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                 r = dns_packet_append_opt(p, ADVERTISE_DATAGRAM_SIZE_MAX, edns0_do, rcode, NULL);
  96                 if (r == -EMSGSIZE) /* Hit the size limit? then indicate truncation */
  97                         tc = true;
  98                 else if (r < 0)
  99                         return r;
 100
 101         } else {
 102                 /* If the client can't to EDNS0, don't do DO either */
 103                 edns0_do = false;
 104
 105                 /* If the client didn't do EDNS, clamp the rcode to 4 bit */
 106                 if (rcode > 0xF)
 107                         rcode = DNS_RCODE_SERVFAIL;
 108         }
 109
 110         /* Don't set the AD bit unless DO is on, too */
 111         if (!edns0_do)
 112                 ad = false;
 113
 114         DNS_PACKET_HEADER(p)->id = id;
 115
 116         DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
 117                                                               1  /* qr */,
 118                                                               0  /* opcode */,
 119                                                               0  /* aa */,
 120                                                               tc /* tc */,
 121                                                               1  /* rd */,
 122                                                               1  /* ra */,
 123                                                               ad /* ad */,
 124                                                               0  /* cd */,
 125                                                               rcode));
 126
 127         return 0;
 128 }
 129
 130 static int dns_stub_send(Manager *m, DnsStream *s, DnsPacket *p, DnsPacket *reply) {
 131         int r;
 132
 133         assert(m);
 134         assert(p);
 135         assert(reply);
 136
 137         if (s)
 138                 r = dns_stream_write_packet(s, reply);
 139         else {
 140                 int fd;
 141
 142                 fd = manager_dns_stub_udp_fd(m);
 143                 if (fd < 0)
 144                         return log_debug_errno(fd, "Failed to get reply socket: %m");
 145
 146                 /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
 147                  * is because otherwise the kernel will choose it automatically based on the routing table and will
 148                  * thus pick 127.0.0.1 rather than 127.0.0.53. */
 149
 150                 r = manager_send(m, fd, LOOPBACK_IFINDEX, p->family, &p->sender, p->sender_port, &p->destination, reply);
 151         }
 152         if (r < 0)
 153                 return log_debug_errno(r, "Failed to send reply packet: %m");
 154
 155         return 0;
 156 }
 157
 158 static int dns_stub_send_failure(Manager *m, DnsStream *s, DnsPacket *p, int rcode, bool authenticated) {
 159         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 160         int r;
 161
 162         assert(m);
 163         assert(p);
 164
 165         r = dns_stub_make_reply_packet(&reply, DNS_PACKET_PAYLOAD_SIZE_MAX(p), p->question, NULL, NULL);
 166         if (r < 0)
 167                 return log_debug_errno(r, "Failed to make failure packet: %m");
 168
 169         r = dns_stub_finish_reply_packet(reply, DNS_PACKET_ID(p), rcode, false, !!p->opt, DNS_PACKET_DO(p), authenticated);
 170         if (r < 0)
 171                 return log_debug_errno(r, "Failed to build failure packet: %m");
 172
 173         return dns_stub_send(m, s, p, reply);
 174 }
 175
 176 static void dns_stub_query_complete(DnsQuery *q) {
 177         int r;
 178
 179         assert(q);
 180         assert(q->request_dns_packet);
 181
 182         switch (q->state) {
 183
 184         case DNS_TRANSACTION_SUCCESS: {
 185                 bool truncated;
 186
 187                 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);
 188                 if (r < 0) {
 189                         log_debug_errno(r, "Failed to build reply packet: %m");
 190                         break;
 191                 }
 192
 193                 if (!truncated) {
 194                         r = dns_query_process_cname(q);
 195                         if (r == -ELOOP) {
 196                                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 197                                 break;
 198                         }
 199                         if (r < 0) {
 200                                 log_debug_errno(r, "Failed to process CNAME: %m");
 201                                 break;
 202                         }
 203                         if (r == DNS_QUERY_RESTARTED)
 204                                 return;
 205                 }
 206
 207                 r = dns_stub_finish_reply_packet(
 208                                 q->reply_dns_packet,
 209                                 DNS_PACKET_ID(q->request_dns_packet),
 210                                 q->answer_rcode,
 211                                 truncated,
 212                                 !!q->request_dns_packet->opt,
 213                                 DNS_PACKET_DO(q->request_dns_packet),
 214                                 dns_query_fully_authenticated(q));
 215                 if (r < 0) {
 216                         log_debug_errno(r, "Failed to finish reply packet: %m");
 217                         break;
 218                 }
 219
 220                 (void) dns_stub_send(q->manager, q->request_dns_stream, q->request_dns_packet, q->reply_dns_packet);
 221                 break;
 222         }
 223
 224         case DNS_TRANSACTION_RCODE_FAILURE:
 225                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, q->answer_rcode, dns_query_fully_authenticated(q));
 226                 break;
 227
 228         case DNS_TRANSACTION_NOT_FOUND:
 229                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_NXDOMAIN, dns_query_fully_authenticated(q));
 230                 break;
 231
 232         case DNS_TRANSACTION_TIMEOUT:
 233         case DNS_TRANSACTION_ATTEMPTS_MAX_REACHED:
 234                 /* Propagate a timeout as a no packet, i.e. that the client also gets a timeout */
 235                 break;
 236
 237         case DNS_TRANSACTION_NO_SERVERS:
 238         case DNS_TRANSACTION_INVALID_REPLY:
 239         case DNS_TRANSACTION_ERRNO:
 240         case DNS_TRANSACTION_ABORTED:
 241         case DNS_TRANSACTION_DNSSEC_FAILED:
 242         case DNS_TRANSACTION_NO_TRUST_ANCHOR:
 243         case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
 244         case DNS_TRANSACTION_NETWORK_DOWN:
 245                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
 246                 break;
 247
 248         case DNS_TRANSACTION_NULL:
 249         case DNS_TRANSACTION_PENDING:
 250         case DNS_TRANSACTION_VALIDATING:
 251         default:
 252                 assert_not_reached("Impossible state");
 253         }
 254
 255         dns_query_free(q);
 256 }
 257
 258 static int dns_stub_stream_complete(DnsStream *s, int error) {
 259         assert(s);
 260
 261         log_debug_errno(error, "DNS TCP connection terminated, destroying queries: %m");
 262
 263         for (;;) {
 264                 DnsQuery *q;
 265
 266                 q = set_first(s->queries);
 267                 if (!q)
 268                         break;
 269
 270                 dns_query_free(q);
 271         }
 272
 273         /* This drops the implicit ref we keep around since it was allocated, as incoming stub connections
 274          * should be kept as long as the client wants to. */
 275         dns_stream_unref(s);
 276         return 0;
 277 }
 278
 279 static void dns_stub_process_query(Manager *m, DnsStream *s, DnsPacket *p) {
 280         DnsQuery *q = NULL;
 281         int r;
 282
 283         assert(m);
 284         assert(p);
 285         assert(p->protocol == DNS_PROTOCOL_DNS);
 286
 287         if (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
 288             in_addr_is_localhost(p->family, &p->destination) <= 0) {
 289                 log_error("Got packet on unexpected IP range, refusing.");
 290                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 291                 goto fail;
 292         }
 293
 294         r = dns_packet_extract(p);
 295         if (r < 0) {
 296                 log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
 297                 dns_stub_send_failure(m, s, p, DNS_RCODE_FORMERR, false);
 298                 goto fail;
 299         }
 300
 301         if (!DNS_PACKET_VERSION_SUPPORTED(p)) {
 302                 log_debug("Got EDNS OPT field with unsupported version number.");
 303                 dns_stub_send_failure(m, s, p, DNS_RCODE_BADVERS, false);
 304                 goto fail;
 305         }
 306
 307         if (dns_type_is_obsolete(p->question->keys[0]->type)) {
 308                 log_debug("Got message with obsolete key type, refusing.");
 309                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 310                 goto fail;
 311         }
 312
 313         if (dns_type_is_zone_transer(p->question->keys[0]->type)) {
 314                 log_debug("Got request for zone transfer, refusing.");
 315                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 316                 goto fail;
 317         }
 318
 319         if (!DNS_PACKET_RD(p))  {
 320                 /* If the "rd" bit is off (i.e. recursion was not requested), then refuse operation */
 321                 log_debug("Got request with recursion disabled, refusing.");
 322                 dns_stub_send_failure(m, s, p, DNS_RCODE_REFUSED, false);
 323                 goto fail;
 324         }
 325
 326         if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
 327                 log_debug("Got request with DNSSEC CD bit set, refusing.");
 328                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP, false);
 329                 goto fail;
 330         }
 331
 332         r = dns_query_new(m, &q, p->question, p->question, 0, SD_RESOLVED_PROTOCOLS_ALL|SD_RESOLVED_NO_SEARCH);
 333         if (r < 0) {
 334                 log_error_errno(r, "Failed to generate query object: %m");
 335                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 336                 goto fail;
 337         }
 338
 339         /* Request that the TTL is corrected by the cached time for this lookup, so that we return vaguely useful TTLs */
 340         q->clamp_ttl = true;
 341
 342         q->request_dns_packet = dns_packet_ref(p);
 343         q->request_dns_stream = dns_stream_ref(s); /* make sure the stream stays around until we can send a reply through it */
 344         q->complete = dns_stub_query_complete;
 345
 346         if (s) {
 347                 /* Remember which queries belong to this stream, so that we can cancel them when the stream
 348                  * is disconnected early */
 349
 350                 r = set_ensure_allocated(&s->queries, &trivial_hash_ops);
 351                 if (r < 0) {
 352                         log_oom();
 353                         goto fail;
 354                 }
 355
 356                 if (set_put(s->queries, q) < 0) {
 357                         log_oom();
 358                         goto fail;
 359                 }
 360         }
 361
 362         r = dns_query_go(q);
 363         if (r < 0) {
 364                 log_error_errno(r, "Failed to start query: %m");
 365                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL, false);
 366                 goto fail;
 367         }
 368
 369         log_debug("Processing query...");
 370         return;
 371
 372 fail:
 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         r = socket_bind_to_ifindex(fd, LOOPBACK_IFINDEX);
 425         if (r < 0)
 426                 return r;
 427
 428         if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
 429                 return -errno;
 430
 431         r = sd_event_add_io(m->event, &m->dns_stub_udp_event_source, fd, EPOLLIN, on_dns_stub_packet, m);
 432         if (r < 0)
 433                 return r;
 434
 435         (void) sd_event_source_set_description(m->dns_stub_udp_event_source, "dns-stub-udp");
 436
 437         return m->dns_stub_udp_fd = TAKE_FD(fd);
 438 }
 439
 440 static int on_dns_stub_stream_packet(DnsStream *s) {
 441         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 442
 443         assert(s);
 444
 445         p = dns_stream_take_read_packet(s);
 446         assert(p);
 447
 448         if (dns_packet_validate_query(p) > 0) {
 449                 log_debug("Got DNS stub TCP query packet for id %u", DNS_PACKET_ID(p));
 450
 451                 dns_stub_process_query(s->manager, s, p);
 452         } else
 453                 log_debug("Invalid DNS stub TCP packet, ignoring.");
 454
 455         return 0;
 456 }
 457
 458 static int on_dns_stub_stream(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 459         DnsStream *stream;
 460         Manager *m = userdata;
 461         int cfd, r;
 462
 463         cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
 464         if (cfd < 0) {
 465                 if (IN_SET(errno, EAGAIN, EINTR))
 466                         return 0;
 467
 468                 return -errno;
 469         }
 470
 471         r = dns_stream_new(m, &stream, DNS_STREAM_STUB, DNS_PROTOCOL_DNS, cfd, NULL);
 472         if (r < 0) {
 473                 safe_close(cfd);
 474                 return r;
 475         }
 476
 477         stream->on_packet = on_dns_stub_stream_packet;
 478         stream->complete = dns_stub_stream_complete;
 479
 480         /* We let the reference to the stream dangle here, it will be dropped later by the complete callback. */
 481
 482         return 0;
 483 }
 484
 485 static int manager_dns_stub_tcp_fd(Manager *m) {
 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         r = setsockopt_int(fd, IPPROTO_IP, IP_TTL, true);
 502         if (r < 0)
 503                 return r;
 504
 505         r = setsockopt_int(fd, SOL_SOCKET, SO_REUSEADDR, true);
 506         if (r < 0)
 507                 return r;
 508
 509         r = setsockopt_int(fd, IPPROTO_IP, IP_PKTINFO, true);
 510         if (r < 0)
 511                 return r;
 512
 513         r = setsockopt_int(fd, IPPROTO_IP, IP_RECVTTL, true);
 514         if (r < 0)
 515                 return r;
 516
 517         /* Make sure no traffic from outside the local host can leak to onto this socket */
 518         r = socket_bind_to_ifindex(fd, LOOPBACK_IFINDEX);
 519         if (r < 0)
 520                 return r;
 521
 522         if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
 523                 return -errno;
 524
 525         if (listen(fd, SOMAXCONN) < 0)
 526                 return -errno;
 527
 528         r = sd_event_add_io(m->event, &m->dns_stub_tcp_event_source, fd, EPOLLIN, on_dns_stub_stream, m);
 529         if (r < 0)
 530                 return r;
 531
 532         (void) sd_event_source_set_description(m->dns_stub_tcp_event_source, "dns-stub-tcp");
 533
 534         return m->dns_stub_tcp_fd = TAKE_FD(fd);
 535 }
 536
 537 int manager_dns_stub_start(Manager *m) {
 538         const char *t = "UDP";
 539         int r = 0;
 540
 541         assert(m);
 542
 543         if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_NO)
 544                 log_debug("Not creating stub listener.");
 545         else
 546                 log_debug("Creating stub listener using %s.",
 547                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP ? "UDP" :
 548                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP ? "TCP" :
 549                           "UDP/TCP");
 550
 551         if (IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_UDP))
 552                 r = manager_dns_stub_udp_fd(m);
 553
 554         if (r >= 0 &&
 555             IN_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_YES, DNS_STUB_LISTENER_TCP)) {
 556                 t = "TCP";
 557                 r = manager_dns_stub_tcp_fd(m);
 558         }
 559
 560         if (IN_SET(r, -EADDRINUSE, -EPERM)) {
 561                 if (r == -EADDRINUSE)
 562                         log_warning_errno(r,
 563                                           "Another process is already listening on %s socket 127.0.0.53:53.\n"
 564                                           "Turning off local DNS stub support.", t);
 565                 else
 566                         log_warning_errno(r,
 567                                           "Failed to listen on %s socket 127.0.0.53:53: %m.\n"
 568                                           "Turning off local DNS stub support.", t);
 569                 manager_dns_stub_stop(m);
 570         } else if (r < 0)
 571                 return log_error_errno(r, "Failed to listen on %s socket 127.0.0.53:53: %m", t);
 572
 573         return 0;
 574 }
 575
 576 void manager_dns_stub_stop(Manager *m) {
 577         assert(m);
 578
 579         m->dns_stub_udp_event_source = sd_event_source_unref(m->dns_stub_udp_event_source);
 580         m->dns_stub_tcp_event_source = sd_event_source_unref(m->dns_stub_tcp_event_source);
 581
 582         m->dns_stub_udp_fd = safe_close(m->dns_stub_udp_fd);
 583         m->dns_stub_tcp_fd = safe_close(m->dns_stub_tcp_fd);
 584 }