dns.c

   1 /*
   2  * Copyright (C) 2014 John Crispin <blogic@openwrt.org>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU Lesser General Public License version 2.1
   6  * as published by the Free Software Foundation
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11  * GNU General Public License for more details.
  12  */
  13
  14 #include <sys/types.h>
  15 #include <sys/stat.h>
  16
  17 #include <fcntl.h>
  18 #include <ifaddrs.h>
  19 #include <time.h>
  20 #include <stdio.h>
  21 #include <unistd.h>
  22 #include <sys/types.h>
  23 #include <sys/socket.h>
  24 #include <netinet/in.h>
  25 #include <arpa/inet.h>
  26 #include <arpa/nameser.h>
  27 #include <resolv.h>
  28 #include <stdlib.h>
  29 #include <string.h>
  30
  31 #include <libubox/uloop.h>
  32 #include <libubox/usock.h>
  33 #include <libubox/utils.h>
  34
  35 #include "announce.h"
  36 #include "util.h"
  37 #include "dns.h"
  38 #include "cache.h"
  39 #include "service.h"
  40 #include "interface.h"
  41
  42 static char name_buffer[MAX_NAME_LEN + 1];
  43 static char dns_buffer[MAX_NAME_LEN];
  44 static struct blob_buf ans_buf;
  45
  46 const char*
  47 dns_type_string(uint16_t type)
  48 {
  49         static const struct {
  50                 uint16_t type;
  51                 char str[5];
  52         } type_str[] = {
  53                 { TYPE_A, "A" },
  54                 { TYPE_AAAA, "AAAA" },
  55                 { TYPE_PTR, "PTR" },
  56                 { TYPE_TXT, "TXT" },
  57                 { TYPE_SRV, "SRV" },
  58                 { TYPE_ANY, "ANY" },
  59         };
  60         int i;
  61
  62         for (i = 0; i < ARRAY_SIZE(type_str); i++) {
  63                 if (type == type_str[i].type)
  64                         return type_str[i].str;
  65         }
  66
  67         return "N/A";
  68 }
  69
  70 void
  71 dns_send_question(struct interface *iface, const char *question, int type, int multicast)
  72 {
  73         static struct dns_header h;
  74         static struct dns_question q;
  75         static struct iovec iov[] = {
  76                 {
  77                         .iov_base = &h,
  78                         .iov_len = sizeof(h),
  79                 },
  80                 {
  81                         .iov_base = dns_buffer,
  82                 },
  83                 {
  84                         .iov_base = &q,
  85                         .iov_len = sizeof(q),
  86                 }
  87         };
  88         int len;
  89
  90         h.questions = cpu_to_be16(1);
  91         q.class = cpu_to_be16((multicast ? 0 : CLASS_UNICAST) | 1);
  92         q.type = cpu_to_be16(type);
  93
  94         len = dn_comp(question, (void *) dns_buffer, sizeof(dns_buffer), NULL, NULL);
  95         if (len < 1)
  96                 return;
  97
  98         iov[1].iov_len = len;
  99
 100         DBG(1, "Q <- %s %s\n", dns_type_string(type), question);
 101         if (interface_send_packet(iface, NULL, iov, ARRAY_SIZE(iov)) < 0)
 102                 perror("failed to send question");
 103 }
 104
 105
 106 struct dns_reply {
 107         int type;
 108         struct dns_answer a;
 109         uint16_t rdlength;
 110         uint8_t *rdata;
 111         char *buffer;
 112 };
 113
 114 static int dns_answer_cnt;
 115
 116 void
 117 dns_init_answer(void)
 118 {
 119         dns_answer_cnt = 0;
 120         blob_buf_init(&ans_buf, 0);
 121 }
 122
 123 void
 124 dns_add_answer(int type, const uint8_t *rdata, uint16_t rdlength, int ttl)
 125 {
 126         struct blob_attr *attr;
 127         struct dns_answer *a;
 128
 129         attr = blob_new(&ans_buf, 0, sizeof(*a) + rdlength);
 130         a = blob_data(attr);
 131         a->type = cpu_to_be16(type);
 132         a->class = cpu_to_be16(1);
 133         a->ttl = cpu_to_be32(ttl);
 134         a->rdlength = cpu_to_be16(rdlength);
 135         memcpy(a + 1, rdata, rdlength);
 136
 137         dns_answer_cnt++;
 138 }
 139
 140 void
 141 dns_send_answer(struct interface *iface, struct sockaddr *to, const char *answer)
 142 {
 143         uint8_t buffer[256];
 144         struct blob_attr *attr;
 145         struct dns_header h = { 0 };
 146         struct iovec *iov;
 147         int answer_len, rem;
 148         int n_iov = 0;
 149
 150         if (!dns_answer_cnt)
 151                 return;
 152
 153         h.answers = cpu_to_be16(dns_answer_cnt);
 154         h.flags = cpu_to_be16(0x8400);
 155
 156         iov = alloca(sizeof(struct iovec) * ((dns_answer_cnt * 2) + 1));
 157
 158         iov[n_iov].iov_base = &h;
 159         iov[n_iov].iov_len = sizeof(struct dns_header);
 160         n_iov++;
 161
 162         answer_len = dn_comp(answer, buffer, sizeof(buffer), NULL, NULL);
 163         if (answer_len < 1)
 164                 return;
 165
 166         blob_for_each_attr(attr, ans_buf.head, rem) {
 167                 struct dns_answer *a = blob_data(attr);
 168
 169                 iov[n_iov].iov_base = buffer;
 170                 iov[n_iov].iov_len = answer_len;
 171                 n_iov++;
 172
 173                 iov[n_iov].iov_base = blob_data(attr);
 174                 iov[n_iov].iov_len = blob_len(attr);
 175                 n_iov++;
 176
 177                 DBG(1, "A <- %s %s\n", dns_type_string(be16_to_cpu(a->type)), answer);
 178         }
 179
 180         if (interface_send_packet(iface, to, iov, n_iov) < 0)
 181                 perror("failed to send answer");
 182 }
 183
 184 void
 185 dns_reply_a(struct interface *iface, struct sockaddr *to, int ttl)
 186 {
 187         struct ifaddrs *ifap, *ifa;
 188         struct sockaddr_in *sa;
 189         struct sockaddr_in6 *sa6;
 190
 191         getifaddrs(&ifap);
 192
 193         dns_init_answer();
 194         for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
 195                 if (strcmp(ifa->ifa_name, iface->name))
 196                         continue;
 197                 if (ifa->ifa_addr->sa_family == AF_INET) {
 198                         sa = (struct sockaddr_in *) ifa->ifa_addr;
 199                         dns_add_answer(TYPE_A, (uint8_t *) &sa->sin_addr, 4, ttl);
 200                 }
 201                 if (ifa->ifa_addr->sa_family == AF_INET6) {
 202                         uint8_t ll_prefix[] = {0xfe, 0x80 };
 203                         sa6 = (struct sockaddr_in6 *) ifa->ifa_addr;
 204                         if (!memcmp(&sa6->sin6_addr, &ll_prefix, 2))
 205                                 dns_add_answer(TYPE_AAAA, (uint8_t *) &sa6->sin6_addr, 16, ttl);
 206                 }
 207         }
 208         dns_send_answer(iface, to, mdns_hostname_local);
 209
 210         freeifaddrs(ifap);
 211 }
 212
 213 static int
 214 scan_name(const uint8_t *buffer, int len)
 215 {
 216         int offset = 0;
 217
 218         while (len && (*buffer != '\0')) {
 219                 int l = *buffer;
 220
 221                 if (IS_COMPRESSED(l))
 222                         return offset + 2;
 223
 224                 len -= l + 1;
 225                 offset += l + 1;
 226                 buffer += l + 1;
 227         }
 228
 229         if (!len || !offset || (*buffer != '\0'))
 230                 return -1;
 231
 232         return offset + 1;
 233 }
 234
 235 static struct dns_header*
 236 dns_consume_header(uint8_t **data, int *len)
 237 {
 238         struct dns_header *h = (struct dns_header *) *data;
 239         uint16_t *swap = (uint16_t *) h;
 240         int endianess = 6;
 241
 242         if (*len < sizeof(struct dns_header))
 243                 return NULL;
 244
 245         while (endianess--) {
 246                 *swap = be16_to_cpu(*swap);
 247                 swap++;
 248         }
 249
 250         *len -= sizeof(struct dns_header);
 251         *data += sizeof(struct dns_header);
 252
 253         return h;
 254 }
 255
 256 static struct dns_question*
 257 dns_consume_question(uint8_t **data, int *len)
 258 {
 259         struct dns_question *q = (struct dns_question *) *data;
 260         uint16_t *swap = (uint16_t *) q;
 261         int endianess = 2;
 262
 263         if (*len < sizeof(struct dns_question))
 264                 return NULL;
 265
 266         while (endianess--) {
 267                 *swap = be16_to_cpu(*swap);
 268                 swap++;
 269         }
 270
 271         *len -= sizeof(struct dns_question);
 272         *data += sizeof(struct dns_question);
 273
 274         return q;
 275 }
 276
 277 static struct dns_answer*
 278 dns_consume_answer(uint8_t **data, int *len)
 279 {
 280         struct dns_answer *a = (struct dns_answer *) *data;
 281
 282         if (*len < sizeof(struct dns_answer))
 283                 return NULL;
 284
 285         a->type = be16_to_cpu(a->type);
 286         a->class = be16_to_cpu(a->class);
 287         a->ttl = be32_to_cpu(a->ttl);
 288         a->rdlength = be16_to_cpu(a->rdlength);
 289
 290         *len -= sizeof(struct dns_answer);
 291         *data += sizeof(struct dns_answer);
 292
 293         return a;
 294 }
 295
 296 static char *
 297 dns_consume_name(const uint8_t *base, int blen, uint8_t **data, int *len)
 298 {
 299         int nlen = scan_name(*data, *len);
 300
 301         if (nlen < 1)
 302                 return NULL;
 303
 304         if (dn_expand(base, base + blen, *data, name_buffer, MAX_NAME_LEN) < 0) {
 305                 perror("dns_consume_name/dn_expand");
 306                 return NULL;
 307         }
 308
 309         *len -= nlen;
 310         *data += nlen;
 311
 312         return name_buffer;
 313 }
 314
 315 static int
 316 parse_answer(struct interface *iface, uint8_t *buffer, int len, uint8_t **b, int *rlen, int cache)
 317 {
 318         char *name = dns_consume_name(buffer, len, b, rlen);
 319         struct dns_answer *a;
 320         uint8_t *rdata;
 321
 322         if (!name) {
 323                 fprintf(stderr, "dropping: bad question\n");
 324                 return -1;
 325         }
 326
 327         a = dns_consume_answer(b, rlen);
 328         if (!a) {
 329                 fprintf(stderr, "dropping: bad question\n");
 330                 return -1;
 331         }
 332
 333         if ((a->class & ~CLASS_FLUSH) != CLASS_IN)
 334                 return -1;
 335
 336         rdata = *b;
 337         if (a->rdlength > *rlen) {
 338                 fprintf(stderr, "dropping: bad question\n");
 339                 return -1;
 340         }
 341
 342         *rlen -= a->rdlength;
 343         *b += a->rdlength;
 344
 345         if (cache)
 346                 cache_answer(iface, buffer, len, name, a, rdata, a->class & CLASS_FLUSH);
 347
 348         return 0;
 349 }
 350
 351 static void
 352 parse_question(struct interface *iface, struct sockaddr *from, char *name, struct dns_question *q)
 353 {
 354         struct sockaddr *to;
 355         char *host;
 356
 357         /* TODO: Multicast if more than one quarter of TTL has passed */
 358         if ((q->class & CLASS_UNICAST) && iface->multicast) {
 359                 iface = iface->peer;
 360                 to = from;
 361         } else {
 362                 to = NULL;
 363         }
 364
 365         DBG(1, "Q -> %s %s\n", dns_type_string(q->type), name);
 366
 367         switch (q->type) {
 368         case TYPE_ANY:
 369                 if (!strcmp(name, mdns_hostname_local)) {
 370                         dns_reply_a(iface, to, announce_ttl);
 371                         service_reply(iface, to, NULL, announce_ttl);
 372                 }
 373                 break;
 374
 375         case TYPE_PTR:
 376                 if (!strcmp(name, sdudp)) {
 377                         dns_reply_a(iface, to, announce_ttl);
 378                         service_announce_services(iface, to, announce_ttl);
 379                 } else {
 380                         /* First dot separates instance name from the rest */
 381                         char *dot = strchr(name, '.');
 382                         /* Length of queried instance */
 383                         size_t len = dot ? dot - name : 0;
 384
 385                         /* Make sure it's query for the instance name we use */
 386                         if (len && len == strlen(mdns_hostname) &&
 387                             !strncmp(name, mdns_hostname, len))
 388                                 service_reply(iface, to, dot + 1, announce_ttl);
 389                 }
 390                 break;
 391
 392         case TYPE_AAAA:
 393         case TYPE_A:
 394                 host = strstr(name, ".local");
 395                 if (host)
 396                         *host = '\0';
 397                 if (!strcmp(mdns_hostname, name))
 398                         dns_reply_a(iface, to, announce_ttl);
 399                 break;
 400         };
 401 }
 402
 403 void
 404 dns_handle_packet(struct interface *iface, struct sockaddr *s, uint16_t port, uint8_t *buffer, int len)
 405 {
 406         struct dns_header *h;
 407         uint8_t *b = buffer;
 408         int rlen = len;
 409
 410         h = dns_consume_header(&b, &rlen);
 411         if (!h) {
 412                 fprintf(stderr, "dropping: bad header\n");
 413                 return;
 414         }
 415
 416         if (h->questions && !iface->multicast && port != 5353)
 417                 /* silently drop unicast questions that dont originate from port 5353 */
 418                 return;
 419
 420         while (h->questions-- > 0) {
 421                 char *name = dns_consume_name(buffer, len, &b, &rlen);
 422                 struct dns_question *q;
 423
 424                 if (!name) {
 425                         fprintf(stderr, "dropping: bad name\n");
 426                         return;
 427                 }
 428
 429                 q = dns_consume_question(&b, &rlen);
 430                 if (!q) {
 431                         fprintf(stderr, "dropping: bad question\n");
 432                         return;
 433                 }
 434
 435                 if (!(h->flags & FLAG_RESPONSE))
 436                         parse_question(iface, s, name, q);
 437         }
 438
 439         if (!(h->flags & FLAG_RESPONSE))
 440                 return;
 441
 442         while (h->answers-- > 0)
 443                 if (parse_answer(iface, buffer, len, &b, &rlen, 1))
 444                         return;
 445
 446         while (h->authority-- > 0)
 447                 if (parse_answer(iface, buffer, len, &b, &rlen, 1))
 448                         return;
 449
 450         while (h->additional-- > 0)
 451                 if (parse_answer(iface, buffer, len, &b, &rlen, 1))
 452                         return;
 453
 454 }