src/resolve/resolved-dns-stub.c

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