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