src/resolve/resolved-dns-stub.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   Copyright 2016 Lennart Poettering
   4 ***/
   5
   6 #include "fd-util.h"
   7 #include "resolved-dns-stub.h"
   8 #include "socket-util.h"
   9
  10 /* The MTU of the loopback device is 64K on Linux, advertise that as maximum datagram size, but subtract the Ethernet,
  11  * IP and UDP header sizes */
  12 #define ADVERTISE_DATAGRAM_SIZE_MAX (65536U-14U-20U-8U)
  13
  14 static int manager_dns_stub_udp_fd(Manager *m);
  15 static int manager_dns_stub_tcp_fd(Manager *m);
  16
  17 static int dns_stub_make_reply_packet(
  18                 DnsPacket **p,
  19                 size_t max_size,
  20                 DnsQuestion *q,
  21                 DnsAnswer *answer,
  22                 bool *ret_truncated) {
  23
  24         bool truncated = false;
  25         DnsResourceRecord *rr;
  26         unsigned c = 0;
  27         int r;
  28
  29         assert(p);
  30
  31         /* Note that we don't bother with any additional RRs, as this is stub is for local lookups only, and hence
  32          * roundtrips aren't expensive. */
  33
  34         if (!*p) {
  35                 r = dns_packet_new(p, DNS_PROTOCOL_DNS, 0, max_size);
  36                 if (r < 0)
  37                         return r;
  38
  39                 r = dns_packet_append_question(*p, q);
  40                 if (r < 0)
  41                         return r;
  42
  43                 DNS_PACKET_HEADER(*p)->qdcount = htobe16(dns_question_size(q));
  44         }
  45
  46         DNS_ANSWER_FOREACH(rr, answer) {
  47
  48                 r = dns_question_matches_rr(q, rr, NULL);
  49                 if (r < 0)
  50                         return r;
  51                 if (r > 0)
  52                         goto add;
  53
  54                 r = dns_question_matches_cname_or_dname(q, rr, NULL);
  55                 if (r < 0)
  56                         return r;
  57                 if (r > 0)
  58                         goto add;
  59
  60                 continue;
  61         add:
  62                 r = dns_packet_append_rr(*p, rr, 0, NULL, NULL);
  63                 if (r == -EMSGSIZE) {
  64                         truncated = true;
  65                         break;
  66                 }
  67                 if (r < 0)
  68                         return r;
  69
  70                 c++;
  71         }
  72
  73         if (ret_truncated)
  74                 *ret_truncated = truncated;
  75         else if (truncated)
  76                 return -EMSGSIZE;
  77
  78         DNS_PACKET_HEADER(*p)->ancount = htobe16(be16toh(DNS_PACKET_HEADER(*p)->ancount) + c);
  79
  80         return 0;
  81 }
  82
  83 static int dns_stub_finish_reply_packet(
  84                 DnsPacket *p,
  85                 uint16_t id,
  86                 int rcode,
  87                 bool tc,        /* set the Truncated bit? */
  88                 bool add_opt,   /* add an OPT RR to this packet? */
  89                 bool edns0_do,  /* set the EDNS0 DNSSEC OK bit? */
  90                 bool ad) {      /* set the DNSSEC authenticated data bit? */
  91
  92         int r;
  93
  94         assert(p);
  95
  96         if (!add_opt) {
  97                 /* If the client can't to EDNS0, don't do DO either */
  98                 edns0_do = false;
  99
 100                 /* If the client didn't do EDNS, clamp the rcode to 4 bit */
 101                 if (rcode > 0xF)
 102                         rcode = DNS_RCODE_SERVFAIL;
 103         }
 104
 105         /* Don't set the AD bit unless DO is on, too */
 106         if (!edns0_do)
 107                 ad = false;
 108
 109         DNS_PACKET_HEADER(p)->id = id;
 110
 111         DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
 112                                                               1  /* qr */,
 113                                                               0  /* opcode */,
 114                                                               0  /* aa */,
 115                                                               tc /* tc */,
 116                                                               1  /* rd */,
 117                                                               1  /* ra */,
 118                                                               ad /* ad */,
 119                                                               0  /* cd */,
 120                                                               rcode));
 121
 122         if (add_opt) {
 123                 r = dns_packet_append_opt(p, ADVERTISE_DATAGRAM_SIZE_MAX, edns0_do, rcode, NULL);
 124                 if (r < 0)
 125                         return r;
 126         }
 127
 128         return 0;
 129 }
 130
 131 static void dns_stub_detach_stream(DnsStream *s) {
 132         assert(s);
 133
 134         s->complete = NULL;
 135         s->on_packet = NULL;
 136         s->query = NULL;
 137 }
 138
 139 static int dns_stub_send(Manager *m, DnsStream *s, DnsPacket *p, DnsPacket *reply) {
 140         int r;
 141
 142         assert(m);
 143         assert(p);
 144         assert(reply);
 145
 146         if (s)
 147                 r = dns_stream_write_packet(s, reply);
 148         else {
 149                 int fd;
 150
 151                 fd = manager_dns_stub_udp_fd(m);
 152                 if (fd < 0)
 153                         return log_debug_errno(fd, "Failed to get reply socket: %m");
 154
 155                 /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
 156                  * is because otherwise the kernel will choose it automatically based on the routing table and will
 157                  * thus pick 127.0.0.1 rather than 127.0.0.53. */
 158
 159                 r = manager_send(m, fd, LOOPBACK_IFINDEX, p->family, &p->sender, p->sender_port, &p->destination, reply);
 160         }
 161         if (r < 0)
 162                 return log_debug_errno(r, "Failed to send reply packet: %m");
 163
 164         return 0;
 165 }
 166
 167 static int dns_stub_send_failure(Manager *m, DnsStream *s, DnsPacket *p, int rcode, bool authenticated) {
 168         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 169         int r;
 170
 171         assert(m);
 172         assert(p);
 173
 174         r = dns_stub_make_reply_packet(&reply, DNS_PACKET_PAYLOAD_SIZE_MAX(p), p->question, NULL, NULL);
 175         if (r < 0)
 176                 return log_debug_errno(r, "Failed to make failure packet: %m");
 177
 178         r = dns_stub_finish_reply_packet(reply, DNS_PACKET_ID(p), rcode, false, !!p->opt, DNS_PACKET_DO(p), authenticated);
 179         if (r < 0)
 180                 return log_debug_errno(r, "Failed to build failure packet: %m");
 181
 182         return dns_stub_send(m, s, p, reply);
 183 }
 184
 185 static void dns_stub_query_complete(DnsQuery *q) {
 186         int r;
 187
 188         assert(q);
 189         assert(q->request_dns_packet);
 190
 191         switch (q->state) {
 192
 193         case DNS_TRANSACTION_SUCCESS: {
 194                 bool truncated;
 195
 196                 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);
 197                 if (r < 0) {
 198                         log_debug_errno(r, "Failed to build reply packet: %m");
 199                         break;
 200                 }
 201
 202                 r = dns_query_process_cname(q);
 203                 if (r == -ELOOP) {
 204                         (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 205                         break;
 206                 }
 207                 if (r < 0) {
 208                         log_debug_errno(r, "Failed to process CNAME: %m");
 209                         break;
 210                 }
 211                 if (r == DNS_QUERY_RESTARTED)
 212                         return;
 213
 214                 r = dns_stub_finish_reply_packet(
 215                                 q->reply_dns_packet,
 216                                 DNS_PACKET_ID(q->request_dns_packet),
 217                                 q->answer_rcode,
 218                                 truncated,
 219                                 !!q->request_dns_packet->opt,
 220                                 DNS_PACKET_DO(q->request_dns_packet),
 221                                 dns_query_fully_authenticated(q));
 222                 if (r < 0) {
 223                         log_debug_errno(r, "Failed to finish reply packet: %m");
 224                         break;
 225                 }
 226
 227                 (void) dns_stub_send(q->manager, q->request_dns_stream, q->request_dns_packet, q->reply_dns_packet);
 228                 break;
 229         }
 230
 231         case DNS_TRANSACTION_RCODE_FAILURE:
 232                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, q->answer_rcode, dns_query_fully_authenticated(q));
 233                 break;
 234
 235         case DNS_TRANSACTION_NOT_FOUND:
 236                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_NXDOMAIN, dns_query_fully_authenticated(q));
 237                 break;
 238
 239         case DNS_TRANSACTION_TIMEOUT:
 240         case DNS_TRANSACTION_ATTEMPTS_MAX_REACHED:
 241                 /* Propagate a timeout as a no packet, i.e. that the client also gets a timeout */
 242                 break;
 243
 244         case DNS_TRANSACTION_NO_SERVERS:
 245         case DNS_TRANSACTION_INVALID_REPLY:
 246         case DNS_TRANSACTION_ERRNO:
 247         case DNS_TRANSACTION_ABORTED:
 248         case DNS_TRANSACTION_DNSSEC_FAILED:
 249         case DNS_TRANSACTION_NO_TRUST_ANCHOR:
 250         case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
 251         case DNS_TRANSACTION_NETWORK_DOWN:
 252                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 253                 break;
 254
 255         case DNS_TRANSACTION_NULL:
 256         case DNS_TRANSACTION_PENDING:
 257         case DNS_TRANSACTION_VALIDATING:
 258         default:
 259                 assert_not_reached("Impossible state");
 260         }
 261
 262         /* If there's a packet to write set, let's leave the stream around */
 263         if (q->request_dns_stream && DNS_STREAM_QUEUED(q->request_dns_stream)) {
 264
 265                 /* Detach the stream from our query (make it an orphan), but do not drop the reference to it. The
 266                  * default completion action of the stream will drop the reference. */
 267
 268                 dns_stub_detach_stream(q->request_dns_stream);
 269                 q->request_dns_stream = NULL;
 270         }
 271
 272         dns_query_free(q);
 273 }
 274
 275 static int dns_stub_stream_complete(DnsStream *s, int error) {
 276         assert(s);
 277
 278         log_debug_errno(error, "DNS TCP connection terminated, destroying query: %m");
 279
 280         assert(s->query);
 281         dns_query_free(s->query);
 282
 283         return 0;
 284 }
 285
 286 static void dns_stub_process_query(Manager *m, DnsStream *s, DnsPacket *p) {
 287         DnsQuery *q = NULL;
 288         int r;
 289
 290         assert(m);
 291         assert(p);
 292         assert(p->protocol == DNS_PROTOCOL_DNS);
 293
 294         /* Takes ownership of the *s stream object */
 295
 296         if (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
 297             in_addr_is_localhost(p->family, &p->destination) <= 0) {
 298                 log_error("Got packet on unexpected IP range, refusing.");
 299                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 300                 goto fail;
 301         }
 302
 303         r = dns_packet_extract(p);
 304         if (r < 0) {
 305                 log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
 306                 dns_stub_send_failure(m, s, p, DNS_RCODE_FORMERR, false);
 307                 goto fail;
 308         }
 309
 310         if (!DNS_PACKET_VERSION_SUPPORTED(p)) {
 311                 log_debug("Got EDNS OPT field with unsupported version number.");
 312                 dns_stub_send_failure(m, s, p, DNS_RCODE_BADVERS, false);
 313                 goto fail;
 314         }
 315
 316         if (dns_type_is_obsolete(p->question->keys[0]->type)) {
 317                 log_debug("Got message with obsolete key type, refusing.");
 318                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 319                 goto fail;
 320         }
 321
 322         if (dns_type_is_zone_transer(p->question->keys[0]->type)) {
 323                 log_debug("Got request for zone transfer, refusing.");
 324                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 325                 goto fail;
 326         }
 327
 328         if (!DNS_PACKET_RD(p))  {
 329                 /* If the "rd" bit is off (i.e. recursion was not requested), then refuse operation */
 330                 log_debug("Got request with recursion disabled, refusing.");
 331                 dns_stub_send_failure(m, s, p, DNS_RCODE_REFUSED, false);
 332                 goto fail;
 333         }
 334
 335         if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
 336                 log_debug("Got request with DNSSEC CD bit set, refusing.");
 337                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 338                 goto fail;
 339         }
 340
 341         r = dns_query_new(m, &q, p->question, p->question, 0, SD_RESOLVED_PROTOCOLS_ALL|SD_RESOLVED_NO_SEARCH);
 342         if (r < 0) {
 343                 log_error_errno(r, "Failed to generate query object: %m");
 344                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 345                 goto fail;
 346         }
 347
 348         /* Request that the TTL is corrected by the cached time for this lookup, so that we return vaguely useful TTLs */
 349         q->clamp_ttl = true;
 350
 351         q->request_dns_packet = dns_packet_ref(p);
 352         q->request_dns_stream = dns_stream_ref(s); /* make sure the stream stays around until we can send a reply through it */
 353         q->complete = dns_stub_query_complete;
 354
 355         if (s) {
 356                 s->on_packet = NULL;
 357                 s->complete = dns_stub_stream_complete;
 358                 s->query = q;
 359         }
 360
 361         r = dns_query_go(q);
 362         if (r < 0) {
 363                 log_error_errno(r, "Failed to start query: %m");
 364                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 365                 goto fail;
 366         }
 367
 368         log_debug("Processing query...");
 369         return;
 370
 371 fail:
 372         if (s && DNS_STREAM_QUEUED(s))
 373                 dns_stub_detach_stream(s);
 374
 375         dns_query_free(q);
 376 }
 377
 378 static int on_dns_stub_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 379         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 380         Manager *m = userdata;
 381         int r;
 382
 383         r = manager_recv(m, fd, DNS_PROTOCOL_DNS, &p);
 384         if (r <= 0)
 385                 return r;
 386
 387         if (dns_packet_validate_query(p) > 0) {
 388                 log_debug("Got DNS stub UDP query packet for id %u", DNS_PACKET_ID(p));
 389
 390                 dns_stub_process_query(m, NULL, p);
 391         } else
 392                 log_debug("Invalid DNS stub UDP packet, ignoring.");
 393
 394         return 0;
 395 }
 396
 397 static int manager_dns_stub_udp_fd(Manager *m) {
 398         static const int one = 1;
 399         union sockaddr_union sa = {
 400                 .in.sin_family = AF_INET,
 401                 .in.sin_port = htobe16(53),
 402                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 403         };
 404         _cleanup_close_ int fd = -1;
 405         int r;
 406
 407         if (m->dns_stub_udp_fd >= 0)
 408                 return m->dns_stub_udp_fd;
 409
 410         fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 411         if (fd < 0)
 412                 return -errno;
 413
 414         if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof one) < 0)
 415                 return -errno;
 416
 417         if (setsockopt(fd, IPPROTO_IP, IP_PKTINFO, &one, sizeof one) < 0)
 418                 return -errno;
 419
 420         if (setsockopt(fd, IPPROTO_IP, IP_RECVTTL, &one, sizeof one) < 0)
 421                 return -errno;
 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         assert(s);
 441         assert(s->read_packet);
 442
 443         if (dns_packet_validate_query(s->read_packet) > 0) {
 444                 log_debug("Got DNS stub TCP query packet for id %u", DNS_PACKET_ID(s->read_packet));
 445
 446                 dns_stub_process_query(s->manager, s, s->read_packet);
 447         } else
 448                 log_debug("Invalid DNS stub TCP packet, ignoring.");
 449
 450         /* Drop the reference to the stream. Either a query was created and added its own reference to the stream now,
 451          * or that didn't happen in which case we want to free the stream */
 452         dns_stream_unref(s);
 453
 454         return 0;
 455 }
 456
 457 static int on_dns_stub_stream(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 458         DnsStream *stream;
 459         Manager *m = userdata;
 460         int cfd, r;
 461
 462         cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
 463         if (cfd < 0) {
 464                 if (IN_SET(errno, EAGAIN, EINTR))
 465                         return 0;
 466
 467                 return -errno;
 468         }
 469
 470         r = dns_stream_new(m, &stream, DNS_PROTOCOL_DNS, cfd, NULL);
 471         if (r < 0) {
 472                 safe_close(cfd);
 473                 return r;
 474         }
 475
 476         stream->on_packet = on_dns_stub_stream_packet;
 477
 478         /* We let the reference to the stream dangling here, it will either be dropped by the default "complete" action
 479          * of the stream, or by our packet callback, or when the manager is shut down. */
 480
 481         return 0;
 482 }
 483
 484 static int manager_dns_stub_tcp_fd(Manager *m) {
 485         static const int one = 1;
 486         union sockaddr_union sa = {
 487                 .in.sin_family = AF_INET,
 488                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 489                 .in.sin_port = htobe16(53),
 490         };
 491         _cleanup_close_ int fd = -1;
 492         int r;
 493
 494         if (m->dns_stub_tcp_fd >= 0)
 495                 return m->dns_stub_tcp_fd;
 496
 497         fd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 498         if (fd < 0)
 499                 return -errno;
 500
 501         if (setsockopt(fd, IPPROTO_IP, IP_TTL, &one, sizeof one) < 0)
 502                 return -errno;
 503
 504         if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof one) < 0)
 505                 return -errno;
 506
 507         if (setsockopt(fd, IPPROTO_IP, IP_PKTINFO, &one, sizeof one) < 0)
 508                 return -errno;
 509
 510         if (setsockopt(fd, IPPROTO_IP, IP_RECVTTL, &one, sizeof one) < 0)
 511                 return -errno;
 512
 513         /* Make sure no traffic from outside the local host can leak to onto this socket */
 514         if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, "lo", 3) < 0)
 515                 return -errno;
 516
 517         if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
 518                 return -errno;
 519
 520         if (listen(fd, SOMAXCONN) < 0)
 521                 return -errno;
 522
 523         r = sd_event_add_io(m->event, &m->dns_stub_tcp_event_source, fd, EPOLLIN, on_dns_stub_stream, m);
 524         if (r < 0)
 525                 return r;
 526
 527         (void) sd_event_source_set_description(m->dns_stub_tcp_event_source, "dns-stub-tcp");
 528
 529         return m->dns_stub_tcp_fd = TAKE_FD(fd);
 530 }
 531
 532 int manager_dns_stub_start(Manager *m) {
 533         const char *t = "UDP";
 534         int r = 0;
 535
 536         assert(m);
 537
 538         if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_NO)
 539                 log_debug("Not creating stub listener.");
 540         else
 541                 log_debug("Creating stub listener using %s.",
 542                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP ? "UDP" :
 543                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP ? "TCP" :
 544                           "UDP/TCP");
 545
 546         if (IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_UDP))
 547                 r = manager_dns_stub_udp_fd(m);
 548
 549         if (r >= 0 &&
 550             IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_TCP)) {
 551                 t = "TCP";
 552                 r = manager_dns_stub_tcp_fd(m);
 553         }
 554
 555         if (IN_SET(r, -EADDRINUSE, -EPERM)) {
 556                 if (r == -EADDRINUSE)
 557                         log_warning_errno(r,
 558                                           "Another process is already listening on %s socket 127.0.0.53:53.\n"
 559                                           "Turning off local DNS stub support.", t);
 560                 else
 561                         log_warning_errno(r,
 562                                           "Failed to listen on %s socket 127.0.0.53:53: %m.\n"
 563                                           "Turning off local DNS stub support.", t);
 564                 manager_dns_stub_stop(m);
 565         } else if (r < 0)
 566                 return log_error_errno(r, "Failed to listen on %s socket 127.0.0.53:53: %m", t);
 567
 568         return 0;
 569 }
 570
 571 void manager_dns_stub_stop(Manager *m) {
 572         assert(m);
 573
 574         m->dns_stub_udp_event_source = sd_event_source_unref(m->dns_stub_udp_event_source);
 575         m->dns_stub_tcp_event_source = sd_event_source_unref(m->dns_stub_tcp_event_source);
 576
 577         m->dns_stub_udp_fd = safe_close(m->dns_stub_udp_fd);
 578         m->dns_stub_tcp_fd = safe_close(m->dns_stub_tcp_fd);
 579 }