summaryrefslogtreecommitdiff
path: root/src/resolve/resolved-dns-server.c
blob: 7282848e35de1bf13f346c604d17f6b2e560c2ec (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
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
/***
  This file is part of systemd.

  Copyright 2014 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd 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
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/

#include <sd-messages.h>

#include "alloc-util.h"
#include "resolved-dns-server.h"
#include "resolved-dns-stub.h"
#include "resolved-resolv-conf.h"
#include "siphash24.h"
#include "string-table.h"
#include "string-util.h"

/* After how much time to repeat classic DNS requests */
#define DNS_TIMEOUT_MIN_USEC (500 * USEC_PER_MSEC)
#define DNS_TIMEOUT_MAX_USEC (5 * USEC_PER_SEC)

/* The amount of time to wait before retrying with a full feature set */
#define DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC (6 * USEC_PER_HOUR)
#define DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC (5 * USEC_PER_MINUTE)

/* The number of times we will attempt a certain feature set before degrading */
#define DNS_SERVER_FEATURE_RETRY_ATTEMPTS 3

int dns_server_new(
                Manager *m,
                DnsServer **ret,
                DnsServerType type,
                Link *l,
                int family,
                const union in_addr_union *in_addr,
                int ifindex) {

        DnsServer *s;

        assert(m);
        assert((type == DNS_SERVER_LINK) == !!l);
        assert(in_addr);

        if (!IN_SET(family, AF_INET, AF_INET6))
                return -EAFNOSUPPORT;

        if (l) {
                if (l->n_dns_servers >= LINK_DNS_SERVERS_MAX)
                        return -E2BIG;
        } else {
                if (m->n_dns_servers >= MANAGER_DNS_SERVERS_MAX)
                        return -E2BIG;
        }

        s = new0(DnsServer, 1);
        if (!s)
                return -ENOMEM;

        s->n_ref = 1;
        s->manager = m;
        s->verified_feature_level = _DNS_SERVER_FEATURE_LEVEL_INVALID;
        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST;
        s->features_grace_period_usec = DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC;
        s->received_udp_packet_max = DNS_PACKET_UNICAST_SIZE_MAX;
        s->type = type;
        s->family = family;
        s->address = *in_addr;
        s->ifindex = ifindex;
        s->resend_timeout = DNS_TIMEOUT_MIN_USEC;

        switch (type) {

        case DNS_SERVER_LINK:
                s->link = l;
                LIST_APPEND(servers, l->dns_servers, s);
                l->n_dns_servers++;
                break;

        case DNS_SERVER_SYSTEM:
                LIST_APPEND(servers, m->dns_servers, s);
                m->n_dns_servers++;
                break;

        case DNS_SERVER_FALLBACK:
                LIST_APPEND(servers, m->fallback_dns_servers, s);
                m->n_dns_servers++;
                break;

        default:
                assert_not_reached("Unknown server type");
        }

        s->linked = true;

        /* A new DNS server that isn't fallback is added and the one
         * we used so far was a fallback one? Then let's try to pick
         * the new one */
        if (type != DNS_SERVER_FALLBACK &&
            m->current_dns_server &&
            m->current_dns_server->type == DNS_SERVER_FALLBACK)
                manager_set_dns_server(m, NULL);

        if (ret)
                *ret = s;

        return 0;
}

DnsServer* dns_server_ref(DnsServer *s)  {
        if (!s)
                return NULL;

        assert(s->n_ref > 0);
        s->n_ref++;

        return s;
}

DnsServer* dns_server_unref(DnsServer *s)  {
        if (!s)
                return NULL;

        assert(s->n_ref > 0);
        s->n_ref--;

        if (s->n_ref > 0)
                return NULL;

        free(s->server_string);
        return mfree(s);
}

void dns_server_unlink(DnsServer *s) {
        assert(s);
        assert(s->manager);

        /* This removes the specified server from the linked list of
         * servers, but any server might still stay around if it has
         * refs, for example from an ongoing transaction. */

        if (!s->linked)
                return;

        switch (s->type) {

        case DNS_SERVER_LINK:
                assert(s->link);
                assert(s->link->n_dns_servers > 0);
                LIST_REMOVE(servers, s->link->dns_servers, s);
                s->link->n_dns_servers--;
                break;

        case DNS_SERVER_SYSTEM:
                assert(s->manager->n_dns_servers > 0);
                LIST_REMOVE(servers, s->manager->dns_servers, s);
                s->manager->n_dns_servers--;
                break;

        case DNS_SERVER_FALLBACK:
                assert(s->manager->n_dns_servers > 0);
                LIST_REMOVE(servers, s->manager->fallback_dns_servers, s);
                s->manager->n_dns_servers--;
                break;
        }

        s->linked = false;

        if (s->link && s->link->current_dns_server == s)
                link_set_dns_server(s->link, NULL);

        if (s->manager->current_dns_server == s)
                manager_set_dns_server(s->manager, NULL);

        dns_server_unref(s);
}

void dns_server_move_back_and_unmark(DnsServer *s) {
        DnsServer *tail;

        assert(s);

        if (!s->marked)
                return;

        s->marked = false;

        if (!s->linked || !s->servers_next)
                return;

        /* Move us to the end of the list, so that the order is
         * strictly kept, if we are not at the end anyway. */

        switch (s->type) {

        case DNS_SERVER_LINK:
                assert(s->link);
                LIST_FIND_TAIL(servers, s, tail);
                LIST_REMOVE(servers, s->link->dns_servers, s);
                LIST_INSERT_AFTER(servers, s->link->dns_servers, tail, s);
                break;

        case DNS_SERVER_SYSTEM:
                LIST_FIND_TAIL(servers, s, tail);
                LIST_REMOVE(servers, s->manager->dns_servers, s);
                LIST_INSERT_AFTER(servers, s->manager->dns_servers, tail, s);
                break;

        case DNS_SERVER_FALLBACK:
                LIST_FIND_TAIL(servers, s, tail);
                LIST_REMOVE(servers, s->manager->fallback_dns_servers, s);
                LIST_INSERT_AFTER(servers, s->manager->fallback_dns_servers, tail, s);
                break;

        default:
                assert_not_reached("Unknown server type");
        }
}

static void dns_server_verified(DnsServer *s, DnsServerFeatureLevel level) {
        assert(s);

        if (s->verified_feature_level > level)
                return;

        if (s->verified_feature_level != level) {
                log_debug("Verified we get a response at feature level %s from DNS server %s.",
                          dns_server_feature_level_to_string(level),
                          dns_server_string(s));
                s->verified_feature_level = level;
        }

        assert_se(sd_event_now(s->manager->event, clock_boottime_or_monotonic(), &s->verified_usec) >= 0);
}

static void dns_server_reset_counters(DnsServer *s) {
        assert(s);

        s->n_failed_udp = 0;
        s->n_failed_tcp = 0;
        s->packet_truncated = false;
        s->verified_usec = 0;

        /* Note that we do not reset s->packet_bad_opt and s->packet_rrsig_missing here. We reset them only when the
         * grace period ends, but not when lowering the possible feature level, as a lower level feature level should
         * not make RRSIGs appear or OPT appear, but rather make them disappear. If the reappear anyway, then that's
         * indication for a differently broken OPT/RRSIG implementation, and we really don't want to support that
         * either.
         *
         * This is particularly important to deal with certain Belkin routers which break OPT for certain lookups (A),
         * but pass traffic through for others (AAAA). If we detect the broken behaviour on one lookup we should not
         * reenable it for another, because we cannot validate things anyway, given that the RRSIG/OPT data will be
         * incomplete. */
}

void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t rtt, size_t size) {
        assert(s);

        if (protocol == IPPROTO_UDP) {
                if (s->possible_feature_level == level)
                        s->n_failed_udp = 0;

                /* If the RRSIG data is missing, then we can only validate EDNS0 at max */
                if (s->packet_rrsig_missing && level >= DNS_SERVER_FEATURE_LEVEL_DO)
                        level = DNS_SERVER_FEATURE_LEVEL_DO - 1;

                /* If the OPT RR got lost, then we can only validate UDP at max */
                if (s->packet_bad_opt && level >= DNS_SERVER_FEATURE_LEVEL_EDNS0)
                        level = DNS_SERVER_FEATURE_LEVEL_EDNS0 - 1;

                /* Even if we successfully receive a reply to a request announcing support for large packets,
                   that does not mean we can necessarily receive large packets. */
                if (level == DNS_SERVER_FEATURE_LEVEL_LARGE)
                        level = DNS_SERVER_FEATURE_LEVEL_LARGE - 1;

        } else if (protocol == IPPROTO_TCP) {

                if (s->possible_feature_level == level)
                        s->n_failed_tcp = 0;

                /* Successful TCP connections are only useful to verify the TCP feature level. */
                level = DNS_SERVER_FEATURE_LEVEL_TCP;
        }

        dns_server_verified(s, level);

        /* Remember the size of the largest UDP packet we received from a server,
           we know that we can always announce support for packets with at least
           this size. */
        if (protocol == IPPROTO_UDP && s->received_udp_packet_max < size)
                s->received_udp_packet_max = size;

        if (s->max_rtt < rtt) {
                s->max_rtt = rtt;
                s->resend_timeout = CLAMP(s->max_rtt * 2, DNS_TIMEOUT_MIN_USEC, DNS_TIMEOUT_MAX_USEC);
        }
}

void dns_server_packet_lost(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t usec) {
        assert(s);
        assert(s->manager);

        if (s->possible_feature_level == level) {
                if (protocol == IPPROTO_UDP)
                        s->n_failed_udp++;
                else if (protocol == IPPROTO_TCP)
                        s->n_failed_tcp++;
        }

        if (s->resend_timeout > usec)
                return;

        s->resend_timeout = MIN(s->resend_timeout * 2, DNS_TIMEOUT_MAX_USEC);
}

void dns_server_packet_truncated(DnsServer *s, DnsServerFeatureLevel level) {
        assert(s);

        /* Invoked whenever we get a packet with TC bit set. */

        if (s->possible_feature_level != level)
                return;

        s->packet_truncated = true;
}

void dns_server_packet_rrsig_missing(DnsServer *s, DnsServerFeatureLevel level) {
        assert(s);

        if (level < DNS_SERVER_FEATURE_LEVEL_DO)
                return;

        /* If the RRSIG RRs are missing, we have to downgrade what we previously verified */
        if (s->verified_feature_level >= DNS_SERVER_FEATURE_LEVEL_DO)
                s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_DO-1;

        s->packet_rrsig_missing = true;
}

void dns_server_packet_bad_opt(DnsServer *s, DnsServerFeatureLevel level) {
        assert(s);

        if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0)
                return;

        /* If the OPT RR got lost, we have to downgrade what we previously verified */
        if (s->verified_feature_level >= DNS_SERVER_FEATURE_LEVEL_EDNS0)
                s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_EDNS0-1;

        s->packet_bad_opt = true;
}

void dns_server_packet_rcode_downgrade(DnsServer *s, DnsServerFeatureLevel level) {
        assert(s);

        /* Invoked whenever we got a FORMERR, SERVFAIL or NOTIMP rcode from a server and downgrading the feature level
         * for the transaction made it go away. In this case we immediately downgrade to the feature level that made
         * things work. */

        if (s->verified_feature_level > level)
                s->verified_feature_level = level;

        if (s->possible_feature_level > level) {
                s->possible_feature_level = level;
                dns_server_reset_counters(s);
        }

        log_debug("Downgrading transaction feature level fixed an RCODE error, downgrading server %s too.", dns_server_string(s));
}

static bool dns_server_grace_period_expired(DnsServer *s) {
        usec_t ts;

        assert(s);
        assert(s->manager);

        if (s->verified_usec == 0)
                return false;

        assert_se(sd_event_now(s->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);

        if (s->verified_usec + s->features_grace_period_usec > ts)
                return false;

        s->features_grace_period_usec = MIN(s->features_grace_period_usec * 2, DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC);

        return true;
}

DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) {
        assert(s);

        if (s->possible_feature_level != DNS_SERVER_FEATURE_LEVEL_BEST &&
            dns_server_grace_period_expired(s)) {

                s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST;

                dns_server_reset_counters(s);

                s->packet_bad_opt = false;
                s->packet_rrsig_missing = false;

                log_info("Grace period over, resuming full feature set (%s) for DNS server %s.",
                         dns_server_feature_level_to_string(s->possible_feature_level),
                         dns_server_string(s));

        } else if (s->possible_feature_level <= s->verified_feature_level)
                s->possible_feature_level = s->verified_feature_level;
        else {
                DnsServerFeatureLevel p = s->possible_feature_level;

                if (s->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS &&
                    s->possible_feature_level == DNS_SERVER_FEATURE_LEVEL_TCP) {

                        /* We are at the TCP (lowest) level, and we tried a couple of TCP connections, and it didn't
                         * work. Upgrade back to UDP again. */
                        log_debug("Reached maximum number of failed TCP connection attempts, trying UDP again...");
                        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP;

                } else if (s->packet_bad_opt &&
                           s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_EDNS0) {

                        /* A reply to one of our EDNS0 queries didn't carry a valid OPT RR, then downgrade to below
                         * EDNS0 levels. After all, some records generate different responses with and without OPT RR
                         * in the request. Example:
                         * https://open.nlnetlabs.nl/pipermail/dnssec-trigger/2014-November/000376.html */

                        log_debug("Server doesn't support EDNS(0) properly, downgrading feature level...");
                        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP;

                } else if (s->packet_rrsig_missing &&
                           s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_DO) {

                        /* RRSIG data was missing on a EDNS0 packet with DO bit set. This means the server doesn't
                         * augment responses with DNSSEC RRs. If so, let's better not ask the server for it anymore,
                         * after all some servers generate different replies depending if an OPT RR is in the query or
                         * not. */

                        log_debug("Detected server responses lack RRSIG records, downgrading feature level...");
                        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_EDNS0;

                } else if (s->n_failed_udp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS &&
                            s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_UDP) {

                        /* We lost too many UDP packets in a row, and are on a feature level of UDP or higher. If the
                         * packets are lost, maybe the server cannot parse them, hence downgrading sounds like a good
                         * idea. We might downgrade all the way down to TCP this way. */

                        log_debug("Lost too many UDP packets, downgrading feature level...");
                        s->possible_feature_level--;

                } else if (s->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS &&
                           s->packet_truncated &&
                           s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_UDP) {

                         /* We got too many TCP connection failures in a row, we had at least one truncated packet, and
                          * are on a feature level above UDP. By downgrading things and getting rid of DNSSEC or EDNS0
                          * data we hope to make the packet smaller, so that it still works via UDP given that TCP
                          * appears not to be a fallback. Note that if we are already at the lowest UDP level, we don't
                          * go further down, since that's TCP, and TCP failed too often after all. */

                        log_debug("Got too many failed TCP connection failures and truncated UDP packets, downgrading feature level...");
                        s->possible_feature_level--;
                }

                if (p != s->possible_feature_level) {

                        /* We changed the feature level, reset the counting */
                        dns_server_reset_counters(s);

                        log_warning("Using degraded feature set (%s) for DNS server %s.",
                                    dns_server_feature_level_to_string(s->possible_feature_level),
                                    dns_server_string(s));
                }
        }

        return s->possible_feature_level;
}

int dns_server_adjust_opt(DnsServer *server, DnsPacket *packet, DnsServerFeatureLevel level) {
        size_t packet_size;
        bool edns_do;
        int r;

        assert(server);
        assert(packet);
        assert(packet->protocol == DNS_PROTOCOL_DNS);

        /* Fix the OPT field in the packet to match our current feature level. */

        r = dns_packet_truncate_opt(packet);
        if (r < 0)
                return r;

        if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0)
                return 0;

        edns_do = level >= DNS_SERVER_FEATURE_LEVEL_DO;

        if (level >= DNS_SERVER_FEATURE_LEVEL_LARGE)
                packet_size = DNS_PACKET_UNICAST_SIZE_LARGE_MAX;
        else
                packet_size = server->received_udp_packet_max;

        return dns_packet_append_opt(packet, packet_size, edns_do, 0, NULL);
}

int dns_server_ifindex(const DnsServer *s) {
        assert(s);

        /* The link ifindex always takes precedence */
        if (s->link)
                return s->link->ifindex;

        if (s->ifindex > 0)
                return s->ifindex;

        return 0;
}

const char *dns_server_string(DnsServer *server) {
        assert(server);

        if (!server->server_string)
                (void) in_addr_ifindex_to_string(server->family, &server->address, dns_server_ifindex(server), &server->server_string);

        return strna(server->server_string);
}

bool dns_server_dnssec_supported(DnsServer *server) {
        assert(server);

        /* Returns whether the server supports DNSSEC according to what we know about it */

        if (server->possible_feature_level < DNS_SERVER_FEATURE_LEVEL_DO)
                return false;

        if (server->packet_bad_opt)
                return false;

        if (server->packet_rrsig_missing)
                return false;

        /* DNSSEC servers need to support TCP properly (see RFC5966), if they don't, we assume DNSSEC is borked too */
        if (server->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS)
                return false;

        return true;
}

void dns_server_warn_downgrade(DnsServer *server) {
        assert(server);

        if (server->warned_downgrade)
                return;

        log_struct(LOG_NOTICE,
                   LOG_MESSAGE_ID(SD_MESSAGE_DNSSEC_DOWNGRADE),
                   LOG_MESSAGE("Server %s does not support DNSSEC, downgrading to non-DNSSEC mode.", dns_server_string(server)),
                   "DNS_SERVER=%s", dns_server_string(server),
                   "DNS_SERVER_FEATURE_LEVEL=%s", dns_server_feature_level_to_string(server->possible_feature_level),
                   NULL);

        server->warned_downgrade = true;
}

bool dns_server_limited_domains(DnsServer *server)
{
        DnsSearchDomain *domain;
        bool domain_restricted = false;

        /* Check if the server has route-only domains without ~., i. e. whether
         * it should only be used for particular domains */
        if (!server->link)
                return false;

        LIST_FOREACH(domains, domain, server->link->search_domains)
                if (domain->route_only) {
                        domain_restricted = true;
                        /* ~. means "any domain", thus it is a global server */
                        if (streq(DNS_SEARCH_DOMAIN_NAME(domain), "."))
                                return false;
                }

        return domain_restricted;
}

static void dns_server_hash_func(const void *p, struct siphash *state) {
        const DnsServer *s = p;

        assert(s);

        siphash24_compress(&s->family, sizeof(s->family), state);
        siphash24_compress(&s->address, FAMILY_ADDRESS_SIZE(s->family), state);
        siphash24_compress(&s->ifindex, sizeof(s->ifindex), state);
}

static int dns_server_compare_func(const void *a, const void *b) {
        const DnsServer *x = a, *y = b;
        int r;

        if (x->family < y->family)
                return -1;
        if (x->family > y->family)
                return 1;

        r = memcmp(&x->address, &y->address, FAMILY_ADDRESS_SIZE(x->family));
        if (r != 0)
                return r;

        if (x->ifindex < y->ifindex)
                return -1;
        if (x->ifindex > y->ifindex)
                return 1;

        return 0;
}

const struct hash_ops dns_server_hash_ops = {
        .hash = dns_server_hash_func,
        .compare = dns_server_compare_func
};

void dns_server_unlink_all(DnsServer *first) {
        DnsServer *next;

        if (!first)
                return;

        next = first->servers_next;
        dns_server_unlink(first);

        dns_server_unlink_all(next);
}

void dns_server_unlink_marked(DnsServer *first) {
        DnsServer *next;

        if (!first)
                return;

        next = first->servers_next;

        if (first->marked)
                dns_server_unlink(first);

        dns_server_unlink_marked(next);
}

void dns_server_mark_all(DnsServer *first) {
        if (!first)
                return;

        first->marked = true;
        dns_server_mark_all(first->servers_next);
}

DnsServer *dns_server_find(DnsServer *first, int family, const union in_addr_union *in_addr, int ifindex) {
        DnsServer *s;

        LIST_FOREACH(servers, s, first)
                if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0 && s->ifindex == ifindex)
                        return s;

        return NULL;
}

DnsServer *manager_get_first_dns_server(Manager *m, DnsServerType t) {
        assert(m);

        switch (t) {

        case DNS_SERVER_SYSTEM:
                return m->dns_servers;

        case DNS_SERVER_FALLBACK:
                return m->fallback_dns_servers;

        default:
                return NULL;
        }
}

DnsServer *manager_set_dns_server(Manager *m, DnsServer *s) {
        assert(m);

        if (m->current_dns_server == s)
                return s;

        if (s)
                log_info("Switching to %s DNS server %s.",
                         dns_server_type_to_string(s->type),
                         dns_server_string(s));

        dns_server_unref(m->current_dns_server);
        m->current_dns_server = dns_server_ref(s);

        if (m->unicast_scope)
                dns_cache_flush(&m->unicast_scope->cache);

        return s;
}

DnsServer *manager_get_dns_server(Manager *m) {
        Link *l;
        assert(m);

        /* Try to read updates resolv.conf */
        manager_read_resolv_conf(m);

        /* If no DNS server was chosen so far, pick the first one */
        if (!m->current_dns_server)
                manager_set_dns_server(m, m->dns_servers);

        if (!m->current_dns_server) {
                bool found = false;
                Iterator i;

                /* No DNS servers configured, let's see if there are
                 * any on any links. If not, we use the fallback
                 * servers */

                HASHMAP_FOREACH(l, m->links, i)
                        if (l->dns_servers) {
                                found = true;
                                break;
                        }

                if (!found)
                        manager_set_dns_server(m, m->fallback_dns_servers);
        }

        return m->current_dns_server;
}

void manager_next_dns_server(Manager *m) {
        assert(m);

        /* If there's currently no DNS server set, then the next
         * manager_get_dns_server() will find one */
        if (!m->current_dns_server)
                return;

        /* Change to the next one, but make sure to follow the linked
         * list only if the server is still linked. */
        if (m->current_dns_server->linked && m->current_dns_server->servers_next) {
                manager_set_dns_server(m, m->current_dns_server->servers_next);
                return;
        }

        /* If there was no next one, then start from the beginning of
         * the list */
        if (m->current_dns_server->type == DNS_SERVER_FALLBACK)
                manager_set_dns_server(m, m->fallback_dns_servers);
        else
                manager_set_dns_server(m, m->dns_servers);
}

bool dns_server_address_valid(int family, const union in_addr_union *sa) {

        /* Refuses the 0 IP addresses as well as 127.0.0.53 (which is our own DNS stub) */

        if (in_addr_is_null(family, sa))
                return false;

        if (family == AF_INET && sa->in.s_addr == htobe32(INADDR_DNS_STUB))
                return false;

        return true;
}

static const char* const dns_server_type_table[_DNS_SERVER_TYPE_MAX] = {
        [DNS_SERVER_SYSTEM] = "system",
        [DNS_SERVER_FALLBACK] = "fallback",
        [DNS_SERVER_LINK] = "link",
};
DEFINE_STRING_TABLE_LOOKUP(dns_server_type, DnsServerType);

static const char* const dns_server_feature_level_table[_DNS_SERVER_FEATURE_LEVEL_MAX] = {
        [DNS_SERVER_FEATURE_LEVEL_TCP] = "TCP",
        [DNS_SERVER_FEATURE_LEVEL_UDP] = "UDP",
        [DNS_SERVER_FEATURE_LEVEL_EDNS0] = "UDP+EDNS0",
        [DNS_SERVER_FEATURE_LEVEL_DO] = "UDP+EDNS0+DO",
        [DNS_SERVER_FEATURE_LEVEL_LARGE] = "UDP+EDNS0+DO+LARGE",
};
DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level, DnsServerFeatureLevel);