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Sources/mdnsd/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, struct sockaddr *to,
 72                   const char *question, int type, int multicast)
 73 {
 74         static struct dns_header h;
 75         static struct dns_question q;
 76         static struct iovec iov[] = {
 77                 {
 78                         .iov_base = &h,
 79                         .iov_len = sizeof(h),
 80                 },
 81                 {
 82                         .iov_base = dns_buffer,
 83                 },
 84                 {
 85                         .iov_base = &q,
 86                         .iov_len = sizeof(q),
 87                 }
 88         };
 89         int len;
 90 
 91         h.questions = cpu_to_be16(1);
 92         q.class = cpu_to_be16((multicast ? 0 : CLASS_UNICAST) | 1);
 93         q.type = cpu_to_be16(type);
 94 
 95         len = dn_comp(question, (void *) dns_buffer, sizeof(dns_buffer), NULL, NULL);
 96         if (len < 1)
 97                 return;
 98 
 99         iov[1].iov_len = len;
100 
101         DBG(1, "Q <- %s %s\n", dns_type_string(type), question);
102         if (interface_send_packet(iface, to, iov, ARRAY_SIZE(iov)) < 0)
103                 perror("failed to send question");
104 }
105 
106 
107 struct dns_reply {
108         int type;
109         struct dns_answer a;
110         uint16_t rdlength;
111         uint8_t *rdata;
112         char *buffer;
113 };
114 
115 static int dns_answer_cnt;
116 
117 void
118 dns_init_answer(void)
119 {
120         dns_answer_cnt = 0;
121         blob_buf_init(&ans_buf, 0);
122 }
123 
124 void
125 dns_add_answer(int type, const uint8_t *rdata, uint16_t rdlength, int ttl)
126 {
127         struct blob_attr *attr;
128         struct dns_answer *a;
129 
130         attr = blob_new(&ans_buf, 0, sizeof(*a) + rdlength);
131         a = blob_data(attr);
132         a->type = cpu_to_be16(type);
133         a->class = cpu_to_be16(1);
134         a->ttl = cpu_to_be32(ttl);
135         a->rdlength = cpu_to_be16(rdlength);
136         memcpy(a + 1, rdata, rdlength);
137 
138         dns_answer_cnt++;
139 }
140 
141 void
142 dns_send_answer(struct interface *iface, struct sockaddr *to, const char *answer)
143 {
144         uint8_t buffer[256];
145         struct blob_attr *attr;
146         struct dns_header h = { 0 };
147         struct iovec *iov;
148         int answer_len, rem;
149         int n_iov = 0;
150 
151         if (!dns_answer_cnt)
152                 return;
153 
154         h.answers = cpu_to_be16(dns_answer_cnt);
155         h.flags = cpu_to_be16(0x8400);
156 
157         iov = alloca(sizeof(struct iovec) * ((dns_answer_cnt * 2) + 1));
158 
159         iov[n_iov].iov_base = &h;
160         iov[n_iov].iov_len = sizeof(struct dns_header);
161         n_iov++;
162 
163         answer_len = dn_comp(answer, buffer, sizeof(buffer), NULL, NULL);
164         if (answer_len < 1)
165                 return;
166 
167         blob_for_each_attr(attr, ans_buf.head, rem) {
168                 struct dns_answer *a = blob_data(attr);
169 
170                 iov[n_iov].iov_base = buffer;
171                 iov[n_iov].iov_len = answer_len;
172                 n_iov++;
173 
174                 iov[n_iov].iov_base = blob_data(attr);
175                 iov[n_iov].iov_len = blob_len(attr);
176                 n_iov++;
177 
178                 DBG(1, "A <- %s %s\n", dns_type_string(be16_to_cpu(a->type)), answer);
179         }
180 
181         if (interface_send_packet(iface, to, iov, n_iov) < 0)
182                 perror("failed to send answer");
183 }
184 
185 void
186 dns_reply_a(struct interface *iface, struct sockaddr *to, int ttl)
187 {
188         struct ifaddrs *ifap, *ifa;
189         struct sockaddr_in *sa;
190         struct sockaddr_in6 *sa6;
191 
192         getifaddrs(&ifap);
193 
194         dns_init_answer();
195         for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
196                 if (strcmp(ifa->ifa_name, iface->name))
197                         continue;
198                 if (ifa->ifa_addr->sa_family == AF_INET) {
199                         sa = (struct sockaddr_in *) ifa->ifa_addr;
200                         dns_add_answer(TYPE_A, (uint8_t *) &sa->sin_addr, 4, ttl);
201                 }
202                 if (ifa->ifa_addr->sa_family == AF_INET6) {
203                         uint8_t ll_prefix[] = {0xfe, 0x80 };
204                         sa6 = (struct sockaddr_in6 *) ifa->ifa_addr;
205                         if (!memcmp(&sa6->sin6_addr, &ll_prefix, 2))
206                                 dns_add_answer(TYPE_AAAA, (uint8_t *) &sa6->sin6_addr, 16, ttl);
207                 }
208         }
209         dns_send_answer(iface, to, mdns_hostname_local);
210 
211         freeifaddrs(ifap);
212 }
213 
214 static int
215 scan_name(const uint8_t *buffer, int len)
216 {
217         int offset = 0;
218 
219         while (len && (*buffer != '\0')) {
220                 int l = *buffer;
221 
222                 if (IS_COMPRESSED(l))
223                         return offset + 2;
224 
225                 len -= l + 1;
226                 offset += l + 1;
227                 buffer += l + 1;
228         }
229 
230         if (!len || !offset || (*buffer != '\0'))
231                 return -1;
232 
233         return offset + 1;
234 }
235 
236 static struct dns_header*
237 dns_consume_header(uint8_t **data, int *len)
238 {
239         struct dns_header *h = (struct dns_header *) *data;
240 
241         if (*len < sizeof(struct dns_header))
242                 return NULL;
243 
244         h->id = be16_to_cpu(h->id);
245         h->flags = be16_to_cpu(h->flags);
246         h->questions = be16_to_cpu(h->questions);
247         h->answers = be16_to_cpu(h->answers);
248         h->authority = be16_to_cpu(h->authority);
249         h->additional = be16_to_cpu(h->additional);
250 
251         *len -= sizeof(struct dns_header);
252         *data += sizeof(struct dns_header);
253 
254         return h;
255 }
256 
257 static struct dns_question*
258 dns_consume_question(uint8_t **data, int *len)
259 {
260         struct dns_question *q = (struct dns_question *) *data;
261 
262         if (*len < sizeof(struct dns_question))
263                 return NULL;
264 
265         q->type = be16_to_cpu(q->type);
266         q->class = be16_to_cpu(q->class);
267 
268         *len -= sizeof(struct dns_question);
269         *data += sizeof(struct dns_question);
270 
271         return q;
272 }
273 
274 static struct dns_answer*
275 dns_consume_answer(uint8_t **data, int *len)
276 {
277         struct dns_answer *a = (struct dns_answer *) *data;
278 
279         if (*len < sizeof(struct dns_answer))
280                 return NULL;
281 
282         a->type = be16_to_cpu(a->type);
283         a->class = be16_to_cpu(a->class);
284         a->ttl = be32_to_cpu(a->ttl);
285         a->rdlength = be16_to_cpu(a->rdlength);
286 
287         *len -= sizeof(struct dns_answer);
288         *data += sizeof(struct dns_answer);
289 
290         return a;
291 }
292 
293 static char *
294 dns_consume_name(const uint8_t *base, int blen, uint8_t **data, int *len)
295 {
296         int nlen = scan_name(*data, *len);
297 
298         if (nlen < 1)
299                 return NULL;
300 
301         if (dn_expand(base, base + blen, *data, name_buffer, MAX_NAME_LEN) < 0) {
302                 perror("dns_consume_name/dn_expand");
303                 return NULL;
304         }
305 
306         *len -= nlen;
307         *data += nlen;
308 
309         return name_buffer;
310 }
311 
312 static int parse_answer(struct interface *iface, struct sockaddr *from,
313                         uint8_t *buffer, int len, uint8_t **b, int *rlen,
314                         int cache)
315 {
316         char *name = dns_consume_name(buffer, len, b, rlen);
317         struct dns_answer *a;
318         uint8_t *rdata;
319 
320         if (!name) {
321                 fprintf(stderr, "dropping: bad question\n");
322                 return -1;
323         }
324 
325         a = dns_consume_answer(b, rlen);
326         if (!a) {
327                 fprintf(stderr, "dropping: bad question\n");
328                 return -1;
329         }
330 
331         if ((a->class & ~CLASS_FLUSH) != CLASS_IN)
332                 return -1;
333 
334         rdata = *b;
335         if (a->rdlength > *rlen) {
336                 fprintf(stderr, "dropping: bad question\n");
337                 return -1;
338         }
339 
340         *rlen -= a->rdlength;
341         *b += a->rdlength;
342 
343         if (cache)
344                 cache_answer(iface, from, buffer, len, name, a, rdata, a->class & CLASS_FLUSH);
345 
346         return 0;
347 }
348 
349 static void
350 parse_question(struct interface *iface, struct sockaddr *from, char *name, struct dns_question *q)
351 {
352         struct sockaddr *to = NULL;
353         char *host;
354 
355         /* TODO: Multicast if more than one quarter of TTL has passed */
356         if (q->class & CLASS_UNICAST) {
357                 to = from;
358                 if (iface->multicast)
359                         iface = iface->peer;
360         }
361 
362         DBG(1, "Q -> %s %s\n", dns_type_string(q->type), name);
363 
364         switch (q->type) {
365         case TYPE_ANY:
366                 if (!strcmp(name, mdns_hostname_local)) {
367                         dns_reply_a(iface, to, announce_ttl);
368                         service_reply(iface, to, NULL, NULL, announce_ttl);
369                 }
370                 break;
371 
372         case TYPE_PTR:
373                 if (!strcmp(name, C_DNS_SD)) {
374                         dns_reply_a(iface, to, announce_ttl);
375                         service_announce_services(iface, to, announce_ttl);
376                 } else {
377                         if (name[0] == '_') {
378                                 service_reply(iface, to, NULL, name, announce_ttl);
379                         } else {
380                                 /* First dot separates instance name from the rest */
381                                 char *dot = strchr(name, '.');
382 
383                                 if (dot) {
384                                         *dot = '\0';
385                                         service_reply(iface, to, name, dot + 1, announce_ttl);
386                                         *dot = '.';
387                                 }
388                         }
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(umdns_host_label, 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 *from, 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 != MCAST_PORT)
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, from, name, q);
437         }
438 
439         if (!(h->flags & FLAG_RESPONSE))
440                 return;
441 
442         while (h->answers-- > 0)
443                 if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
444                         return;
445 
446         while (h->authority-- > 0)
447                 if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
448                         return;
449 
450         while (h->additional-- > 0)
451                 if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
452                         return;
453 
454 }
455 

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