* (struct connectdata *) */
                               int status,
#ifdef HAVE_CARES_CALLBACK_TIMEOUTS
                               int timeouts,
#endif
                               struct hostent *hostent)
{
  struct connectdata *conn = (struct connectdata *)arg;
  struct ResolverResults *res;

#ifdef HAVE_CARES_CALLBACK_TIMEOUTS
  (void)timeouts; /* ignored */
#endif

  if(ARES_EDESTRUCTION == status)
    /* when this ares handle is getting destroyed, the 'arg' pointer may not
       be valid so only defer it when we know the 'status' says its fine! */
    return;

  res = (struct ResolverResults *)conn->async.os_specific;
  res->num_pending--;

  if(CURL_ASYNC_SUCCESS == status) {
    Curl_addrinfo *ai = Curl_he2ai(hostent, conn->async.port);
    if(ai) {
      compound_results(res, ai);
    }
  }
  /* A successful result overwrites any previous error */
  if(res->last_status != ARES_SUCCESS)
    res->last_status = status;
}

/*
 * Curl_resolver_getaddrinfo() - when using ares
 *
 * Returns name information about the given hostname and port number. If
 * successful, the 'hostent' is returned and the forth argument will point to
 * memory we need to free after use. That memory *MUST* be freed with
 * Curl_freeaddrinfo(), nothing else.
 */
Curl_addrinfo *Curl_resolver_getaddrinfo(struct connectdata *conn,
                                         const char *hostname,
                                         int port,
                                         int *waitp)
{
  char *bufp;
  struct Curl_easy *data = conn->data;
  struct in_addr in;
  int family = PF_INET;
#ifdef ENABLE_IPV6 /* CURLRES_IPV6 */
  struct in6_addr in6;
#endif /* CURLRES_IPV6 */

  *waitp = 0; /* default to synchronous response */

  /* First check if this is an IPv4 address string */
  if(Curl_inet_pton(AF_INET, hostname, &in) > 0) {
    /* This is a dotted IP address 123.123.123.123-style */
    return Curl_ip2addr(AF_INET, &in, hostname, port);
  }

#ifdef ENABLE_IPV6 /* CURLRES_IPV6 */
  /* Otherwise, check if this is an IPv6 address string */
  if(Curl_inet_pton (AF_INET6, hostname, &in6) > 0)
    /* This must be an IPv6 address literal.  */
    return Curl_ip2addr(AF_INET6, &in6, hostname, port);

  switch(conn->ip_version) {
  default:
#if ARES_VERSION >= 0x010601
    family = PF_UNSPEC; /* supported by c-ares since 1.6.1, so for older
                           c-ares versions this just falls through and defaults
                           to PF_INET */
    break;
#endif
  case CURL_IPRESOLVE_V4:
    family = PF_INET;
    break;
  case CURL_IPRESOLVE_V6:
    family = PF_INET6;
    break;
  }
#endif /* CURLRES_IPV6 */

  bufp = strdup(hostname);
  if(bufp) {
    struct ResolverResults *res = NULL;
    free(conn->async.hostname);
    conn->async.hostname = bufp;
    conn->async.port = port;
    conn->async.done = FALSE;   /* not done */
    conn->async.status = 0;     /* clear */
    conn->async.dns = NULL;     /* clear */
    res = calloc(sizeof(struct ResolverResults), 1);
    if(!res) {
      free(conn->async.hostname);
      conn->async.hostname = NULL;
      return NULL;
    }
    conn->async.os_specific = res;

    /* initial status - failed */
    res->last_status = ARES_ENOTFOUND;
#ifdef ENABLE_IPV6 /* CURLRES_IPV6 */
    if(family == PF_UNSPEC) {
      if(Curl_ipv6works()) {
        res->num_pending = 2;

        /* areschannel is already setup in the Curl_open() function */
        ares_gethostbyname((ares_channel)data->state.resolver, hostname,
                            PF_INET, query_completed_cb, conn);
        ares_gethostbyname((ares_channel)data->state.resolver, hostname,
                            PF_INET6, query_completed_cb, conn);
      }
      else {
        res->num_pending = 1;

        /* areschannel is already setup in the Curl_open() function */
        ares_gethostbyname((ares_channel)data->state.resolver, hostname,
                            PF_INET, query_completed_cb, conn);
      }
    }
    else
#endif /* CURLRES_IPV6 */
    {
      res->num_pending = 1;

      /* areschannel is already setup in the Curl_open() function */
      ares_gethostbyname((ares_channel)data->state.resolver, hostname, family,
                         query_completed_cb, conn);
    }

    *waitp = 1; /* expect asynchronous response */
  }
  return NULL; /* no struct yet */
}

CURLcode Curl_set_dns_servers(struct Curl_easy *data,
                              char *servers)
{
  CURLcode result = CURLE_NOT_BUILT_IN;
  int ares_result;

  /* If server is NULL or empty, this would purge all DNS servers
   * from ares library, which will cause any and all queries to fail.
   * So, just return OK if none are configured and don't actually make
   * any changes to c-ares.  This lets c-ares use it's defaults, which
   * it gets from the OS (for instance from /etc/resolv.conf on Linux).
   */
  if(!(servers && servers[0]))
    return CURLE_OK;

#if (ARES_VERSION >= 0x010704)
  ares_result = ares_set_servers_csv(data->state.resolver, servers);
  switch(ares_result) {
  case ARES_SUCCESS:
    result = CURLE_OK;
    break;
  case ARES_ENOMEM:
    result = CURLE_OUT_OF_MEMORY;
    break;
  case ARES_ENOTINITIALIZED:
  case ARES_ENODATA:
  case ARES_EBADSTR:
  default:
    result = CURLE_BAD_FUNCTION_ARGUMENT;
    break;
  }
#else /* too old c-ares version! */
  (void)data;
  (void)(ares_result);
#endif
  return result;
}

CURLcode Curl_set_dns_interface(struct Curl_easy *data,
                                const char *interf)
{
#if (ARES_VERSION >= 0x010704)
  if(!interf)
    interf = "";

  ares_set_local_dev((ares_channel)data->state.resolver, interf);

  return CURLE_OK;
#else /* c-ares version too old! */
  (void)data;
  (void)interf;
  return CURLE_NOT_BUILT_IN;
#endif
}

CURLcode Curl_set_dns_local_ip4(struct Curl_easy *data,
                                const char *local_ip4)
{
#if (ARES_VERSION >= 0x010704)
  struct in_addr a4;

  if((!local_ip4) || (local_ip4[0] == 0)) {
    a4.s_addr = 0; /* disabled: do not bind to a specific address */
  }
  else {
    if(Curl_inet_pton(AF_INET, local_ip4, &a4) != 1) {
      return CURLE_BAD_FUNCTION_ARGUMENT;
    }
  }

  ares_set_local_ip4((ares_channel)data->state.resolver, ntohl(a4.s_addr));

  return CURLE_OK;
#else /* c-ares version too old! */
  (void)data;
  (void)local_ip4;
  return CURLE_NOT_BUILT_IN;
#endif
}

CURLcode Curl_set_dns_local_ip6(struct Curl_easy *data,
                                const char *local_ip6)
{
#if (ARES_VERSION >= 0x010704) && defined(ENABLE_IPV6)
  unsigned char a6[INET6_ADDRSTRLEN];

  if((!local_ip6) || (local_ip6[0] == 0)) {
    /* disabled: do not bind to a specific address */
    memset(a6, 0, sizeof(a6));
  }
  else {
    if(Curl_inet_pton(AF_INET6, local_ip6, a6) != 1) {
      return CURLE_BAD_FUNCTION_ARGUMENT;
    }
  }

  ares_set_local_ip6((ares_channel)data->state.resolver, a6);

  return CURLE_OK;
#else /* c-ares version too old! */
  (void)data;
  (void)local_ip6;
  return CURLE_NOT_BUILT_IN;
#endif
}
#endif /* CURLRES_ARES */