src/resolve/resolved-dns-stub.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2016 Lennart Poettering
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18 ***/
  19
  20 #include "fd-util.h"
  21 #include "resolved-dns-stub.h"
  22 #include "socket-util.h"
  23
  24 /* The MTU of the loopback device is 64K on Linux, advertise that as maximum datagram size, but subtract the Ethernet,
  25  * IP and UDP header sizes */
  26 #define ADVERTISE_DATAGRAM_SIZE_MAX (65536U-14U-20U-8U)
  27
  28 static int dns_stub_make_reply_packet(
  29                 uint16_t id,
  30                 int rcode,
  31                 DnsQuestion *q,
  32                 DnsAnswer *answer,
  33                 bool add_opt,   /* add an OPT RR to this packet */
  34                 bool edns0_do,  /* set the EDNS0 DNSSEC OK bit */
  35                 bool ad,        /* set the DNSSEC authenticated data bit */
  36                 DnsPacket **ret) {
  37
  38         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
  39         DnsResourceRecord *rr;
  40         unsigned c = 0;
  41         int r;
  42
  43         /* Note that we don't bother with any additional RRs, as this is stub is for local lookups only, and hence
  44          * roundtrips aren't expensive. */
  45
  46         r = dns_packet_new(&p, DNS_PROTOCOL_DNS, 0);
  47         if (r < 0)
  48                 return r;
  49
  50         /* If the client didn't do EDNS, clamp the rcode to 4 bit */
  51         if (!add_opt && rcode > 0xF)
  52                 rcode = DNS_RCODE_SERVFAIL;
  53
  54         DNS_PACKET_HEADER(p)->id = id;
  55         DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
  56                                                               1 /* qr */,
  57                                                               0 /* opcode */,
  58                                                               0 /* aa */,
  59                                                               0 /* tc */,
  60                                                               1 /* rd */,
  61                                                               1 /* ra */,
  62                                                               ad /* ad */,
  63                                                               0 /* cd */,
  64                                                               rcode));
  65
  66         r = dns_packet_append_question(p, q);
  67         if (r < 0)
  68                 return r;
  69         DNS_PACKET_HEADER(p)->qdcount = htobe16(dns_question_size(q));
  70
  71         DNS_ANSWER_FOREACH(rr, answer) {
  72                 r = dns_question_matches_rr(q, rr, NULL);
  73                 if (r < 0)
  74                         return r;
  75                 if (r > 0)
  76                         goto add;
  77
  78                 r = dns_question_matches_cname_or_dname(q, rr, NULL);
  79                 if (r < 0)
  80                         return r;
  81                 if (r > 0)
  82                         goto add;
  83
  84                 continue;
  85         add:
  86                 r = dns_packet_append_rr(p, rr, NULL, NULL);
  87                 if (r < 0)
  88                         return r;
  89
  90                 c++;
  91         }
  92         DNS_PACKET_HEADER(p)->ancount = htobe16(c);
  93
  94         if (add_opt) {
  95                 r = dns_packet_append_opt(p, ADVERTISE_DATAGRAM_SIZE_MAX, edns0_do, rcode, NULL);
  96                 if (r < 0)
  97                         return r;
  98         }
  99
 100         *ret = p;
 101         p = NULL;
 102
 103         return 0;
 104 }
 105
 106 static void dns_stub_detach_stream(DnsStream *s) {
 107         assert(s);
 108
 109         s->complete = NULL;
 110         s->on_packet = NULL;
 111         s->query = NULL;
 112 }
 113
 114 static int dns_stub_send(Manager *m, DnsStream *s, DnsPacket *p, DnsPacket *reply) {
 115         int r;
 116
 117         assert(m);
 118         assert(p);
 119         assert(reply);
 120
 121         if (s)
 122                 r = dns_stream_write_packet(s, reply);
 123         else {
 124                 int fd;
 125
 126                 /* Truncate the message to the right size */
 127                 if (reply->size > DNS_PACKET_PAYLOAD_SIZE_MAX(p)) {
 128                         dns_packet_truncate(reply, DNS_PACKET_UNICAST_SIZE_MAX);
 129                         DNS_PACKET_HEADER(reply)->flags = htobe16(be16toh(DNS_PACKET_HEADER(reply)->flags) | DNS_PACKET_FLAG_TC);
 130                 }
 131
 132                 fd = manager_dns_stub_udp_fd(m);
 133                 if (fd < 0)
 134                         return log_debug_errno(fd, "Failed to get reply socket: %m");
 135
 136                 /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
 137                  * is because otherwise the kernel will choose it automatically based on the routing table and will
 138                  * thus pick 127.0.0.1 rather than 127.0.0.53. */
 139
 140                 r = manager_send(m, fd, LOOPBACK_IFINDEX, p->family, &p->sender, p->sender_port, &p->destination, reply);
 141         }
 142         if (r < 0)
 143                 return log_debug_errno(r, "Failed to send reply packet: %m");
 144
 145         return 0;
 146 }
 147
 148 static int dns_stub_send_failure(Manager *m, DnsStream *s, DnsPacket *p, int rcode) {
 149         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 150         int r;
 151
 152         assert(m);
 153         assert(p);
 154
 155         r = dns_stub_make_reply_packet(DNS_PACKET_ID(p), rcode, p->question, NULL, !!p->opt, DNS_PACKET_DO(p), false, &reply);
 156         if (r < 0)
 157                 return log_debug_errno(r, "Failed to build failure packet: %m");
 158
 159         return dns_stub_send(m, s, p, reply);
 160 }
 161
 162 static void dns_stub_query_complete(DnsQuery *q) {
 163         int r;
 164
 165         assert(q);
 166         assert(q->request_dns_packet);
 167
 168         switch (q->state) {
 169
 170         case DNS_TRANSACTION_SUCCESS: {
 171                 _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 172
 173                 r = dns_stub_make_reply_packet(
 174                                 DNS_PACKET_ID(q->request_dns_packet),
 175                                 q->answer_rcode,
 176                                 q->question_idna,
 177                                 q->answer,
 178                                 !!q->request_dns_packet->opt,
 179                                 DNS_PACKET_DO(q->request_dns_packet),
 180                                 DNS_PACKET_DO(q->request_dns_packet) && q->answer_authenticated,
 181                                 &reply);
 182                 if (r < 0) {
 183                         log_debug_errno(r, "Failed to build reply packet: %m");
 184                         break;
 185                 }
 186
 187                 (void) dns_stub_send(q->manager, q->request_dns_stream, q->request_dns_packet, reply);
 188                 break;
 189         }
 190
 191         case DNS_TRANSACTION_RCODE_FAILURE:
 192                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, q->answer_rcode);
 193                 break;
 194
 195         case DNS_TRANSACTION_NOT_FOUND:
 196                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_NXDOMAIN);
 197                 break;
 198
 199         case DNS_TRANSACTION_TIMEOUT:
 200         case DNS_TRANSACTION_ATTEMPTS_MAX_REACHED:
 201                 /* Propagate a timeout as a no packet, i.e. that the client also gets a timeout */
 202                 break;
 203
 204         case DNS_TRANSACTION_NO_SERVERS:
 205         case DNS_TRANSACTION_INVALID_REPLY:
 206         case DNS_TRANSACTION_ERRNO:
 207         case DNS_TRANSACTION_ABORTED:
 208         case DNS_TRANSACTION_DNSSEC_FAILED:
 209         case DNS_TRANSACTION_NO_TRUST_ANCHOR:
 210         case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
 211         case DNS_TRANSACTION_NETWORK_DOWN:
 212                 (void) dns_stub_send_failure(q->manager, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL);
 213                 break;
 214
 215         case DNS_TRANSACTION_NULL:
 216         case DNS_TRANSACTION_PENDING:
 217         case DNS_TRANSACTION_VALIDATING:
 218         default:
 219                 assert_not_reached("Impossible state");
 220         }
 221
 222         /* If there's a packet to write set, let's leave the stream around */
 223         if (q->request_dns_stream && DNS_STREAM_QUEUED(q->request_dns_stream)) {
 224
 225                 /* Detach the stream from our query (make it an orphan), but do not drop the reference to it. The
 226                  * default completion action of the stream will drop the reference. */
 227
 228                 dns_stub_detach_stream(q->request_dns_stream);
 229                 q->request_dns_stream = NULL;
 230         }
 231
 232         dns_query_free(q);
 233 }
 234
 235 static int dns_stub_stream_complete(DnsStream *s, int error) {
 236         assert(s);
 237
 238         log_debug_errno(error, "DNS TCP connection terminated, destroying query: %m");
 239
 240         assert(s->query);
 241         dns_query_free(s->query);
 242
 243         return 0;
 244 }
 245
 246 static void dns_stub_process_query(Manager *m, DnsStream *s, DnsPacket *p) {
 247         DnsQuery *q = NULL;
 248         int r;
 249
 250         assert(m);
 251         assert(p);
 252         assert(p->protocol == DNS_PROTOCOL_DNS);
 253
 254         /* Takes ownership of the *s stream object */
 255
 256         if (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
 257             in_addr_is_localhost(p->family, &p->destination) <= 0) {
 258                 log_error("Got packet on unexpected IP range, refusing.");
 259                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL);
 260                 goto fail;
 261         }
 262
 263         r = dns_packet_extract(p);
 264         if (r < 0) {
 265                 log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
 266                 dns_stub_send_failure(m, s, p, DNS_RCODE_FORMERR);
 267                 goto fail;
 268         }
 269
 270         if (!DNS_PACKET_VERSION_SUPPORTED(p)) {
 271                 log_debug("Got EDNS OPT field with unsupported version number.");
 272                 dns_stub_send_failure(m, s, p, DNS_RCODE_BADVERS);
 273                 goto fail;
 274         }
 275
 276         if (dns_type_is_obsolete(p->question->keys[0]->type)) {
 277                 log_debug("Got message with obsolete key type, refusing.");
 278                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP);
 279                 goto fail;
 280         }
 281
 282         if (dns_type_is_zone_transer(p->question->keys[0]->type)) {
 283                 log_debug("Got request for zone transfer, refusing.");
 284                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP);
 285                 goto fail;
 286         }
 287
 288         if (!DNS_PACKET_RD(p))  {
 289                 /* If the "rd" bit is off (i.e. recursion was not requested), then refuse operation */
 290                 log_debug("Got request with recursion disabled, refusing.");
 291                 dns_stub_send_failure(m, s, p, DNS_RCODE_REFUSED);
 292                 goto fail;
 293         }
 294
 295         if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
 296                 log_debug("Got request with DNSSEC CD bit set, refusing.");
 297                 dns_stub_send_failure(m, s, p, DNS_RCODE_NOTIMP);
 298                 goto fail;
 299         }
 300
 301         r = dns_query_new(m, &q, p->question, p->question, 0, SD_RESOLVED_PROTOCOLS_ALL|SD_RESOLVED_NO_SEARCH|SD_RESOLVED_NO_CNAME);
 302         if (r < 0) {
 303                 log_error_errno(r, "Failed to generate query object: %m");
 304                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL);
 305                 goto fail;
 306         }
 307
 308         /* Request that the TTL is corrected by the cached time for this lookup, so that we return vaguely useful TTLs */
 309         q->clamp_ttl = true;
 310
 311         q->request_dns_packet = dns_packet_ref(p);
 312         q->request_dns_stream = dns_stream_ref(s); /* make sure the stream stays around until we can send a reply through it */
 313         q->complete = dns_stub_query_complete;
 314
 315         if (s) {
 316                 s->on_packet = NULL;
 317                 s->complete = dns_stub_stream_complete;
 318                 s->query = q;
 319         }
 320
 321         r = dns_query_go(q);
 322         if (r < 0) {
 323                 log_error_errno(r, "Failed to start query: %m");
 324                 dns_stub_send_failure(m, s, p, DNS_RCODE_SERVFAIL);
 325                 goto fail;
 326         }
 327
 328         log_info("Processing query...");
 329         return;
 330
 331 fail:
 332         if (s && DNS_STREAM_QUEUED(s))
 333                 dns_stub_detach_stream(s);
 334
 335         dns_query_free(q);
 336 }
 337
 338 static int on_dns_stub_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 339         _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
 340         Manager *m = userdata;
 341         int r;
 342
 343         r = manager_recv(m, fd, DNS_PROTOCOL_DNS, &p);
 344         if (r <= 0)
 345                 return r;
 346
 347         if (dns_packet_validate_query(p) > 0) {
 348                 log_debug("Got DNS stub UDP query packet for id %u", DNS_PACKET_ID(p));
 349
 350                 dns_stub_process_query(m, NULL, p);
 351         } else
 352                 log_debug("Invalid DNS stub UDP packet, ignoring.");
 353
 354         return 0;
 355 }
 356
 357 int manager_dns_stub_udp_fd(Manager *m) {
 358         static const int one = 1;
 359
 360         union sockaddr_union sa = {
 361                 .in.sin_family = AF_INET,
 362                 .in.sin_port = htobe16(53),
 363                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 364         };
 365
 366         int r;
 367
 368         if (m->dns_stub_udp_fd >= 0)
 369                 return m->dns_stub_udp_fd;
 370
 371         m->dns_stub_udp_fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 372         if (m->dns_stub_udp_fd < 0)
 373                 return -errno;
 374
 375         r = setsockopt(m->dns_stub_udp_fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
 376         if (r < 0) {
 377                 r = -errno;
 378                 goto fail;
 379         }
 380
 381         r = setsockopt(m->dns_stub_udp_fd, IPPROTO_IP, IP_PKTINFO, &one, sizeof(one));
 382         if (r < 0) {
 383                 r = -errno;
 384                 goto fail;
 385         }
 386
 387         r = setsockopt(m->dns_stub_udp_fd, IPPROTO_IP, IP_RECVTTL, &one, sizeof(one));
 388         if (r < 0) {
 389                 r = -errno;
 390                 goto fail;
 391         }
 392
 393         /* Make sure no traffic from outside the local host can leak to onto this socket */
 394         r = setsockopt(m->dns_stub_udp_fd, SOL_SOCKET, SO_BINDTODEVICE, "lo", 3);
 395         if (r < 0) {
 396                 r = -errno;
 397                 goto fail;
 398         }
 399
 400         r = bind(m->dns_stub_udp_fd, &sa.sa, sizeof(sa.in));
 401         if (r < 0) {
 402                 r = -errno;
 403                 goto fail;
 404         }
 405
 406         r = sd_event_add_io(m->event, &m->dns_stub_udp_event_source, m->dns_stub_udp_fd, EPOLLIN, on_dns_stub_packet, m);
 407         if (r < 0)
 408                 goto fail;
 409
 410         (void) sd_event_source_set_description(m->dns_stub_udp_event_source, "dns-stub-udp");
 411
 412         return m->dns_stub_udp_fd;
 413
 414 fail:
 415         m->dns_stub_udp_fd = safe_close(m->dns_stub_udp_fd);
 416         return r;
 417 }
 418
 419 static int on_dns_stub_stream_packet(DnsStream *s) {
 420         assert(s);
 421         assert(s->read_packet);
 422
 423         if (dns_packet_validate_query(s->read_packet) > 0) {
 424                 log_debug("Got DNS stub TCP query packet for id %u", DNS_PACKET_ID(s->read_packet));
 425
 426                 dns_stub_process_query(s->manager, s, s->read_packet);
 427         } else
 428                 log_debug("Invalid DNS stub TCP packet, ignoring.");
 429
 430         /* Drop the reference to the stream. Either a query was created and added its own reference to the stream now,
 431          * or that didn't happen in which case we want to free the stream */
 432         dns_stream_unref(s);
 433
 434         return 0;
 435 }
 436
 437 static int on_dns_stub_stream(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
 438         DnsStream *stream;
 439         Manager *m = userdata;
 440         int cfd, r;
 441
 442         cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
 443         if (cfd < 0) {
 444                 if (errno == EAGAIN || errno == EINTR)
 445                         return 0;
 446
 447                 return -errno;
 448         }
 449
 450         r = dns_stream_new(m, &stream, DNS_PROTOCOL_DNS, cfd);
 451         if (r < 0) {
 452                 safe_close(cfd);
 453                 return r;
 454         }
 455
 456         stream->on_packet = on_dns_stub_stream_packet;
 457
 458         /* We let the reference to the stream dangling here, it will either be dropped by the default "complete" action
 459          * of the stream, or by our packet callback, or when the manager is shut down. */
 460
 461         return 0;
 462 }
 463
 464 int manager_dns_stub_tcp_fd(Manager *m) {
 465         static const int one = 1;
 466
 467         union sockaddr_union sa = {
 468                 .in.sin_family = AF_INET,
 469                 .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
 470                 .in.sin_port = htobe16(53),
 471         };
 472
 473         int r;
 474
 475         if (m->dns_stub_tcp_fd >= 0)
 476                 return m->dns_stub_tcp_fd;
 477
 478         m->dns_stub_tcp_fd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 479         if (m->dns_stub_tcp_fd < 0)
 480                 return -errno;
 481
 482         r = setsockopt(m->dns_stub_tcp_fd, IPPROTO_IP, IP_TTL, &one, sizeof(one));
 483         if (r < 0) {
 484                 r = -errno;
 485                 goto fail;
 486         }
 487
 488         r = setsockopt(m->dns_stub_tcp_fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
 489         if (r < 0) {
 490                 r = -errno;
 491                 goto fail;
 492         }
 493
 494         r = setsockopt(m->dns_stub_tcp_fd, IPPROTO_IP, IP_PKTINFO, &one, sizeof(one));
 495         if (r < 0) {
 496                 r = -errno;
 497                 goto fail;
 498         }
 499
 500         r = setsockopt(m->dns_stub_tcp_fd, IPPROTO_IP, IP_RECVTTL, &one, sizeof(one));
 501         if (r < 0) {
 502                 r = -errno;
 503                 goto fail;
 504         }
 505
 506         /* Make sure no traffic from outside the local host can leak to onto this socket */
 507         r = setsockopt(m->dns_stub_tcp_fd, SOL_SOCKET, SO_BINDTODEVICE, "lo", 3);
 508         if (r < 0) {
 509                 r = -errno;
 510                 goto fail;
 511         }
 512
 513         r = bind(m->dns_stub_tcp_fd, &sa.sa, sizeof(sa.in));
 514         if (r < 0) {
 515                 r = -errno;
 516                 goto fail;
 517         }
 518
 519         r = listen(m->dns_stub_tcp_fd, SOMAXCONN);
 520         if (r < 0) {
 521                 r = -errno;
 522                 goto fail;
 523         }
 524
 525         r = sd_event_add_io(m->event, &m->dns_stub_tcp_event_source, m->dns_stub_tcp_fd, EPOLLIN, on_dns_stub_stream, m);
 526         if (r < 0)
 527                 goto fail;
 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;
 532
 533 fail:
 534         m->dns_stub_tcp_fd = safe_close(m->dns_stub_tcp_fd);
 535         return r;
 536 }
 537
 538 int manager_dns_stub_start(Manager *m) {
 539         int r;
 540
 541         assert(m);
 542
 543         r = manager_dns_stub_udp_fd(m);
 544         if (r == -EADDRINUSE)
 545                 goto eaddrinuse;
 546         if (r < 0)
 547                 return r;
 548
 549         r = manager_dns_stub_tcp_fd(m);
 550         if (r == -EADDRINUSE)
 551                 goto eaddrinuse;
 552         if (r < 0)
 553                 return r;
 554
 555         return 0;
 556
 557 eaddrinuse:
 558         log_warning("Another process is already listening on 127.0.0.53:53. Turning off local DNS stub support.");
 559         manager_dns_stub_stop(m);
 560
 561         return 0;
 562 }
 563
 564 void manager_dns_stub_stop(Manager *m) {
 565         assert(m);
 566
 567         m->dns_stub_udp_event_source = sd_event_source_unref(m->dns_stub_udp_event_source);
 568         m->dns_stub_tcp_event_source = sd_event_source_unref(m->dns_stub_tcp_event_source);
 569
 570         m->dns_stub_udp_fd = safe_close(m->dns_stub_udp_fd);
 571         m->dns_stub_tcp_fd = safe_close(m->dns_stub_tcp_fd);
 572 }