src/resolve/resolved-dns-stub.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include "fd-util.h"
   4 #include "missing_network.h"
   5 #include "resolved-dns-stub.h"
   6 #include "socket-util.h"
   7
   8 /* The MTU of the loopback device is 64K on Linux, advertise that as maximum datagram size, but subtract the Ethernet,
   9  * IP and UDP header sizes */
  10 #define ADVERTISE_DATAGRAM_SIZE_MAX (65536U-14U-20U-8U)
  11
  12 static int manager_dns_stub_udp_fd(Manager *m);
  13 static int manager_dns_stub_tcp_fd(Manager *m);
  14
  15 static int dns_stub_make_reply_packet(
  16                 DnsPacket **p,
  17                 size_t max_size,
  18                 DnsQuestion *q,
  19                 DnsAnswer *answer,
  20                 bool *ret_truncated) {
  21
  22         bool truncated = false;
  23         DnsResourceRecord *rr;
  24         unsigned c = 0;
  25         int r;
  26
  27         assert(p);
  28
  29         /* Note that we don't bother with any additional RRs, as this is stub is for local lookups only, and hence
  30          * roundtrips aren't expensive. */
  31
  32         if (!*p) {
  33                 r = dns_packet_new(p, DNS_PROTOCOL_DNS, 0, max_size);
  34                 if (r < 0)
  35                         return r;
  36
  37                 r = dns_packet_append_question(*p, q);
  38                 if (r < 0)
  39                         return r;
  40
  41                 DNS_PACKET_HEADER(*p)->qdcount = htobe16(dns_question_size(q));
  42         }
  43
  44         DNS_ANSWER_FOREACH(rr, answer) {
  45
  46                 r = dns_question_matches_rr(q, rr, NULL);
  47                 if (r < 0)
  48                         return r;
  49                 if (r > 0)
  50                         goto add;
  51
  52                 r = dns_question_matches_cname_or_dname(q, rr, NULL);
  53                 if (r < 0)
  54                         return r;
  55                 if (r > 0)
  56                         goto add;
  57
  58                 continue;
  59         add:
  60                 r = dns_packet_append_rr(*p, rr, 0, NULL, NULL);
  61                 if (r == -EMSGSIZE) {
  62                         truncated = true;
  63                         break;
  64                 }
  65                 if (r < 0)
  66                         return r;
  67
  68                 c++;
  69         }
  70
  71         if (ret_truncated)
  72                 *ret_truncated = truncated;
  73         else if (truncated)
  74                 return -EMSGSIZE;
  75
  76         DNS_PACKET_HEADER(*p)->ancount = htobe16(be16toh(DNS_PACKET_HEADER(*p)->ancount) + c);
  77
  78         return 0;
  79 }
  80
  81 static int dns_stub_finish_reply_packet(
  82                 DnsPacket *p,
  83                 uint16_t id,
  84                 int rcode,
  85                 bool tc,        /* set the Truncated bit? */
  86                 bool add_opt,   /* add an OPT RR to this packet? */
  87                 bool edns0_do,  /* set the EDNS0 DNSSEC OK bit? */
  88                 bool ad) {      /* set the DNSSEC authenticated data bit? */
  89
  90         int r;
  91
  92         assert(p);
  93
  94         if (!add_opt) {
  95                 /* If the client can't to EDNS0, don't do DO either */
  96                 edns0_do = false;
  97
  98                 /* If the client didn't do EDNS, clamp the rcode to 4 bit */
  99                 if (rcode > 0xF)
 100                         rcode = DNS_RCODE_SERVFAIL;
 101         }
 102
 103         /* Don't set the AD bit unless DO is on, too */
 104         if (!edns0_do)
 105                 ad = false;
 106
 107         DNS_PACKET_HEADER(p)->id = id;
 108
 109         DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
 110                                                               1  /* qr */,
 111                                                               0  /* opcode */,
 112                                                               0  /* aa */,
 113                                                               tc /* tc */,
 114                                                               1  /* rd */,
 115                                                               1  /* ra */,
 116                                                               ad /* ad */,
 117                                                               0  /* cd */,
 118                                                               rcode));
 119
 120         if (add_opt) {
 121                 r = dns_packet_append_opt(p, ADVERTISE_DATAGRAM_SIZE_MAX, edns0_do, rcode, NULL);
 122                 if (r < 0)
 123                         return r;
 124         }
 125
 126         return 0;
 127 }
 128
 129 static void dns_stub_detach_stream(DnsStream *s) {
 130         assert(s);
 131
 132         s->complete = NULL;
 133         s->on_packet = NULL;
 134         s->query = NULL;
 135 }
 136
 137 static int dns_stub_send(Manager *m, DnsStream *s, DnsPacket *p, DnsPacket *reply) {
 138         int r;
 139
 140         assert(m);
 141         assert(p);
 142         assert(reply);
 143
 144         if (s)
 145                 r = dns_stream_write_packet(s, reply);
 146         else {
 147                 int fd;
 148
 149                 fd = manager_dns_stub_udp_fd(m);
 150                 if (fd < 0)
 151                         return log_debug_errno(fd, "Failed to get reply socket: %m");
 152
 153                 /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
 154                  * is because otherwise the kernel will choose it automatically based on the routing table and will
 155                  * thus pick 127.0.0.1 rather than 127.0.0.53. */
 156
 157                 r = manager_send(m, fd, LOOPBACK_IFINDEX, p->family, &p->sender, p->sender_port, &p->destination, reply);
 158         }
 159         if (r < 0)
 160                 return log_debug_errno(r, "Failed to send reply packet: %m");
 161
 162         return 0;
 163 }
 164
 165 static int dns_stub_send_failure(Manager *m, DnsStream *s, DnsPacket *p, int rcode, bool authenticated) {
 166         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 167         int r;
 168
 169         assert(m);
 170         assert(p);
 171
 172         r = dns_stub_make_reply_packet(&reply, DNS_PACKET_PAYLOAD_SIZE_MAX(p), p->question, NULL, NULL);
 173         if (r < 0)
 174                 return log_debug_errno(r, "Failed to make failure packet: %m");
 175
 176         r = dns_stub_finish_reply_packet(reply, DNS_PACKET_ID(p), rcode, false, !!p->opt, DNS_PACKET_DO(p), authenticated);
 177         if (r < 0)
 178                 return log_debug_errno(r, "Failed to build failure packet: %m");
 179
 180         return dns_stub_send(m, s, p, reply);
 181 }
 182
 183 static void dns_stub_query_complete(DnsQuery *q) {
 184         int r;
 185
 186         assert(q);
 187         assert(q->request_dns_packet);
 188
 189         switch (q->state) {
 190
 191         case DNS_TRANSACTION_SUCCESS: {
 192                 bool truncated;
 193
 194                 r = dns_stub_make_reply_packet(&q->reply_dns_packet, DNS_PACKET_PAYLOAD_SIZE_MAX(q->request_dns_packet), q->question_idna, q->answer, &truncated);
 195                 if (r < 0) {
 196                         log_debug_errno(r, "Failed to build reply packet: %m");
 197                         break;
 198                 }
 199
 200                 r = dns_query_process_cname(q);
 201                 if (r == -ELOOP) {
 202                         (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 203                         break;
 204                 }
 205                 if (r < 0) {
 206                         log_debug_errno(r, "Failed to process CNAME: %m");
 207                         break;
 208                 }
 209                 if (r == DNS_QUERY_RESTARTED)
 210                         return;
 211
 212                 r = dns_stub_finish_reply_packet(
 213                                 q->reply_dns_packet,
 214                                 DNS_PACKET_ID(q->request_dns_packet),
 215                                 q->answer_rcode,
 216                                 truncated,
 217                                 !!q->request_dns_packet->opt,
 218                                 DNS_PACKET_DO(q->request_dns_packet),
 219                                 dns_query_fully_authenticated(q));
 220                 if (r < 0) {
 221                         log_debug_errno(r, "Failed to finish reply packet: %m");
 222                         break;
 223                 }
 224
 225                 (void) dns_stub_send(q->manager, q->request_dns_stream, q->request_dns_packet, q->reply_dns_packet);
 226                 break;
 227         }
 228
 229         case DNS_TRANSACTION_RCODE_FAILURE:
 230                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, q->answer_rcode, dns_query_fully_authenticated(q));
 231                 break;
 232
 233         case DNS_TRANSACTION_NOT_FOUND:
 234                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_NXDOMAIN, dns_query_fully_authenticated(q));
 235                 break;
 236
 237         case DNS_TRANSACTION_TIMEOUT:
 238         case DNS_TRANSACTION_ATTEMPTS_MAX_REACHED:
 239                 /* Propagate a timeout as a no packet, i.e. that the client also gets a timeout */
 240                 break;
 241
 242         case DNS_TRANSACTION_NO_SERVERS:
 243         case DNS_TRANSACTION_INVALID_REPLY:
 244         case DNS_TRANSACTION_ERRNO:
 245         case DNS_TRANSACTION_ABORTED:
 246         case DNS_TRANSACTION_DNSSEC_FAILED:
 247         case DNS_TRANSACTION_NO_TRUST_ANCHOR:
 248         case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
 249         case DNS_TRANSACTION_NETWORK_DOWN:
 250                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 251                 break;
 252
 253         case DNS_TRANSACTION_NULL:
 254         case DNS_TRANSACTION_PENDING:
 255         case DNS_TRANSACTION_VALIDATING:
 256         default:
 257                 assert_not_reached("Impossible state");
 258         }
 259
 260         /* If there's a packet to write set, let's leave the stream around */
 261         if (q->request_dns_stream && DNS_STREAM_QUEUED(q->request_dns_stream)) {
 262
 263                 /* Detach the stream from our query (make it an orphan), but do not drop the reference to it. The
 264                  * default completion action of the stream will drop the reference. */
 265
 266                 dns_stub_detach_stream(q->request_dns_stream);
 267                 q->request_dns_stream = NULL;
 268         }
 269
 270         dns_query_free(q);
 271 }
 272
 273 static int dns_stub_stream_complete(DnsStream *s, int error) {
 274         assert(s);
 275
 276         log_debug_errno(error, "DNS TCP connection terminated, destroying query: %m");
 277
 278         assert(s->query);
 279         dns_query_free(s->query);
 280
 281         return 0;
 282 }
 283
 284 static void dns_stub_process_query(Manager *m, DnsStream *s, DnsPacket *p) {
 285         DnsQuery *q = NULL;
 286         int r;
 287
 288         assert(m);
 289         assert(p);
 290         assert(p->protocol == DNS_PROTOCOL_DNS);
 291
 292         /* Takes ownership of the *s stream object */
 293
 294         if (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
 295             in_addr_is_localhost(p->family, &p->destination) <= 0) {
 296                 log_error("Got packet on unexpected IP range, refusing.");
 297                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 298                 goto fail;
 299         }
 300
 301         r = dns_packet_extract(p);
 302         if (r < 0) {
 303                 log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
 304                 dns_stub_send_failure(m, s, p, DNS_RCODE_FORMERR, false);
 305                 goto fail;
 306         }
 307
 308         if (!DNS_PACKET_VERSION_SUPPORTED(p)) {
 309                 log_debug("Got EDNS OPT field with unsupported version number.");
 310                 dns_stub_send_failure(m, s, p, DNS_RCODE_BADVERS, false);
 311                 goto fail;
 312         }
 313
 314         if (dns_type_is_obsolete(p->question->keys[0]->type)) {
 315                 log_debug("Got message with obsolete key type, refusing.");
 316                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 317                 goto fail;
 318         }
 319
 320         if (dns_type_is_zone_transer(p->question->keys[0]->type)) {
 321                 log_debug("Got request for zone transfer, refusing.");
 322                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 323                 goto fail;
 324         }
 325
 326         if (!DNS_PACKET_RD(p))  {
 327                 /* If the "rd" bit is off (i.e. recursion was not requested), then refuse operation */
 328                 log_debug("Got request with recursion disabled, refusing.");
 329                 dns_stub_send_failure(m, s, p, DNS_RCODE_REFUSED, false);
 330                 goto fail;
 331         }
 332
 333         if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
 334                 log_debug("Got request with DNSSEC CD bit set, refusing.");
 335                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 336                 goto fail;
 337         }
 338
 339         r = dns_query_new(m, &q, p->question, p->question, 0, SD_RESOLVED_PROTOCOLS_ALL|SD_RESOLVED_NO_SEARCH);
 340         if (r < 0) {
 341                 log_error_errno(r, "Failed to generate query object: %m");
 342                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 343                 goto fail;
 344         }
 345
 346         /* Request that the TTL is corrected by the cached time for this lookup, so that we return vaguely useful TTLs */
 347         q->clamp_ttl = true;
 348
 349         q->request_dns_packet = dns_packet_ref(p);
 350         q->request_dns_stream = dns_stream_ref(s); /* make sure the stream stays around until we can send a reply through it */
 351         q->complete = dns_stub_query_complete;
 352
 353         if (s) {
 354                 s->on_packet = NULL;
 355                 s->complete = dns_stub_stream_complete;
 356                 s->query = q;
 357         }
 358
 359         r = dns_query_go(q);
 360         if (r < 0) {
 361                 log_error_errno(r, "Failed to start query: %m");
 362                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 363                 goto fail;
 364         }
 365
 366         log_debug("Processing query...");
 367         return;
 368
 369 fail:
 370         if (s && DNS_STREAM_QUEUED(s))
 371                 dns_stub_detach_stream(s);
 372
 373         dns_query_free(q);
 374 }
 375
 376 static int on_dns_stub_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 377         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 378         Manager *m = userdata;
 379         int r;
 380
 381         r = manager_recv(m, fd, DNS_PROTOCOL_DNS, &p);
 382         if (r <= 0)
 383                 return r;
 384
 385         if (dns_packet_validate_query(p) > 0) {
 386                 log_debug("Got DNS stub UDP query packet for id %u", DNS_PACKET_ID(p));
 387
 388                 dns_stub_process_query(m, NULL, p);
 389         } else
 390                 log_debug("Invalid DNS stub UDP packet, ignoring.");
 391
 392         return 0;
 393 }
 394
 395 static int manager_dns_stub_udp_fd(Manager *m) {
 396         union sockaddr_union sa = {
 397                 .in.sin_family = AF_INET,
 398                 .in.sin_port = htobe16(53),
 399                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 400         };
 401         _cleanup_close_ int fd = -1;
 402         int r;
 403
 404         if (m->dns_stub_udp_fd >= 0)
 405                 return m->dns_stub_udp_fd;
 406
 407         fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 408         if (fd < 0)
 409                 return -errno;
 410
 411         r = setsockopt_int(fd, SOL_SOCKET, SO_REUSEADDR, true);
 412         if (r < 0)
 413                 return r;
 414
 415         r = setsockopt_int(fd, IPPROTO_IP, IP_PKTINFO, true);
 416         if (r < 0)
 417                 return r;
 418
 419         r = setsockopt_int(fd, IPPROTO_IP, IP_RECVTTL, true);
 420         if (r < 0)
 421                 return r;
 422
 423         /* Make sure no traffic from outside the local host can leak to onto this socket */
 424         if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, "lo", 3) < 0)
 425                 return -errno;
 426
 427         if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
 428                 return -errno;
 429
 430         r = sd_event_add_io(m->event, &m->dns_stub_udp_event_source, fd, EPOLLIN, on_dns_stub_packet, m);
 431         if (r < 0)
 432                 return r;
 433
 434         (void) sd_event_source_set_description(m->dns_stub_udp_event_source, "dns-stub-udp");
 435
 436         return m->dns_stub_udp_fd = TAKE_FD(fd);
 437 }
 438
 439 static int on_dns_stub_stream_packet(DnsStream *s) {
 440         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 441
 442         assert(s);
 443
 444         p = dns_stream_take_read_packet(s);
 445         assert(p);
 446
 447         if (dns_packet_validate_query(p) > 0) {
 448                 log_debug("Got DNS stub TCP query packet for id %u", DNS_PACKET_ID(p));
 449
 450                 dns_stub_process_query(s->manager, s, p);
 451         } else
 452                 log_debug("Invalid DNS stub TCP packet, ignoring.");
 453
 454         /* Drop the reference to the stream. Either a query was created and added its own reference to the stream now,
 455          * or that didn't happen in which case we want to free the stream */
 456         dns_stream_unref(s);
 457
 458         return 0;
 459 }
 460
 461 static int on_dns_stub_stream(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 462         DnsStream *stream;
 463         Manager *m = userdata;
 464         int cfd, r;
 465
 466         cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
 467         if (cfd < 0) {
 468                 if (IN_SET(errno, EAGAIN, EINTR))
 469                         return 0;
 470
 471                 return -errno;
 472         }
 473
 474         r = dns_stream_new(m, &stream, DNS_STREAM_STUB, DNS_PROTOCOL_DNS, cfd, NULL);
 475         if (r < 0) {
 476                 safe_close(cfd);
 477                 return r;
 478         }
 479
 480         stream->on_packet = on_dns_stub_stream_packet;
 481
 482         /* We let the reference to the stream dangling here, it will either be dropped by the default "complete" action
 483          * of the stream, or by our packet callback, or when the manager is shut down. */
 484
 485         return 0;
 486 }
 487
 488 static int manager_dns_stub_tcp_fd(Manager *m) {
 489         union sockaddr_union sa = {
 490                 .in.sin_family = AF_INET,
 491                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 492                 .in.sin_port = htobe16(53),
 493         };
 494         _cleanup_close_ int fd = -1;
 495         int r;
 496
 497         if (m->dns_stub_tcp_fd >= 0)
 498                 return m->dns_stub_tcp_fd;
 499
 500         fd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 501         if (fd < 0)
 502                 return -errno;
 503
 504         r = setsockopt_int(fd, IPPROTO_IP, IP_TTL, true);
 505         if (r < 0)
 506                 return r;
 507
 508         r = setsockopt_int(fd, SOL_SOCKET, SO_REUSEADDR, true);
 509         if (r < 0)
 510                 return r;
 511
 512         r = setsockopt_int(fd, IPPROTO_IP, IP_PKTINFO, true);
 513         if (r < 0)
 514                 return r;
 515
 516         r = setsockopt_int(fd, IPPROTO_IP, IP_RECVTTL, true);
 517         if (r < 0)
 518                 return r;
 519
 520         /* Make sure no traffic from outside the local host can leak to onto this socket */
 521         if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, "lo", 3) < 0)
 522                 return -errno;
 523
 524         if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
 525                 return -errno;
 526
 527         if (listen(fd, SOMAXCONN) < 0)
 528                 return -errno;
 529
 530         r = sd_event_add_io(m->event, &m->dns_stub_tcp_event_source, fd, EPOLLIN, on_dns_stub_stream, m);
 531         if (r < 0)
 532                 return r;
 533
 534         (void) sd_event_source_set_description(m->dns_stub_tcp_event_source, "dns-stub-tcp");
 535
 536         return m->dns_stub_tcp_fd = TAKE_FD(fd);
 537 }
 538
 539 int manager_dns_stub_start(Manager *m) {
 540         const char *t = "UDP";
 541         int r = 0;
 542
 543         assert(m);
 544
 545         if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_NO)
 546                 log_debug("Not creating stub listener.");
 547         else
 548                 log_debug("Creating stub listener using %s.",
 549                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP ? "UDP" :
 550                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP ? "TCP" :
 551                           "UDP/TCP");
 552
 553         if (IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_UDP))
 554                 r = manager_dns_stub_udp_fd(m);
 555
 556         if (r >= 0 &&
 557             IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_TCP)) {
 558                 t = "TCP";
 559                 r = manager_dns_stub_tcp_fd(m);
 560         }
 561
 562         if (IN_SET(r, -EADDRINUSE, -EPERM)) {
 563                 if (r == -EADDRINUSE)
 564                         log_warning_errno(r,
 565                                           "Another process is already listening on %s socket 127.0.0.53:53.\n"
 566                                           "Turning off local DNS stub support.", t);
 567                 else
 568                         log_warning_errno(r,
 569                                           "Failed to listen on %s socket 127.0.0.53:53: %m.\n"
 570                                           "Turning off local DNS stub support.", t);
 571                 manager_dns_stub_stop(m);
 572         } else if (r < 0)
 573                 return log_error_errno(r, "Failed to listen on %s socket 127.0.0.53:53: %m", t);
 574
 575         return 0;
 576 }
 577
 578 void manager_dns_stub_stop(Manager *m) {
 579         assert(m);
 580
 581         m->dns_stub_udp_event_source = sd_event_source_unref(m->dns_stub_udp_event_source);
 582         m->dns_stub_tcp_event_source = sd_event_source_unref(m->dns_stub_tcp_event_source);
 583
 584         m->dns_stub_udp_fd = safe_close(m->dns_stub_udp_fd);
 585         m->dns_stub_tcp_fd = safe_close(m->dns_stub_tcp_fd);
 586 }