2 This file is part of systemd.
4 Copyright 2014 Lennart Poettering
6 systemd is free software; you can redistribute it and/or modify it
7 under the terms of the GNU Lesser General Public License as published by
8 the Free Software Foundation; either version 2.1 of the License, or
9 (at your option) any later version.
11 systemd is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
16 You should have received a copy of the GNU Lesser General Public License
17 along with systemd; If not, see <http://www.gnu.org/licenses/>.
20 #include <sd-messages.h>
22 #include "alloc-util.h"
23 #include "resolved-dns-server.h"
24 #include "resolved-resolv-conf.h"
25 #include "siphash24.h"
26 #include "string-table.h"
27 #include "string-util.h"
29 /* After how much time to repeat classic DNS requests */
30 #define DNS_TIMEOUT_MIN_USEC (500 * USEC_PER_MSEC)
31 #define DNS_TIMEOUT_MAX_USEC (5 * USEC_PER_SEC)
33 /* The amount of time to wait before retrying with a full feature set */
34 #define DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC (6 * USEC_PER_HOUR)
35 #define DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC (5 * USEC_PER_MINUTE)
37 /* The number of times we will attempt a certain feature set before degrading */
38 #define DNS_SERVER_FEATURE_RETRY_ATTEMPTS 3
46 const union in_addr_union
*in_addr
) {
51 assert((type
== DNS_SERVER_LINK
) == !!l
);
54 if (!IN_SET(family
, AF_INET
, AF_INET6
))
58 if (l
->n_dns_servers
>= LINK_DNS_SERVERS_MAX
)
61 if (m
->n_dns_servers
>= MANAGER_DNS_SERVERS_MAX
)
65 s
= new0(DnsServer
, 1);
71 s
->verified_feature_level
= _DNS_SERVER_FEATURE_LEVEL_INVALID
;
72 s
->possible_feature_level
= DNS_SERVER_FEATURE_LEVEL_BEST
;
73 s
->features_grace_period_usec
= DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC
;
74 s
->received_udp_packet_max
= DNS_PACKET_UNICAST_SIZE_MAX
;
77 s
->address
= *in_addr
;
78 s
->resend_timeout
= DNS_TIMEOUT_MIN_USEC
;
84 LIST_APPEND(servers
, l
->dns_servers
, s
);
88 case DNS_SERVER_SYSTEM
:
89 LIST_APPEND(servers
, m
->dns_servers
, s
);
93 case DNS_SERVER_FALLBACK
:
94 LIST_APPEND(servers
, m
->fallback_dns_servers
, s
);
99 assert_not_reached("Unknown server type");
104 /* A new DNS server that isn't fallback is added and the one
105 * we used so far was a fallback one? Then let's try to pick
107 if (type
!= DNS_SERVER_FALLBACK
&&
108 m
->current_dns_server
&&
109 m
->current_dns_server
->type
== DNS_SERVER_FALLBACK
)
110 manager_set_dns_server(m
, NULL
);
118 DnsServer
* dns_server_ref(DnsServer
*s
) {
122 assert(s
->n_ref
> 0);
128 DnsServer
* dns_server_unref(DnsServer
*s
) {
132 assert(s
->n_ref
> 0);
138 free(s
->server_string
);
143 void dns_server_unlink(DnsServer
*s
) {
147 /* This removes the specified server from the linked list of
148 * servers, but any server might still stay around if it has
149 * refs, for example from an ongoing transaction. */
156 case DNS_SERVER_LINK
:
158 assert(s
->link
->n_dns_servers
> 0);
159 LIST_REMOVE(servers
, s
->link
->dns_servers
, s
);
162 case DNS_SERVER_SYSTEM
:
163 assert(s
->manager
->n_dns_servers
> 0);
164 LIST_REMOVE(servers
, s
->manager
->dns_servers
, s
);
165 s
->manager
->n_dns_servers
--;
168 case DNS_SERVER_FALLBACK
:
169 assert(s
->manager
->n_dns_servers
> 0);
170 LIST_REMOVE(servers
, s
->manager
->fallback_dns_servers
, s
);
171 s
->manager
->n_dns_servers
--;
177 if (s
->link
&& s
->link
->current_dns_server
== s
)
178 link_set_dns_server(s
->link
, NULL
);
180 if (s
->manager
->current_dns_server
== s
)
181 manager_set_dns_server(s
->manager
, NULL
);
186 void dns_server_move_back_and_unmark(DnsServer
*s
) {
196 if (!s
->linked
|| !s
->servers_next
)
199 /* Move us to the end of the list, so that the order is
200 * strictly kept, if we are not at the end anyway. */
204 case DNS_SERVER_LINK
:
206 LIST_FIND_TAIL(servers
, s
, tail
);
207 LIST_REMOVE(servers
, s
->link
->dns_servers
, s
);
208 LIST_INSERT_AFTER(servers
, s
->link
->dns_servers
, tail
, s
);
211 case DNS_SERVER_SYSTEM
:
212 LIST_FIND_TAIL(servers
, s
, tail
);
213 LIST_REMOVE(servers
, s
->manager
->dns_servers
, s
);
214 LIST_INSERT_AFTER(servers
, s
->manager
->dns_servers
, tail
, s
);
217 case DNS_SERVER_FALLBACK
:
218 LIST_FIND_TAIL(servers
, s
, tail
);
219 LIST_REMOVE(servers
, s
->manager
->fallback_dns_servers
, s
);
220 LIST_INSERT_AFTER(servers
, s
->manager
->fallback_dns_servers
, tail
, s
);
224 assert_not_reached("Unknown server type");
228 static void dns_server_verified(DnsServer
*s
, DnsServerFeatureLevel level
) {
231 if (s
->verified_feature_level
> level
)
234 if (s
->verified_feature_level
!= level
) {
235 log_debug("Verified we get a response at feature level %s from DNS server %s.",
236 dns_server_feature_level_to_string(level
),
237 dns_server_string(s
));
238 s
->verified_feature_level
= level
;
241 assert_se(sd_event_now(s
->manager
->event
, clock_boottime_or_monotonic(), &s
->verified_usec
) >= 0);
244 void dns_server_packet_received(DnsServer
*s
, int protocol
, DnsServerFeatureLevel level
, usec_t rtt
, size_t size
) {
247 if (protocol
== IPPROTO_UDP
) {
248 if (s
->possible_feature_level
== level
)
251 /* If the RRSIG data is missing, then we can only validate EDNS0 at max */
252 if (s
->packet_rrsig_missing
&& level
>= DNS_SERVER_FEATURE_LEVEL_DO
)
253 level
= DNS_SERVER_FEATURE_LEVEL_DO
- 1;
255 /* If the OPT RR got lost, then we can only validate UDP at max */
256 if (s
->packet_bad_opt
&& level
>= DNS_SERVER_FEATURE_LEVEL_EDNS0
)
257 level
= DNS_SERVER_FEATURE_LEVEL_EDNS0
- 1;
259 /* Even if we successfully receive a reply to a request announcing support for large packets,
260 that does not mean we can necessarily receive large packets. */
261 if (level
== DNS_SERVER_FEATURE_LEVEL_LARGE
)
262 level
= DNS_SERVER_FEATURE_LEVEL_LARGE
- 1;
264 } else if (protocol
== IPPROTO_TCP
) {
266 if (s
->possible_feature_level
== level
)
269 /* Successful TCP connections are only useful to verify the TCP feature level. */
270 level
= DNS_SERVER_FEATURE_LEVEL_TCP
;
273 dns_server_verified(s
, level
);
275 /* Remember the size of the largest UDP packet we received from a server,
276 we know that we can always announce support for packets with at least
278 if (protocol
== IPPROTO_UDP
&& s
->received_udp_packet_max
< size
)
279 s
->received_udp_packet_max
= size
;
281 if (s
->max_rtt
< rtt
) {
283 s
->resend_timeout
= CLAMP(s
->max_rtt
* 2, DNS_TIMEOUT_MIN_USEC
, DNS_TIMEOUT_MAX_USEC
);
287 void dns_server_packet_lost(DnsServer
*s
, int protocol
, DnsServerFeatureLevel level
, usec_t usec
) {
291 if (s
->possible_feature_level
== level
) {
292 if (protocol
== IPPROTO_UDP
)
294 else if (protocol
== IPPROTO_TCP
)
298 if (s
->resend_timeout
> usec
)
301 s
->resend_timeout
= MIN(s
->resend_timeout
* 2, DNS_TIMEOUT_MAX_USEC
);
304 void dns_server_packet_failed(DnsServer
*s
, DnsServerFeatureLevel level
) {
307 /* Invoked whenever we get a FORMERR, SERVFAIL or NOTIMP rcode from a server. */
309 if (s
->possible_feature_level
!= level
)
312 s
->packet_failed
= true;
315 void dns_server_packet_truncated(DnsServer
*s
, DnsServerFeatureLevel level
) {
318 /* Invoked whenever we get a packet with TC bit set. */
320 if (s
->possible_feature_level
!= level
)
323 s
->packet_truncated
= true;
326 void dns_server_packet_rrsig_missing(DnsServer
*s
, DnsServerFeatureLevel level
) {
329 if (level
< DNS_SERVER_FEATURE_LEVEL_DO
)
332 /* If the RRSIG RRs are missing, we have to downgrade what we previously verified */
333 if (s
->verified_feature_level
>= DNS_SERVER_FEATURE_LEVEL_DO
)
334 s
->verified_feature_level
= DNS_SERVER_FEATURE_LEVEL_DO
-1;
336 s
->packet_rrsig_missing
= true;
339 void dns_server_packet_bad_opt(DnsServer
*s
, DnsServerFeatureLevel level
) {
342 if (level
< DNS_SERVER_FEATURE_LEVEL_EDNS0
)
345 /* If the OPT RR got lost, we have to downgrade what we previously verified */
346 if (s
->verified_feature_level
>= DNS_SERVER_FEATURE_LEVEL_EDNS0
)
347 s
->verified_feature_level
= DNS_SERVER_FEATURE_LEVEL_EDNS0
-1;
349 s
->packet_bad_opt
= true;
352 static bool dns_server_grace_period_expired(DnsServer
*s
) {
358 if (s
->verified_usec
== 0)
361 assert_se(sd_event_now(s
->manager
->event
, clock_boottime_or_monotonic(), &ts
) >= 0);
363 if (s
->verified_usec
+ s
->features_grace_period_usec
> ts
)
366 s
->features_grace_period_usec
= MIN(s
->features_grace_period_usec
* 2, DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC
);
371 static void dns_server_reset_counters(DnsServer
*s
) {
376 s
->packet_failed
= false;
377 s
->packet_truncated
= false;
378 s
->verified_usec
= 0;
380 /* Note that we do not reset s->packet_bad_opt and s->packet_rrsig_missing here. We reset them only when the
381 * grace period ends, but not when lowering the possible feature level, as a lower level feature level should
382 * not make RRSIGs appear or OPT appear, but rather make them disappear. If the reappear anyway, then that's
383 * indication for a differently broken OPT/RRSIG implementation, and we really don't want to support that
386 * This is particularly important to deal with certain Belkin routers which break OPT for certain lookups (A),
387 * but pass traffic through for others (AAAA). If we detect the broken behaviour on one lookup we should not
388 * reenable it for another, because we cannot validate things anyway, given that the RRSIG/OPT data will be
392 DnsServerFeatureLevel
dns_server_possible_feature_level(DnsServer
*s
) {
395 if (s
->possible_feature_level
!= DNS_SERVER_FEATURE_LEVEL_BEST
&&
396 dns_server_grace_period_expired(s
)) {
398 s
->possible_feature_level
= DNS_SERVER_FEATURE_LEVEL_BEST
;
400 dns_server_reset_counters(s
);
402 s
->packet_bad_opt
= false;
403 s
->packet_rrsig_missing
= false;
405 log_info("Grace period over, resuming full feature set (%s) for DNS server %s.",
406 dns_server_feature_level_to_string(s
->possible_feature_level
),
407 dns_server_string(s
));
409 } else if (s
->possible_feature_level
<= s
->verified_feature_level
)
410 s
->possible_feature_level
= s
->verified_feature_level
;
412 DnsServerFeatureLevel p
= s
->possible_feature_level
;
414 if (s
->n_failed_tcp
>= DNS_SERVER_FEATURE_RETRY_ATTEMPTS
&&
415 s
->possible_feature_level
== DNS_SERVER_FEATURE_LEVEL_TCP
) {
417 /* We are at the TCP (lowest) level, and we tried a couple of TCP connections, and it didn't
418 * work. Upgrade back to UDP again. */
419 log_debug("Reached maximum number of failed TCP connection attempts, trying UDP again...");
420 s
->possible_feature_level
= DNS_SERVER_FEATURE_LEVEL_UDP
;
422 } else if (s
->packet_bad_opt
&&
423 s
->possible_feature_level
>= DNS_SERVER_FEATURE_LEVEL_EDNS0
) {
425 /* A reply to one of our EDNS0 queries didn't carry a valid OPT RR, then downgrade to below
426 * EDNS0 levels. After all, some records generate different responses with and without OPT RR
427 * in the request. Example:
428 * https://open.nlnetlabs.nl/pipermail/dnssec-trigger/2014-November/000376.html */
430 log_debug("Server doesn't support EDNS(0) properly, downgrading feature level...");
431 s
->possible_feature_level
= DNS_SERVER_FEATURE_LEVEL_UDP
;
433 } else if (s
->packet_rrsig_missing
&&
434 s
->possible_feature_level
>= DNS_SERVER_FEATURE_LEVEL_DO
) {
436 /* RRSIG data was missing on a EDNS0 packet with DO bit set. This means the server doesn't
437 * augment responses with DNSSEC RRs. If so, let's better not ask the server for it anymore,
438 * after all some servers generate different replies depending if an OPT RR is in the query or
441 log_debug("Detected server responses lack RRSIG records, downgrading feature level...");
442 s
->possible_feature_level
= DNS_SERVER_FEATURE_LEVEL_EDNS0
;
444 } else if (s
->n_failed_udp
>= DNS_SERVER_FEATURE_RETRY_ATTEMPTS
&&
445 s
->possible_feature_level
>= DNS_SERVER_FEATURE_LEVEL_UDP
) {
447 /* We lost too many UDP packets in a row, and are on a feature level of UDP or higher. If the
448 * packets are lost, maybe the server cannot parse them, hence downgrading sounds like a good
449 * idea. We might downgrade all the way down to TCP this way. */
451 log_debug("Lost too many UDP packets, downgrading feature level...");
452 s
->possible_feature_level
--;
454 } else if (s
->packet_failed
&&
455 s
->possible_feature_level
> DNS_SERVER_FEATURE_LEVEL_UDP
) {
457 /* We got a failure packet, and are at a feature level above UDP. Note that in this case we
458 * downgrade no further than UDP, under the assumption that a failure packet indicates an
459 * incompatible packet contents, but not a problem with the transport. */
461 log_debug("Got server failure, downgrading feature level...");
462 s
->possible_feature_level
--;
464 } else if (s
->n_failed_tcp
>= DNS_SERVER_FEATURE_RETRY_ATTEMPTS
&&
465 s
->packet_truncated
&&
466 s
->possible_feature_level
> DNS_SERVER_FEATURE_LEVEL_UDP
) {
468 /* We got too many TCP connection failures in a row, we had at least one truncated packet, and
469 * are on a feature level above UDP. By downgrading things and getting rid of DNSSEC or EDNS0
470 * data we hope to make the packet smaller, so that it still works via UDP given that TCP
471 * appears not to be a fallback. Note that if we are already at the lowest UDP level, we don't
472 * go further down, since that's TCP, and TCP failed too often after all. */
474 log_debug("Got too many failed TCP connection failures and truncated UDP packets, downgrading feature level...");
475 s
->possible_feature_level
--;
478 if (p
!= s
->possible_feature_level
) {
480 /* We changed the feature level, reset the counting */
481 dns_server_reset_counters(s
);
483 log_warning("Using degraded feature set (%s) for DNS server %s.",
484 dns_server_feature_level_to_string(s
->possible_feature_level
),
485 dns_server_string(s
));
489 return s
->possible_feature_level
;
492 int dns_server_adjust_opt(DnsServer
*server
, DnsPacket
*packet
, DnsServerFeatureLevel level
) {
499 assert(packet
->protocol
== DNS_PROTOCOL_DNS
);
501 /* Fix the OPT field in the packet to match our current feature level. */
503 r
= dns_packet_truncate_opt(packet
);
507 if (level
< DNS_SERVER_FEATURE_LEVEL_EDNS0
)
510 edns_do
= level
>= DNS_SERVER_FEATURE_LEVEL_DO
;
512 if (level
>= DNS_SERVER_FEATURE_LEVEL_LARGE
)
513 packet_size
= DNS_PACKET_UNICAST_SIZE_LARGE_MAX
;
515 packet_size
= server
->received_udp_packet_max
;
517 return dns_packet_append_opt(packet
, packet_size
, edns_do
, NULL
);
520 const char *dns_server_string(DnsServer
*server
) {
523 if (!server
->server_string
)
524 (void) in_addr_to_string(server
->family
, &server
->address
, &server
->server_string
);
526 return strna(server
->server_string
);
529 bool dns_server_dnssec_supported(DnsServer
*server
) {
532 /* Returns whether the server supports DNSSEC according to what we know about it */
534 if (server
->possible_feature_level
< DNS_SERVER_FEATURE_LEVEL_DO
)
537 if (server
->packet_bad_opt
)
540 if (server
->packet_rrsig_missing
)
543 /* DNSSEC servers need to support TCP properly (see RFC5966), if they don't, we assume DNSSEC is borked too */
544 if (server
->n_failed_tcp
>= DNS_SERVER_FEATURE_RETRY_ATTEMPTS
)
550 void dns_server_warn_downgrade(DnsServer
*server
) {
553 if (server
->warned_downgrade
)
556 log_struct(LOG_NOTICE
,
557 LOG_MESSAGE_ID(SD_MESSAGE_DNSSEC_DOWNGRADE
),
558 LOG_MESSAGE("Server %s does not support DNSSEC, downgrading to non-DNSSEC mode.", dns_server_string(server
)),
559 "DNS_SERVER=%s", dns_server_string(server
),
560 "DNS_SERVER_FEATURE_LEVEL=%s", dns_server_feature_level_to_string(server
->possible_feature_level
),
563 server
->warned_downgrade
= true;
566 static void dns_server_hash_func(const void *p
, struct siphash
*state
) {
567 const DnsServer
*s
= p
;
571 siphash24_compress(&s
->family
, sizeof(s
->family
), state
);
572 siphash24_compress(&s
->address
, FAMILY_ADDRESS_SIZE(s
->family
), state
);
575 static int dns_server_compare_func(const void *a
, const void *b
) {
576 const DnsServer
*x
= a
, *y
= b
;
578 if (x
->family
< y
->family
)
580 if (x
->family
> y
->family
)
583 return memcmp(&x
->address
, &y
->address
, FAMILY_ADDRESS_SIZE(x
->family
));
586 const struct hash_ops dns_server_hash_ops
= {
587 .hash
= dns_server_hash_func
,
588 .compare
= dns_server_compare_func
591 void dns_server_unlink_all(DnsServer
*first
) {
597 next
= first
->servers_next
;
598 dns_server_unlink(first
);
600 dns_server_unlink_all(next
);
603 void dns_server_unlink_marked(DnsServer
*first
) {
609 next
= first
->servers_next
;
612 dns_server_unlink(first
);
614 dns_server_unlink_marked(next
);
617 void dns_server_mark_all(DnsServer
*first
) {
621 first
->marked
= true;
622 dns_server_mark_all(first
->servers_next
);
625 DnsServer
*dns_server_find(DnsServer
*first
, int family
, const union in_addr_union
*in_addr
) {
628 LIST_FOREACH(servers
, s
, first
)
629 if (s
->family
== family
&& in_addr_equal(family
, &s
->address
, in_addr
) > 0)
635 DnsServer
*manager_get_first_dns_server(Manager
*m
, DnsServerType t
) {
640 case DNS_SERVER_SYSTEM
:
641 return m
->dns_servers
;
643 case DNS_SERVER_FALLBACK
:
644 return m
->fallback_dns_servers
;
651 DnsServer
*manager_set_dns_server(Manager
*m
, DnsServer
*s
) {
654 if (m
->current_dns_server
== s
)
658 log_info("Switching to %s DNS server %s.",
659 dns_server_type_to_string(s
->type
),
660 dns_server_string(s
));
662 dns_server_unref(m
->current_dns_server
);
663 m
->current_dns_server
= dns_server_ref(s
);
665 if (m
->unicast_scope
)
666 dns_cache_flush(&m
->unicast_scope
->cache
);
671 DnsServer
*manager_get_dns_server(Manager
*m
) {
675 /* Try to read updates resolv.conf */
676 manager_read_resolv_conf(m
);
678 /* If no DNS server was chosen so far, pick the first one */
679 if (!m
->current_dns_server
)
680 manager_set_dns_server(m
, m
->dns_servers
);
682 if (!m
->current_dns_server
) {
686 /* No DNS servers configured, let's see if there are
687 * any on any links. If not, we use the fallback
690 HASHMAP_FOREACH(l
, m
->links
, i
)
691 if (l
->dns_servers
) {
697 manager_set_dns_server(m
, m
->fallback_dns_servers
);
700 return m
->current_dns_server
;
703 void manager_next_dns_server(Manager
*m
) {
706 /* If there's currently no DNS server set, then the next
707 * manager_get_dns_server() will find one */
708 if (!m
->current_dns_server
)
711 /* Change to the next one, but make sure to follow the linked
712 * list only if the server is still linked. */
713 if (m
->current_dns_server
->linked
&& m
->current_dns_server
->servers_next
) {
714 manager_set_dns_server(m
, m
->current_dns_server
->servers_next
);
718 /* If there was no next one, then start from the beginning of
720 if (m
->current_dns_server
->type
== DNS_SERVER_FALLBACK
)
721 manager_set_dns_server(m
, m
->fallback_dns_servers
);
723 manager_set_dns_server(m
, m
->dns_servers
);
726 static const char* const dns_server_type_table
[_DNS_SERVER_TYPE_MAX
] = {
727 [DNS_SERVER_SYSTEM
] = "system",
728 [DNS_SERVER_FALLBACK
] = "fallback",
729 [DNS_SERVER_LINK
] = "link",
731 DEFINE_STRING_TABLE_LOOKUP(dns_server_type
, DnsServerType
);
733 static const char* const dns_server_feature_level_table
[_DNS_SERVER_FEATURE_LEVEL_MAX
] = {
734 [DNS_SERVER_FEATURE_LEVEL_TCP
] = "TCP",
735 [DNS_SERVER_FEATURE_LEVEL_UDP
] = "UDP",
736 [DNS_SERVER_FEATURE_LEVEL_EDNS0
] = "UDP+EDNS0",
737 [DNS_SERVER_FEATURE_LEVEL_DO
] = "UDP+EDNS0+DO",
738 [DNS_SERVER_FEATURE_LEVEL_LARGE
] = "UDP+EDNS0+DO+LARGE",
740 DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level
, DnsServerFeatureLevel
);