src/resolve/resolved-mdns.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <resolv.h>
   4 #include <netinet/in.h>
   5 #include <arpa/inet.h>
   6
   7 #include "alloc-util.h"
   8 #include "fd-util.h"
   9 #include "resolved-manager.h"
  10 #include "resolved-mdns.h"
  11
  12 #define CLEAR_CACHE_FLUSH(x) (~MDNS_RR_CACHE_FLUSH & (x))
  13
  14 void manager_mdns_stop(Manager *m) {
  15         assert(m);
  16
  17         m->mdns_ipv4_event_source = sd_event_source_unref(m->mdns_ipv4_event_source);
  18         m->mdns_ipv4_fd = safe_close(m->mdns_ipv4_fd);
  19
  20         m->mdns_ipv6_event_source = sd_event_source_unref(m->mdns_ipv6_event_source);
  21         m->mdns_ipv6_fd = safe_close(m->mdns_ipv6_fd);
  22 }
  23
  24 int manager_mdns_start(Manager *m) {
  25         int r;
  26
  27         assert(m);
  28
  29         if (m->mdns_support == RESOLVE_SUPPORT_NO)
  30                 return 0;
  31
  32         r = manager_mdns_ipv4_fd(m);
  33         if (r == -EADDRINUSE)
  34                 goto eaddrinuse;
  35         if (r < 0)
  36                 return r;
  37
  38         if (socket_ipv6_is_supported()) {
  39                 r = manager_mdns_ipv6_fd(m);
  40                 if (r == -EADDRINUSE)
  41                         goto eaddrinuse;
  42                 if (r < 0)
  43                         return r;
  44         }
  45
  46         return 0;
  47
  48 eaddrinuse:
  49         log_warning("Another mDNS responder prohibits binding the socket to the same port. Turning off mDNS support.");
  50         m->mdns_support = RESOLVE_SUPPORT_NO;
  51         manager_mdns_stop(m);
  52
  53         return 0;
  54 }
  55
  56 static int mdns_rr_compare(DnsResourceRecord * const *a, DnsResourceRecord * const *b) {
  57         DnsResourceRecord *x = *(DnsResourceRecord **) a, *y = *(DnsResourceRecord **) b;
  58         size_t m;
  59         int r;
  60
  61         assert(x);
  62         assert(y);
  63
  64         r = CMP(CLEAR_CACHE_FLUSH(x->key->class), CLEAR_CACHE_FLUSH(y->key->class));
  65         if (r != 0)
  66                 return r;
  67
  68         r = CMP(x->key->type, y->key->type);
  69         if (r != 0)
  70                 return r;
  71
  72         r = dns_resource_record_to_wire_format(x, false);
  73         if (r < 0) {
  74                 log_warning_errno(r, "Can't wire-format RR: %m");
  75                 return 0;
  76         }
  77
  78         r = dns_resource_record_to_wire_format(y, false);
  79         if (r < 0) {
  80                 log_warning_errno(r, "Can't wire-format RR: %m");
  81                 return 0;
  82         }
  83
  84         m = MIN(DNS_RESOURCE_RECORD_RDATA_SIZE(x), DNS_RESOURCE_RECORD_RDATA_SIZE(y));
  85
  86         r = memcmp(DNS_RESOURCE_RECORD_RDATA(x), DNS_RESOURCE_RECORD_RDATA(y), m);
  87         if (r != 0)
  88                 return r;
  89
  90         return CMP(DNS_RESOURCE_RECORD_RDATA_SIZE(x), DNS_RESOURCE_RECORD_RDATA_SIZE(y));
  91 }
  92
  93 static int proposed_rrs_cmp(DnsResourceRecord **x, unsigned x_size, DnsResourceRecord **y, unsigned y_size) {
  94         unsigned m;
  95         int r;
  96
  97         m = MIN(x_size, y_size);
  98         for (unsigned i = 0; i < m; i++) {
  99                 r = mdns_rr_compare(&x[i], &y[i]);
 100                 if (r != 0)
 101                         return r;
 102         }
 103
 104         return CMP(x_size, y_size);
 105 }
 106
 107 static int mdns_packet_extract_matching_rrs(DnsPacket *p, DnsResourceKey *key, DnsResourceRecord ***ret_rrs) {
 108         _cleanup_free_ DnsResourceRecord **list = NULL;
 109         unsigned n = 0, size = 0;
 110         int r;
 111
 112         assert(p);
 113         assert(key);
 114         assert(ret_rrs);
 115         assert_return(DNS_PACKET_NSCOUNT(p) > 0, -EINVAL);
 116
 117         for (size_t i = DNS_PACKET_ANCOUNT(p); i < (DNS_PACKET_ANCOUNT(p) + DNS_PACKET_NSCOUNT(p)); i++) {
 118                 r = dns_resource_key_match_rr(key, p->answer->items[i].rr, NULL);
 119                 if (r < 0)
 120                         return r;
 121                 if (r > 0)
 122                         size++;
 123         }
 124
 125         if (size == 0)
 126                 return 0;
 127
 128         list = new(DnsResourceRecord *, size);
 129         if (!list)
 130                 return -ENOMEM;
 131
 132         for (size_t i = DNS_PACKET_ANCOUNT(p); i < (DNS_PACKET_ANCOUNT(p) + DNS_PACKET_NSCOUNT(p)); i++) {
 133                 r = dns_resource_key_match_rr(key, p->answer->items[i].rr, NULL);
 134                 if (r < 0)
 135                         return r;
 136                 if (r > 0)
 137                         list[n++] = p->answer->items[i].rr;
 138         }
 139         assert(n == size);
 140         typesafe_qsort(list, size, mdns_rr_compare);
 141
 142         *ret_rrs = TAKE_PTR(list);
 143
 144         return size;
 145 }
 146
 147 static int mdns_do_tiebreak(DnsResourceKey *key, DnsAnswer *answer, DnsPacket *p) {
 148         _cleanup_free_ DnsResourceRecord **our = NULL, **remote = NULL;
 149         DnsResourceRecord *rr;
 150         size_t i = 0, size;
 151         int r;
 152
 153         size = dns_answer_size(answer);
 154         our = new(DnsResourceRecord *, size);
 155         if (!our)
 156                 return -ENOMEM;
 157
 158         DNS_ANSWER_FOREACH(rr, answer)
 159                 our[i++] = rr;
 160
 161         typesafe_qsort(our, size, mdns_rr_compare);
 162
 163         r = mdns_packet_extract_matching_rrs(p, key, &remote);
 164         if (r < 0)
 165                 return r;
 166
 167         assert(r > 0);
 168
 169         if (proposed_rrs_cmp(remote, r, our, size) > 0)
 170                 return 1;
 171
 172         return 0;
 173 }
 174
 175 static int mdns_scope_process_query(DnsScope *s, DnsPacket *p) {
 176         _cleanup_(dns_answer_unrefp) DnsAnswer *full_answer = NULL;
 177         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 178         DnsResourceKey *key = NULL;
 179         DnsResourceRecord *rr;
 180         bool tentative = false;
 181         int r;
 182
 183         assert(s);
 184         assert(p);
 185
 186         r = dns_packet_extract(p);
 187         if (r < 0)
 188                 return log_debug_errno(r, "Failed to extract resource records from incoming packet: %m");
 189
 190         assert_return((dns_question_size(p->question) > 0), -EINVAL);
 191
 192         DNS_QUESTION_FOREACH(key, p->question) {
 193                 _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL, *soa = NULL;
 194
 195                 r = dns_zone_lookup(&s->zone, key, 0, &answer, &soa, &tentative);
 196                 if (r < 0)
 197                         return log_debug_errno(r, "Failed to lookup key: %m");
 198
 199                 if (tentative && DNS_PACKET_NSCOUNT(p) > 0) {
 200                         /*
 201                          * A race condition detected with the probe packet from
 202                          * a remote host.
 203                          * Do simultaneous probe tiebreaking as described in
 204                          * RFC 6762, Section 8.2. In case we lost don't reply
 205                          * the question and withdraw conflicting RRs.
 206                          */
 207                         r = mdns_do_tiebreak(key, answer, p);
 208                         if (r < 0)
 209                                 return log_debug_errno(r, "Failed to do tiebreaking");
 210
 211                         if (r > 0) { /* we lost */
 212                                 DNS_ANSWER_FOREACH(rr, answer) {
 213                                         DnsZoneItem *i;
 214
 215                                         i = dns_zone_get(&s->zone, rr);
 216                                         if (i)
 217                                                 dns_zone_item_conflict(i);
 218                                 }
 219
 220                                 continue;
 221                         }
 222                 }
 223
 224                 r = dns_answer_extend(&full_answer, answer);
 225                 if (r < 0)
 226                         return log_debug_errno(r, "Failed to extend answer: %m");
 227         }
 228
 229         if (dns_answer_isempty(full_answer))
 230                 return 0;
 231
 232         r = dns_scope_make_reply_packet(s, DNS_PACKET_ID(p), DNS_RCODE_SUCCESS, NULL, full_answer, NULL, false, &reply);
 233         if (r < 0)
 234                 return log_debug_errno(r, "Failed to build reply packet: %m");
 235
 236         if (!ratelimit_below(&s->ratelimit))
 237                 return 0;
 238
 239         r = dns_scope_emit_udp(s, -1, reply);
 240         if (r < 0)
 241                 return log_debug_errno(r, "Failed to send reply packet: %m");
 242
 243         return 0;
 244 }
 245
 246 static int on_mdns_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 247         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 248         Manager *m = userdata;
 249         DnsScope *scope;
 250         int r;
 251
 252         r = manager_recv(m, fd, DNS_PROTOCOL_MDNS, &p);
 253         if (r <= 0)
 254                 return r;
 255
 256         if (manager_our_packet(m, p))
 257                 return 0;
 258
 259         scope = manager_find_scope(m, p);
 260         if (!scope) {
 261                 log_debug("Got mDNS UDP packet on unknown scope. Ignoring.");
 262                 return 0;
 263         }
 264
 265         if (dns_packet_validate_reply(p) > 0) {
 266                 DnsResourceRecord *rr;
 267
 268                 log_debug("Got mDNS reply packet");
 269
 270                 /*
 271                  * mDNS is different from regular DNS and LLMNR with regard to handling responses.
 272                  * While on other protocols, we can ignore every answer that doesn't match a question
 273                  * we broadcast earlier, RFC6762, section 18.1 recommends looking at and caching all
 274                  * incoming information, regardless of the DNS packet ID.
 275                  *
 276                  * Hence, extract the packet here, and try to find a transaction for answer the we got
 277                  * and complete it. Also store the new information in scope's cache.
 278                  */
 279                 r = dns_packet_extract(p);
 280                 if (r < 0) {
 281                         log_debug("mDNS packet extraction failed.");
 282                         return 0;
 283                 }
 284
 285                 dns_scope_check_conflicts(scope, p);
 286
 287                 DNS_ANSWER_FOREACH(rr, p->answer) {
 288                         const char *name = dns_resource_key_name(rr->key);
 289                         DnsTransaction *t;
 290
 291                         /* If the received reply packet contains ANY record that is not .local or .in-addr.arpa,
 292                          * we assume someone's playing tricks on us and discard the packet completely. */
 293                         if (!(dns_name_endswith(name, "in-addr.arpa") > 0 ||
 294                               dns_name_endswith(name, "local") > 0))
 295                                 return 0;
 296
 297                         if (rr->ttl == 0) {
 298                                 log_debug("Got a goodbye packet");
 299                                 /* See the section 10.1 of RFC6762 */
 300                                 rr->ttl = 1;
 301                         }
 302
 303                         t = dns_scope_find_transaction(scope, rr->key, false);
 304                         if (t)
 305                                 dns_transaction_process_reply(t, p);
 306
 307                         /* Also look for the various types of ANY transactions */
 308                         t = dns_scope_find_transaction(scope, &DNS_RESOURCE_KEY_CONST(rr->key->class, DNS_TYPE_ANY, dns_resource_key_name(rr->key)), false);
 309                         if (t)
 310                                 dns_transaction_process_reply(t, p);
 311
 312                         t = dns_scope_find_transaction(scope, &DNS_RESOURCE_KEY_CONST(DNS_CLASS_ANY, rr->key->type, dns_resource_key_name(rr->key)), false);
 313                         if (t)
 314                                 dns_transaction_process_reply(t, p);
 315
 316                         t = dns_scope_find_transaction(scope, &DNS_RESOURCE_KEY_CONST(DNS_CLASS_ANY, DNS_TYPE_ANY, dns_resource_key_name(rr->key)), false);
 317                         if (t)
 318                                 dns_transaction_process_reply(t, p);
 319                 }
 320
 321                 dns_cache_put(&scope->cache, NULL, DNS_PACKET_RCODE(p), p->answer, false, (uint32_t) -1, 0, p->family, &p->sender);
 322
 323         } else if (dns_packet_validate_query(p) > 0)  {
 324                 log_debug("Got mDNS query packet for id %u", DNS_PACKET_ID(p));
 325
 326                 r = mdns_scope_process_query(scope, p);
 327                 if (r < 0) {
 328                         log_debug_errno(r, "mDNS query processing failed: %m");
 329                         return 0;
 330                 }
 331         } else
 332                 log_debug("Invalid mDNS UDP packet.");
 333
 334         return 0;
 335 }
 336
 337 int manager_mdns_ipv4_fd(Manager *m) {
 338         union sockaddr_union sa = {
 339                 .in.sin_family = AF_INET,
 340                 .in.sin_port = htobe16(MDNS_PORT),
 341         };
 342         _cleanup_close_ int s = -1;
 343         int r;
 344
 345         assert(m);
 346
 347         if (m->mdns_ipv4_fd >= 0)
 348                 return m->mdns_ipv4_fd;
 349
 350         s = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 351         if (s < 0)
 352                 return log_error_errno(errno, "mDNS-IPv4: Failed to create socket: %m");
 353
 354         r = setsockopt_int(s, IPPROTO_IP, IP_TTL, 255);
 355         if (r < 0)
 356                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_TTL: %m");
 357
 358         r = setsockopt_int(s, IPPROTO_IP, IP_MULTICAST_TTL, 255);
 359         if (r < 0)
 360                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_MULTICAST_TTL: %m");
 361
 362         r = setsockopt_int(s, IPPROTO_IP, IP_MULTICAST_LOOP, true);
 363         if (r < 0)
 364                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_MULTICAST_LOOP: %m");
 365
 366         r = setsockopt_int(s, IPPROTO_IP, IP_PKTINFO, true);
 367         if (r < 0)
 368                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_PKTINFO: %m");
 369
 370         r = setsockopt_int(s, IPPROTO_IP, IP_RECVTTL, true);
 371         if (r < 0)
 372                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_RECVTTL: %m");
 373
 374         /* Disable Don't-Fragment bit in the IP header */
 375         r = setsockopt_int(s, IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DONT);
 376         if (r < 0)
 377                 return log_error_errno(r, "mDNS-IPv4: Failed to set IP_MTU_DISCOVER: %m");
 378
 379         /* See the section 15.1 of RFC6762 */
 380         /* first try to bind without SO_REUSEADDR to detect another mDNS responder */
 381         r = bind(s, &sa.sa, sizeof(sa.in));
 382         if (r < 0) {
 383                 if (errno != EADDRINUSE)
 384                         return log_error_errno(errno, "mDNS-IPv4: Failed to bind socket: %m");
 385
 386                 log_warning("mDNS-IPv4: There appears to be another mDNS responder running, or previously systemd-resolved crashed with some outstanding transfers.");
 387
 388                 /* try again with SO_REUSEADDR */
 389                 r = setsockopt_int(s, SOL_SOCKET, SO_REUSEADDR, true);
 390                 if (r < 0)
 391                         return log_error_errno(r, "mDNS-IPv4: Failed to set SO_REUSEADDR: %m");
 392
 393                 r = bind(s, &sa.sa, sizeof(sa.in));
 394                 if (r < 0)
 395                         return log_error_errno(errno, "mDNS-IPv4: Failed to bind socket: %m");
 396         } else {
 397                 /* enable SO_REUSEADDR for the case that the user really wants multiple mDNS responders */
 398                 r = setsockopt_int(s, SOL_SOCKET, SO_REUSEADDR, true);
 399                 if (r < 0)
 400                         return log_error_errno(r, "mDNS-IPv4: Failed to set SO_REUSEADDR: %m");
 401         }
 402
 403         r = sd_event_add_io(m->event, &m->mdns_ipv4_event_source, s, EPOLLIN, on_mdns_packet, m);
 404         if (r < 0)
 405                 return log_error_errno(r, "mDNS-IPv4: Failed to create event source: %m");
 406
 407         return m->mdns_ipv4_fd = TAKE_FD(s);
 408 }
 409
 410 int manager_mdns_ipv6_fd(Manager *m) {
 411         union sockaddr_union sa = {
 412                 .in6.sin6_family = AF_INET6,
 413                 .in6.sin6_port = htobe16(MDNS_PORT),
 414         };
 415         _cleanup_close_ int s = -1;
 416         int r;
 417
 418         assert(m);
 419
 420         if (m->mdns_ipv6_fd >= 0)
 421                 return m->mdns_ipv6_fd;
 422
 423         s = socket(AF_INET6, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 424         if (s < 0)
 425                 return log_error_errno(errno, "mDNS-IPv6: Failed to create socket: %m");
 426
 427         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, 255);
 428         if (r < 0)
 429                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_UNICAST_HOPS: %m");
 430
 431         /* RFC 4795, section 2.5 recommends setting the TTL of UDP packets to 255. */
 432         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, 255);
 433         if (r < 0)
 434                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_MULTICAST_HOPS: %m");
 435
 436         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, true);
 437         if (r < 0)
 438                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_MULTICAST_LOOP: %m");
 439
 440         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_V6ONLY, true);
 441         if (r < 0)
 442                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_V6ONLY: %m");
 443
 444         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_RECVPKTINFO, true);
 445         if (r < 0)
 446                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_RECVPKTINFO: %m");
 447
 448         r = setsockopt_int(s, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, true);
 449         if (r < 0)
 450                 return log_error_errno(r, "mDNS-IPv6: Failed to set IPV6_RECVHOPLIMIT: %m");
 451
 452         /* See the section 15.1 of RFC6762 */
 453         /* first try to bind without SO_REUSEADDR to detect another mDNS responder */
 454         r = bind(s, &sa.sa, sizeof(sa.in6));
 455         if (r < 0) {
 456                 if (errno != EADDRINUSE)
 457                         return log_error_errno(errno, "mDNS-IPv6: Failed to bind socket: %m");
 458
 459                 log_warning("mDNS-IPv6: There appears to be another mDNS responder running, or previously systemd-resolved crashed with some outstanding transfers.");
 460
 461                 /* try again with SO_REUSEADDR */
 462                 r = setsockopt_int(s, SOL_SOCKET, SO_REUSEADDR, true);
 463                 if (r < 0)
 464                         return log_error_errno(r, "mDNS-IPv6: Failed to set SO_REUSEADDR: %m");
 465
 466                 r = bind(s, &sa.sa, sizeof(sa.in6));
 467                 if (r < 0)
 468                         return log_error_errno(errno, "mDNS-IPv6: Failed to bind socket: %m");
 469         } else {
 470                 /* enable SO_REUSEADDR for the case that the user really wants multiple mDNS responders */
 471                 r = setsockopt_int(s, SOL_SOCKET, SO_REUSEADDR, true);
 472                 if (r < 0)
 473                         return log_error_errno(r, "mDNS-IPv6: Failed to set SO_REUSEADDR: %m");
 474         }
 475
 476         r = sd_event_add_io(m->event, &m->mdns_ipv6_event_source, s, EPOLLIN, on_mdns_packet, m);
 477         if (r < 0)
 478                 return log_error_errno(r, "mDNS-IPv6: Failed to create event source: %m");
 479
 480         return m->mdns_ipv6_fd = TAKE_FD(s);
 481 }