src/resolve/resolved-dns-packet.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2014 Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20  ***/
  21
  22 #include "utf8.h"
  23 #include "util.h"
  24 #include "resolved-dns-domain.h"
  25 #include "resolved-dns-packet.h"
  26
  27 int dns_packet_new(DnsPacket **ret, size_t mtu) {
  28         DnsPacket *p;
  29         size_t a;
  30
  31         assert(ret);
  32
  33         if (mtu <= 0)
  34                 a = DNS_PACKET_SIZE_START;
  35         else
  36                 a = mtu;
  37
  38         if (a < DNS_PACKET_HEADER_SIZE)
  39                 a = DNS_PACKET_HEADER_SIZE;
  40
  41         /* round up to next page size */
  42         a = PAGE_ALIGN(ALIGN(sizeof(DnsPacket)) + a) - ALIGN(sizeof(DnsPacket));
  43
  44         /* make sure we never allocate more than useful */
  45         if (a > DNS_PACKET_SIZE_MAX)
  46                 a = DNS_PACKET_SIZE_MAX;
  47
  48         p = malloc0(ALIGN(sizeof(DnsPacket)) + a);
  49         if (!p)
  50                 return -ENOMEM;
  51
  52         p->size = p->rindex = DNS_PACKET_HEADER_SIZE;
  53         p->allocated = a;
  54         p->n_ref = 1;
  55
  56         *ret = p;
  57
  58         return 0;
  59 }
  60
  61 int dns_packet_new_query(DnsPacket **ret, size_t mtu) {
  62         DnsPacket *p;
  63         DnsPacketHeader *h;
  64         int r;
  65
  66         assert(ret);
  67
  68         r = dns_packet_new(&p, mtu);
  69         if (r < 0)
  70                 return r;
  71
  72         h = DNS_PACKET_HEADER(p);
  73         h->flags = htobe16(DNS_PACKET_MAKE_FLAGS(0, 0, 0, 0, 1, 0, 0, 0, 0));
  74
  75         *ret = p;
  76         return 0;
  77 }
  78
  79 DnsPacket *dns_packet_ref(DnsPacket *p) {
  80
  81         if (!p)
  82                 return NULL;
  83
  84         assert(p->n_ref > 0);
  85         p->n_ref++;
  86         return p;
  87 }
  88
  89 static void dns_packet_free(DnsPacket *p) {
  90         char *s;
  91
  92         assert(p);
  93
  94         if (p->rrs)
  95                 dns_resource_record_freev(p->rrs, DNS_PACKET_RRCOUNT(p));
  96
  97         while ((s = hashmap_steal_first_key(p->names)))
  98                 free(s);
  99         hashmap_free(p->names);
 100
 101         free(p->data);
 102         free(p);
 103 }
 104
 105 DnsPacket *dns_packet_unref(DnsPacket *p) {
 106         if (!p)
 107                 return NULL;
 108
 109         assert(p->n_ref > 0);
 110
 111         if (p->n_ref == 1)
 112                 dns_packet_free(p);
 113         else
 114                 p->n_ref--;
 115
 116         return NULL;
 117 }
 118
 119 int dns_packet_validate(DnsPacket *p) {
 120         assert(p);
 121
 122         if (p->size < DNS_PACKET_HEADER_SIZE)
 123                 return -EBADMSG;
 124
 125         if (p->size > DNS_PACKET_SIZE_MAX)
 126                 return -EBADMSG;
 127
 128         return 0;
 129 }
 130
 131 int dns_packet_validate_reply(DnsPacket *p) {
 132         int r;
 133
 134         assert(p);
 135
 136         r = dns_packet_validate(p);
 137         if (r < 0)
 138                 return r;
 139
 140         if (DNS_PACKET_QR(p) == 0)
 141                 return -EBADMSG;
 142
 143         if (DNS_PACKET_OPCODE(p) != 0)
 144                 return -EBADMSG;
 145
 146         return 0;
 147 }
 148
 149 static int dns_packet_extend(DnsPacket *p, size_t add, void **ret, size_t *start) {
 150         assert(p);
 151
 152         if (p->size + add > p->allocated) {
 153                 size_t a;
 154
 155                 a = PAGE_ALIGN((p->size + add) * 2);
 156                 if (a > DNS_PACKET_SIZE_MAX)
 157                         a = DNS_PACKET_SIZE_MAX;
 158
 159                 if (p->size + add > a)
 160                         return -EMSGSIZE;
 161
 162                 if (p->data) {
 163                         void *d;
 164
 165                         d = realloc(p->data, a);
 166                         if (!d)
 167                                 return -ENOMEM;
 168
 169                         p->data = d;
 170                 } else {
 171                         p->data = malloc(a);
 172                         if (!p->data)
 173                                 return -ENOMEM;
 174
 175                         memcpy(p->data, (uint8_t*) p + ALIGN(sizeof(DnsPacket)), p->size);
 176                         memzero((uint8_t*) p->data + p->size, a - p->size);
 177                 }
 178
 179                 p->allocated = a;
 180         }
 181
 182         if (start)
 183                 *start = p->size;
 184
 185         if (ret)
 186                 *ret = (uint8_t*) DNS_PACKET_DATA(p) + p->size;
 187
 188         p->size += add;
 189         return 0;
 190 }
 191
 192 static void dns_packet_truncate(DnsPacket *p, size_t sz) {
 193         Iterator i;
 194         char *s;
 195         void *n;
 196
 197         assert(p);
 198
 199         if (p->size <= sz)
 200                 return;
 201
 202         HASHMAP_FOREACH_KEY(s, n, p->names, i) {
 203
 204                 if (PTR_TO_SIZE(n) < sz)
 205                         continue;
 206
 207                 hashmap_remove(p->names, s);
 208                 free(s);
 209         }
 210
 211         p->size = sz;
 212 }
 213
 214 int dns_packet_append_uint8(DnsPacket *p, uint8_t v, size_t *start) {
 215         void *d;
 216         int r;
 217
 218         assert(p);
 219
 220         r = dns_packet_extend(p, sizeof(uint8_t), &d, start);
 221         if (r < 0)
 222                 return r;
 223
 224         ((uint8_t*) d)[0] = v;
 225
 226         return 0;
 227 }
 228
 229 int dns_packet_append_uint16(DnsPacket *p, uint16_t v, size_t *start) {
 230         void *d;
 231         int r;
 232
 233         assert(p);
 234
 235         r = dns_packet_extend(p, sizeof(uint16_t), &d, start);
 236         if (r < 0)
 237                 return r;
 238
 239         ((uint8_t*) d)[0] = (uint8_t) (v >> 8);
 240         ((uint8_t*) d)[1] = (uint8_t) (v & 255);
 241
 242         return 0;
 243 }
 244
 245 int dns_packet_append_string(DnsPacket *p, const char *s, size_t *start) {
 246         void *d;
 247         size_t l;
 248         int r;
 249
 250         assert(p);
 251         assert(s);
 252
 253         l = strlen(s);
 254         if (l > 255)
 255                 return -E2BIG;
 256
 257         r = dns_packet_extend(p, 1 + l, &d, start);
 258         if (r < 0)
 259                 return r;
 260
 261         ((uint8_t*) d)[0] = (uint8_t) l;
 262         memcpy(((uint8_t*) d) + 1, s, l);
 263
 264         return 0;
 265 }
 266
 267 int dns_packet_append_label(DnsPacket *p, const char *d, size_t l, size_t *start) {
 268         void *w;
 269         int r;
 270
 271         assert(p);
 272         assert(d);
 273
 274         if (l > DNS_LABEL_MAX)
 275                 return -E2BIG;
 276
 277         r = dns_packet_extend(p, 1 + l, &w, start);
 278         if (r < 0)
 279                 return r;
 280
 281         ((uint8_t*) w)[0] = (uint8_t) l;
 282         memcpy(((uint8_t*) w) + 1, d, l);
 283
 284         return 0;
 285 }
 286
 287 int dns_packet_append_name(DnsPacket *p, const char *name, size_t *start) {
 288         size_t saved_size;
 289         int r;
 290
 291         assert(p);
 292         assert(name);
 293
 294         saved_size = p->size;
 295
 296         while (*name) {
 297                 _cleanup_free_ char *s = NULL;
 298                 char label[DNS_LABEL_MAX];
 299                 size_t n;
 300
 301                 n = PTR_TO_SIZE(hashmap_get(p->names, name));
 302                 if (n > 0) {
 303                         assert(n < p->size);
 304
 305                         if (n < 0x4000) {
 306                                 r = dns_packet_append_uint16(p, 0xC000 | n, NULL);
 307                                 if (r < 0)
 308                                         goto fail;
 309
 310                                 goto done;
 311                         }
 312                 }
 313
 314                 s = strdup(name);
 315                 if (!s) {
 316                         r = -ENOMEM;
 317                         goto fail;
 318                 }
 319
 320                 r = dns_label_unescape(&name, label, sizeof(label));
 321                 if (r < 0)
 322                         goto fail;
 323
 324                 r = dns_packet_append_label(p, label, r, &n);
 325                 if (r < 0)
 326                         goto fail;
 327
 328                 r = hashmap_ensure_allocated(&p->names, dns_name_hash_func, dns_name_compare_func);
 329                 if (r < 0)
 330                         goto fail;
 331
 332                 r = hashmap_put(p->names, s, SIZE_TO_PTR(n));
 333                 if (r < 0)
 334                         goto fail;
 335
 336                 s = NULL;
 337         }
 338
 339         r = dns_packet_append_uint8(p, 0, NULL);
 340         if (r < 0)
 341                 return r;
 342
 343 done:
 344         if (start)
 345                 *start = saved_size;
 346
 347         return 0;
 348
 349 fail:
 350         dns_packet_truncate(p, saved_size);
 351         return r;
 352 }
 353
 354 int dns_packet_append_key(DnsPacket *p, const DnsResourceKey *k, size_t *start) {
 355         size_t saved_size;
 356         int r;
 357
 358         assert(p);
 359         assert(k);
 360
 361         saved_size = p->size;
 362
 363         r = dns_packet_append_name(p, k->name, NULL);
 364         if (r < 0)
 365                 goto fail;
 366
 367         r = dns_packet_append_uint16(p, k->type, NULL);
 368         if (r < 0)
 369                 goto fail;
 370
 371         r = dns_packet_append_uint16(p, k->class, NULL);
 372         if (r < 0)
 373                 goto fail;
 374
 375         if (start)
 376                 *start = saved_size;
 377
 378         return 0;
 379
 380 fail:
 381         dns_packet_truncate(p, saved_size);
 382         return r;
 383 }
 384
 385 int dns_packet_read(DnsPacket *p, size_t sz, const void **ret, size_t *start) {
 386         assert(p);
 387
 388         if (p->rindex + sz > p->size)
 389                 return -EMSGSIZE;
 390
 391         if (ret)
 392                 *ret = (uint8_t*) DNS_PACKET_DATA(p) + p->rindex;
 393
 394         if (start)
 395                 *start = p->rindex;
 396
 397         p->rindex += sz;
 398         return 0;
 399 }
 400
 401 void dns_packet_rewind(DnsPacket *p, size_t idx) {
 402         assert(p);
 403         assert(idx <= p->size);
 404         assert(idx >= DNS_PACKET_HEADER_SIZE);
 405
 406         p->rindex = idx;
 407 }
 408
 409 int dns_packet_read_uint8(DnsPacket *p, uint8_t *ret, size_t *start) {
 410         const void *d;
 411         int r;
 412
 413         assert(p);
 414
 415         r = dns_packet_read(p, sizeof(uint8_t), &d, start);
 416         if (r < 0)
 417                 return r;
 418
 419         *ret = ((uint8_t*) d)[0];
 420         return 0;
 421 }
 422
 423 int dns_packet_read_uint16(DnsPacket *p, uint16_t *ret, size_t *start) {
 424         const void *d;
 425         int r;
 426
 427         assert(p);
 428
 429         r = dns_packet_read(p, sizeof(uint16_t), &d, start);
 430         if (r < 0)
 431                 return r;
 432
 433         *ret = (((uint16_t) ((uint8_t*) d)[0]) << 8) |
 434                 ((uint16_t) ((uint8_t*) d)[1]);
 435         return 0;
 436 }
 437
 438 int dns_packet_read_uint32(DnsPacket *p, uint32_t *ret, size_t *start) {
 439         const void *d;
 440         int r;
 441
 442         assert(p);
 443
 444         r = dns_packet_read(p, sizeof(uint32_t), &d, start);
 445         if (r < 0)
 446                 return r;
 447
 448         *ret = (((uint32_t) ((uint8_t*) d)[0]) << 24) |
 449                (((uint32_t) ((uint8_t*) d)[1]) << 16) |
 450                (((uint32_t) ((uint8_t*) d)[2]) << 8) |
 451                 ((uint32_t) ((uint8_t*) d)[3]);
 452
 453         return 0;
 454 }
 455
 456 int dns_packet_read_string(DnsPacket *p, char **ret, size_t *start) {
 457         size_t saved_rindex;
 458         const void *d;
 459         char *t;
 460         uint8_t c;
 461         int r;
 462
 463         assert(p);
 464
 465         saved_rindex = p->rindex;
 466
 467         r = dns_packet_read_uint8(p, &c, NULL);
 468         if (r < 0)
 469                 goto fail;
 470
 471         r = dns_packet_read(p, c, &d, NULL);
 472         if (r < 0)
 473                 goto fail;
 474
 475         if (memchr(d, 0, c)) {
 476                 r = -EBADMSG;
 477                 goto fail;
 478         }
 479
 480         t = strndup(d, c);
 481         if (!t) {
 482                 r = -ENOMEM;
 483                 goto fail;
 484         }
 485
 486         if (!utf8_is_valid(t)) {
 487                 free(t);
 488                 r = -EBADMSG;
 489                 goto fail;
 490         }
 491
 492         *ret = t;
 493
 494         if (start)
 495                 *start = saved_rindex;
 496
 497         return 0;
 498
 499 fail:
 500         dns_packet_rewind(p, saved_rindex);
 501         return r;
 502 }
 503
 504 int dns_packet_read_name(DnsPacket *p, char **_ret, size_t *start) {
 505         size_t saved_rindex, after_rindex = 0;
 506         _cleanup_free_ char *ret = NULL;
 507         size_t n = 0, allocated = 0;
 508         bool first = true;
 509         int r;
 510
 511         assert(p);
 512         assert(_ret);
 513
 514         saved_rindex = p->rindex;
 515
 516         for (;;) {
 517                 uint8_t c, d;
 518
 519                 r = dns_packet_read_uint8(p, &c, NULL);
 520                 if (r < 0)
 521                         goto fail;
 522
 523                 if (c == 0)
 524                         /* End of name */
 525                         break;
 526                 else if (c <= 63) {
 527                         _cleanup_free_ char *t = NULL;
 528                         const char *label;
 529
 530                         /* Literal label */
 531                         r = dns_packet_read(p, c, (const void**) &label, NULL);
 532                         if (r < 0)
 533                                 goto fail;
 534
 535                         r = dns_label_escape(label, c, &t);
 536                         if (r < 0)
 537                                 goto fail;
 538
 539                         if (!GREEDY_REALLOC(ret, allocated, n + !first + strlen(t) + 1)) {
 540                                 r = -ENOMEM;
 541                                 goto fail;
 542                         }
 543
 544                         if (!first)
 545                                 ret[n++] = '.';
 546                         else
 547                                 first = false;
 548
 549                         memcpy(ret + n, t, c);
 550                         n += r;
 551                         continue;
 552                 } else if ((c & 0xc0) == 0xc0) {
 553                         uint16_t ptr;
 554
 555                         /* Pointer */
 556                         r = dns_packet_read_uint8(p, &d, NULL);
 557                         if (r < 0)
 558                                 goto fail;
 559
 560                         ptr = (uint16_t) (c & ~0xc0) << 8 | (uint16_t) d;
 561                         if (ptr < DNS_PACKET_HEADER_SIZE || ptr >= saved_rindex) {
 562                                 r = -EBADMSG;
 563                                 goto fail;
 564                         }
 565
 566                         if (after_rindex == 0)
 567                                 after_rindex = p->rindex;
 568
 569                         p->rindex = ptr;
 570                 } else
 571                         goto fail;
 572         }
 573
 574         if (!GREEDY_REALLOC(ret, allocated, n + 1)) {
 575                 r = -ENOMEM;
 576                 goto fail;
 577         }
 578
 579         ret[n] = 0;
 580
 581         if (after_rindex != 0)
 582                 p->rindex= after_rindex;
 583
 584         *_ret = ret;
 585         ret = NULL;
 586
 587         if (start)
 588                 *start = saved_rindex;
 589
 590         return 0;
 591
 592 fail:
 593         dns_packet_rewind(p, saved_rindex);
 594         return r;
 595 }
 596
 597 int dns_packet_read_key(DnsPacket *p, DnsResourceKey *ret, size_t *start) {
 598         _cleanup_(dns_resource_key_free) DnsResourceKey k = {};
 599         size_t saved_rindex;
 600         int r;
 601
 602         assert(p);
 603         assert(ret);
 604
 605         saved_rindex = p->rindex;
 606
 607         r = dns_packet_read_name(p, &k.name, NULL);
 608         if (r < 0)
 609                 goto fail;
 610
 611         r = dns_packet_read_uint16(p, &k.type, NULL);
 612         if (r < 0)
 613                 goto fail;
 614
 615         r = dns_packet_read_uint16(p, &k.class, NULL);
 616         if (r < 0)
 617                 goto fail;
 618
 619         *ret = k;
 620         zero(k);
 621
 622         if (start)
 623                 *start = saved_rindex;
 624
 625         return 0;
 626 fail:
 627         dns_packet_rewind(p, saved_rindex);
 628         return r;
 629 }
 630
 631 int dns_packet_read_rr(DnsPacket *p, DnsResourceRecord **ret, size_t *start) {
 632         _cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr;
 633         size_t saved_rindex, offset;
 634         uint16_t rdlength;
 635         const void *d;
 636         int r;
 637
 638         assert(p);
 639         assert(ret);
 640
 641         rr = dns_resource_record_new();
 642         if (!rr)
 643                 return -ENOMEM;
 644
 645         saved_rindex = p->rindex;
 646
 647         r = dns_packet_read_key(p, &rr->key, NULL);
 648         if (r < 0)
 649                 goto fail;
 650
 651         r = dns_packet_read_uint32(p, &rr->ttl, NULL);
 652         if (r < 0)
 653                 goto fail;
 654
 655         r = dns_packet_read_uint16(p, &rdlength, NULL);
 656         if (r < 0)
 657                 goto fail;
 658
 659         if (p->rindex + rdlength > p->size) {
 660                 r = -EBADMSG;
 661                 goto fail;
 662         }
 663
 664         offset = p->rindex;
 665
 666         switch (rr->key.type) {
 667
 668         case DNS_TYPE_PTR:
 669         case DNS_TYPE_NS:
 670         case DNS_TYPE_CNAME:
 671                 r = dns_packet_read_name(p, &rr->ptr.name, NULL);
 672                 break;
 673
 674         case DNS_TYPE_HINFO:
 675                 r = dns_packet_read_string(p, &rr->hinfo.cpu, NULL);
 676                 if (r < 0)
 677                         goto fail;
 678
 679                 r = dns_packet_read_string(p, &rr->hinfo.os, NULL);
 680                 break;
 681
 682         case DNS_TYPE_A:
 683                 r = dns_packet_read(p, sizeof(struct in_addr), &d, NULL);
 684                 if (r < 0)
 685                         goto fail;
 686
 687                 memcpy(&rr->a.in_addr, d, sizeof(struct in_addr));
 688                 break;
 689
 690         case DNS_TYPE_AAAA:
 691                 r = dns_packet_read(p, sizeof(struct in6_addr), &d, NULL);
 692                 if (r < 0)
 693                         goto fail;
 694
 695                 memcpy(&rr->aaaa.in6_addr, d, sizeof(struct in6_addr));
 696                 break;
 697
 698         default:
 699                 r = dns_packet_read(p, rdlength, &d, NULL);
 700                 if (r < 0)
 701                         goto fail;
 702
 703                 rr->generic.data = memdup(d, rdlength);
 704                 if (!rr->generic.data) {
 705                         r = -ENOMEM;
 706                         goto fail;
 707                 }
 708
 709                 rr->generic.size = rdlength;
 710                 break;
 711         }
 712         if (r < 0)
 713                 goto fail;
 714         if (p->rindex != offset + rdlength) {
 715                 r = -EBADMSG;
 716                 goto fail;
 717         }
 718
 719         *ret = rr;
 720         rr = NULL;
 721
 722         if (start)
 723                 *start = saved_rindex;
 724
 725         return 0;
 726 fail:
 727         dns_packet_rewind(p, saved_rindex);
 728         return r;
 729 }
 730
 731 int dns_packet_skip_question(DnsPacket *p) {
 732         unsigned i, n;
 733         int r;
 734
 735         assert(p);
 736
 737         dns_packet_rewind(p, DNS_PACKET_HEADER_SIZE);
 738
 739         n = DNS_PACKET_QDCOUNT(p);
 740         for (i = 0; i < n; i++) {
 741                 _cleanup_(dns_resource_key_free) DnsResourceKey key = {};
 742
 743                 r = dns_packet_read_key(p, &key, NULL);
 744                 if (r < 0)
 745                         return r;
 746         }
 747
 748         return 0;
 749 }
 750
 751 int dns_packet_extract_rrs(DnsPacket *p) {
 752         DnsResourceRecord **rrs = NULL;
 753         size_t saved_rindex;
 754         unsigned n, added = 0;
 755         int r;
 756
 757         if (p->rrs)
 758                 return (int) DNS_PACKET_RRCOUNT(p);
 759
 760         saved_rindex = p->rindex;
 761
 762         r = dns_packet_skip_question(p);
 763         if (r < 0)
 764                 goto finish;
 765
 766         n = DNS_PACKET_RRCOUNT(p);
 767         if (n <= 0) {
 768                 r = 0;
 769                 goto finish;
 770         }
 771
 772         rrs = new0(DnsResourceRecord*, n);
 773         if (!rrs) {
 774                 r = -ENOMEM;
 775                 goto finish;
 776         }
 777
 778         for (added = 0; added < n; added++) {
 779                 r = dns_packet_read_rr(p, &rrs[added], NULL);
 780                 if (r < 0) {
 781                         dns_resource_record_freev(rrs, added);
 782                         goto finish;
 783                 }
 784         }
 785
 786         p->rrs = rrs;
 787         r = (int) n;
 788
 789 finish:
 790         p->rindex = saved_rindex;
 791         return r;
 792 }
 793
 794 static const char* const dns_rcode_table[_DNS_RCODE_MAX_DEFINED] = {
 795         [DNS_RCODE_SUCCESS] = "SUCCESS",
 796         [DNS_RCODE_FORMERR] = "FORMERR",
 797         [DNS_RCODE_SERVFAIL] = "SERVFAIL",
 798         [DNS_RCODE_NXDOMAIN] = "NXDOMAIN",
 799         [DNS_RCODE_NOTIMP] = "NOTIMP",
 800         [DNS_RCODE_REFUSED] = "REFUSED",
 801         [DNS_RCODE_YXDOMAIN] = "YXDOMAIN",
 802         [DNS_RCODE_YXRRSET] = "YRRSET",
 803         [DNS_RCODE_NXRRSET] = "NXRRSET",
 804         [DNS_RCODE_NOTAUTH] = "NOTAUTH",
 805         [DNS_RCODE_NOTZONE] = "NOTZONE",
 806         [DNS_RCODE_BADVERS] = "BADVERS",
 807         [DNS_RCODE_BADKEY] = "BADKEY",
 808         [DNS_RCODE_BADTIME] = "BADTIME",
 809         [DNS_RCODE_BADMODE] = "BADMODE",
 810         [DNS_RCODE_BADNAME] = "BADNAME",
 811         [DNS_RCODE_BADALG] = "BADALG",
 812         [DNS_RCODE_BADTRUNC] = "BADTRUNC",
 813 };
 814 DEFINE_STRING_TABLE_LOOKUP(dns_rcode, int);