1 /* vi: set sw=4 ts=4: */
2 /*
3 * Mini DNS server implementation for busybox
4 *
5 * Copyright (C) 2005 Roberto A. Foglietta (me@roberto.foglietta.name)
6 * Copyright (C) 2005 Odd Arild Olsen (oao at fibula dot no)
7 * Copyright (C) 2003 Paul Sheer
8 *
9 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
10 *
11 * Odd Arild Olsen started out with the sheerdns [1] of Paul Sheer and rewrote
12 * it into a shape which I believe is both easier to understand and maintain.
13 * I also reused the input buffer for output and removed services he did not
14 * need. [1] http://threading.2038bug.com/sheerdns/
15 *
16 * Some bugfix and minor changes was applied by Roberto A. Foglietta who made
17 * the first porting of oao' scdns to busybox also.
18 */
19 //config:config DNSD
20 //config: bool "dnsd (9.8 kb)"
21 //config: default y
22 //config: help
23 //config: Small and static DNS server daemon.
24
25 //applet:IF_DNSD(APPLET(dnsd, BB_DIR_USR_SBIN, BB_SUID_DROP))
26
27 //kbuild:lib-$(CONFIG_DNSD) += dnsd.o
28
29 //usage:#define dnsd_trivial_usage
30 //usage: "[-dvs] [-c CONFFILE] [-t TTL_SEC] [-p PORT] [-i ADDR]"
31 //usage:#define dnsd_full_usage "\n\n"
32 //usage: "Small static DNS server daemon\n"
33 //usage: "\n -c FILE Config file"
34 //usage: "\n -t SEC TTL"
35 //usage: "\n -p PORT Listen on PORT"
36 //usage: "\n -i ADDR Listen on ADDR"
37 //usage: "\n -d Daemonize"
38 //usage: "\n -v Verbose"
39 //usage: "\n -s Send successful replies only. Use this if you want"
40 //usage: "\n to use /etc/resolv.conf with two nameserver lines:"
41 //usage: "\n nameserver DNSD_SERVER"
42 //usage: "\n nameserver NORMAL_DNS_SERVER"
43
44 #include "libbb.h"
45 #include <syslog.h>
46
47 //#define DEBUG 1
48 #define DEBUG 0
49
50 enum {
51 /* can tweak this */
52 DEFAULT_TTL = 120,
53
54 /* cannot get bigger packets than 512 per RFC1035. */
55 MAX_PACK_LEN = 512,
56 IP_STRING_LEN = sizeof(".xxx.xxx.xxx.xxx"),
57 MAX_NAME_LEN = IP_STRING_LEN - 1 + sizeof(".in-addr.arpa"),
58 REQ_A = 1,
59 REQ_PTR = 12,
60 };
61
62 /* the message from client and first part of response msg */
63 struct dns_head {
64 uint16_t id;
65 uint16_t flags;
66 uint16_t nquer;
67 uint16_t nansw;
68 uint16_t nauth;
69 uint16_t nadd;
70 };
71 /* Structure used to access type and class fields.
72 * They are totally unaligned, but gcc 4.3.4 thinks that pointer of type uint16_t*
73 * is 16-bit aligned and replaces 16-bit memcpy (in move_from_unaligned16 macro)
74 * with aligned halfword access on arm920t!
75 * Oh well. Slapping PACKED everywhere seems to help: */
76 struct type_and_class {
77 uint16_t type PACKED;
78 uint16_t class PACKED;
79 } PACKED;
80 /* element of known name, ip address and reversed ip address */
81 struct dns_entry {
82 struct dns_entry *next;
83 uint32_t ip;
84 char rip[IP_STRING_LEN]; /* length decimal reversed IP */
85 char name[1];
86 };
87
88 #define OPT_verbose (option_mask32 & 1)
89 #define OPT_silent (option_mask32 & 2)
90
91
92 /*
93 * Insert length of substrings instead of dots
94 */
undot(char * rip)95 static void undot(char *rip)
96 {
97 int i = 0;
98 int s = 0;
99
100 while (rip[i])
101 i++;
102 for (--i; i >= 0; i--) {
103 if (rip[i] == '.') {
104 rip[i] = s;
105 s = 0;
106 } else {
107 s++;
108 }
109 }
110 }
111
112 /*
113 * Read hostname/IP records from file
114 */
parse_conf_file(const char * fileconf)115 static struct dns_entry *parse_conf_file(const char *fileconf)
116 {
117 char *token[2];
118 parser_t *parser;
119 struct dns_entry *m, *conf_data;
120 struct dns_entry **nextp;
121
122 conf_data = NULL;
123 nextp = &conf_data;
124
125 parser = config_open(fileconf);
126 while (config_read(parser, token, 2, 2, "# \t", PARSE_NORMAL)) {
127 struct in_addr ip;
128 uint32_t v32;
129
130 if (inet_aton(token[1], &ip) == 0) {
131 bb_error_msg("error at line %u, skipping", parser->lineno);
132 continue;
133 }
134
135 if (OPT_verbose)
136 bb_info_msg("name:%s, ip:%s", token[0], token[1]);
137
138 /* sizeof(*m) includes 1 byte for m->name[0] */
139 m = xzalloc(sizeof(*m) + strlen(token[0]) + 1);
140 /*m->next = NULL;*/
141 *nextp = m;
142 nextp = &m->next;
143
144 m->name[0] = '.';
145 strcpy(m->name + 1, token[0]);
146 undot(m->name);
147 m->ip = ip.s_addr; /* in network order */
148 v32 = ntohl(m->ip);
149 /* inverted order */
150 sprintf(m->rip, ".%u.%u.%u.%u",
151 (uint8_t)(v32),
152 (uint8_t)(v32 >> 8),
153 (uint8_t)(v32 >> 16),
154 (v32 >> 24)
155 );
156 undot(m->rip);
157 }
158 config_close(parser);
159 return conf_data;
160 }
161
162 /*
163 * Look query up in dns records and return answer if found.
164 */
table_lookup(struct dns_entry * d,uint16_t type,char * query_string)165 static char *table_lookup(struct dns_entry *d,
166 uint16_t type,
167 char* query_string)
168 {
169 while (d) {
170 unsigned len = d->name[0];
171 /* d->name[len] is the last (non NUL) char */
172 #if DEBUG
173 char *p, *q;
174 q = query_string + 1;
175 p = d->name + 1;
176 fprintf(stderr, "%d/%d p:%s q:%s %d\n",
177 (int)strlen(p), len,
178 p, q, (int)strlen(q)
179 );
180 #endif
181 if (type == htons(REQ_A)) {
182 /* search by host name */
183 if (len != 1 || d->name[1] != '*') {
184 /* we are lax, hope no name component is ever >64 so that length
185 * (which will be represented as 'A','B'...) matches a lowercase letter.
186 * Actually, I think false matches are hard to construct.
187 * Example.
188 * [31] len is represented as '1', [65] as 'A', [65+32] as 'a'.
189 * [65] <65 same chars>[31]<31 same chars>NUL
190 * [65+32]<65 same chars>1 <31 same chars>NUL
191 * This example seems to be the minimal case when false match occurs.
192 */
193 if (strcasecmp(d->name, query_string) != 0)
194 goto next;
195 }
196 return (char *)&d->ip;
197 #if DEBUG
198 fprintf(stderr, "Found IP:%x\n", (int)d->ip);
199 #endif
200 return 0;
201 }
202 /* search by IP-address */
203 if ((len != 1 || d->name[1] != '*')
204 /* we assume (do not check) that query_string
205 * ends in ".in-addr.arpa" */
206 && is_prefixed_with(query_string, d->rip)
207 ) {
208 #if DEBUG
209 fprintf(stderr, "Found name:%s\n", d->name);
210 #endif
211 return d->name;
212 }
213 next:
214 d = d->next;
215 }
216
217 return NULL;
218 }
219
220 /*
221 * Decode message and generate answer
222 */
223 /* RFC 1035
224 ...
225 Whenever an octet represents a numeric quantity, the left most bit
226 in the diagram is the high order or most significant bit.
227 That is, the bit labeled 0 is the most significant bit.
228 ...
229
230 4.1.1. Header section format
231 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
232 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
233 | ID |
234 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
235 |QR| OPCODE |AA|TC|RD|RA| 0 0 0| RCODE |
236 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
237 | QDCOUNT |
238 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
239 | ANCOUNT |
240 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
241 | NSCOUNT |
242 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
243 | ARCOUNT |
244 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
245 ID 16 bit random identifier assigned by querying peer.
246 Used to match query/response.
247 QR message is a query (0), or a response (1).
248 OPCODE 0 standard query (QUERY)
249 1 inverse query (IQUERY)
250 2 server status request (STATUS)
251 AA Authoritative Answer - this bit is valid in responses.
252 Responding name server is an authority for the domain name
253 in question section. Answer section may have multiple owner names
254 because of aliases. The AA bit corresponds to the name which matches
255 the query name, or the first owner name in the answer section.
256 TC TrunCation - this message was truncated.
257 RD Recursion Desired - this bit may be set in a query and
258 is copied into the response. If RD is set, it directs
259 the name server to pursue the query recursively.
260 Recursive query support is optional.
261 RA Recursion Available - this be is set or cleared in a
262 response, and denotes whether recursive query support is
263 available in the name server.
264 RCODE Response code.
265 0 No error condition
266 1 Format error
267 2 Server failure - server was unable to process the query
268 due to a problem with the name server.
269 3 Name Error - meaningful only for responses from
270 an authoritative name server. The referenced domain name
271 does not exist.
272 4 Not Implemented.
273 5 Refused.
274 QDCOUNT number of entries in the question section.
275 ANCOUNT number of records in the answer section.
276 NSCOUNT number of records in the authority records section.
277 ARCOUNT number of records in the additional records section.
278
279 4.1.2. Question section format
280
281 The section contains QDCOUNT (usually 1) entries, each of this format:
282 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
283 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
284 / QNAME /
285 / /
286 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
287 | QTYPE |
288 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
289 | QCLASS |
290 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
291 QNAME a domain name represented as a sequence of labels, where
292 each label consists of a length octet followed by that
293 number of octets. The domain name terminates with the
294 zero length octet for the null label of the root. Note
295 that this field may be an odd number of octets; no
296 padding is used.
297 QTYPE a two octet type of the query.
298 1 a host address [REQ_A const]
299 2 an authoritative name server
300 3 a mail destination (Obsolete - use MX)
301 4 a mail forwarder (Obsolete - use MX)
302 5 the canonical name for an alias
303 6 marks the start of a zone of authority
304 7 a mailbox domain name (EXPERIMENTAL)
305 8 a mail group member (EXPERIMENTAL)
306 9 a mail rename domain name (EXPERIMENTAL)
307 10 a null RR (EXPERIMENTAL)
308 11 a well known service description
309 12 a domain name pointer [REQ_PTR const]
310 13 host information
311 14 mailbox or mail list information
312 15 mail exchange
313 16 text strings
314 0x1c IPv6?
315 252 a request for a transfer of an entire zone
316 253 a request for mailbox-related records (MB, MG or MR)
317 254 a request for mail agent RRs (Obsolete - see MX)
318 255 a request for all records
319 QCLASS a two octet code that specifies the class of the query.
320 1 the Internet
321 (others are historic only)
322 255 any class
323
324 4.1.3. Resource Record format
325
326 The answer, authority, and additional sections all share the same format:
327 a variable number of resource records, where the number of records
328 is specified in the corresponding count field in the header.
329 Each resource record has this format:
330 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
331 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
332 / /
333 / NAME /
334 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
335 | TYPE |
336 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
337 | CLASS |
338 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
339 | TTL |
340 | |
341 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
342 | RDLENGTH |
343 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
344 / RDATA /
345 / /
346 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
347 NAME a domain name to which this resource record pertains.
348 TYPE two octets containing one of the RR type codes. This
349 field specifies the meaning of the data in the RDATA field.
350 CLASS two octets which specify the class of the data in the RDATA field.
351 TTL a 32 bit unsigned integer that specifies the time interval
352 (in seconds) that the record may be cached.
353 RDLENGTH a 16 bit integer, length in octets of the RDATA field.
354 RDATA a variable length string of octets that describes the resource.
355 The format of this information varies according to the TYPE
356 and CLASS of the resource record.
357 If the TYPE is A and the CLASS is IN, it's a 4 octet IP address.
358
359 4.1.4. Message compression
360
361 In order to reduce the size of messages, domain names coan be compressed.
362 An entire domain name or a list of labels at the end of a domain name
363 is replaced with a pointer to a prior occurrence of the same name.
364
365 The pointer takes the form of a two octet sequence:
366 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
367 | 1 1| OFFSET |
368 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
369 The first two bits are ones. This allows a pointer to be distinguished
370 from a label, since the label must begin with two zero bits because
371 labels are restricted to 63 octets or less. The OFFSET field specifies
372 an offset from the start of the message (i.e., the first octet
373 of the ID field in the domain header).
374 A zero offset specifies the first byte of the ID field, etc.
375 Domain name in a message can be represented as either:
376 - a sequence of labels ending in a zero octet
377 - a pointer
378 - a sequence of labels ending with a pointer
379 */
process_packet(struct dns_entry * conf_data,uint32_t conf_ttl,uint8_t * buf,unsigned buflen)380 static int process_packet(struct dns_entry *conf_data,
381 uint32_t conf_ttl,
382 uint8_t *buf,
383 unsigned buflen)
384 {
385 struct dns_head *head;
386 struct type_and_class *unaligned_type_class;
387 const char *err_msg;
388 char *query_string;
389 char *answstr;
390 uint8_t *answb;
391 uint16_t outr_rlen;
392 uint16_t outr_flags;
393 uint16_t type;
394 uint16_t class;
395 int query_len;
396
397 head = (struct dns_head *)buf;
398 if (head->nquer == 0) {
399 bb_simple_error_msg("packet has 0 queries, ignored");
400 return 0; /* don't reply */
401 }
402 if (head->flags & htons(0x8000)) { /* QR bit */
403 bb_simple_error_msg("response packet, ignored");
404 return 0; /* don't reply */
405 }
406
407 /* start of query string */
408 query_string = (void *)(head + 1);
409 /* caller guarantees strlen is <= MAX_PACK_LEN */
410 query_len = strlen(query_string) + 1;
411 /* may be unaligned! */
412 unaligned_type_class = (void *)(query_string + query_len);
413 query_len += sizeof(*unaligned_type_class);
414 /* where to append answer block */
415 answb = (void *)(unaligned_type_class + 1);
416
417 if (buflen < answb - buf) {
418 bb_simple_error_msg("packet too short");
419 return 0; /* don't reply */
420 }
421
422 /* QR = 1 "response", RCODE = 4 "Not Implemented" */
423 outr_flags = htons(0x8000 | 4);
424 err_msg = NULL;
425
426 /* OPCODE != 0 "standard query"? */
427 if ((head->flags & htons(0x7800)) != 0) {
428 err_msg = "opcode != 0";
429 goto empty_packet;
430 }
431 move_from_unaligned16(class, &unaligned_type_class->class);
432 if (class != htons(1)) { /* not class INET? */
433 err_msg = "class != 1";
434 goto empty_packet;
435 }
436 move_from_unaligned16(type, &unaligned_type_class->type);
437 if (type != htons(REQ_A) && type != htons(REQ_PTR)) {
438 /* we can't handle this query type */
439 //TODO: happens all the time with REQ_AAAA (0x1c) requests - implement those?
440 err_msg = "type is !REQ_A and !REQ_PTR";
441 goto empty_packet;
442 }
443
444 /* look up the name */
445 answstr = table_lookup(conf_data, type, query_string);
446 #if DEBUG
447 /* Shows lengths instead of dots, unusable for !DEBUG */
448 bb_info_msg("'%s'->'%s'", query_string, answstr);
449 #endif
450 outr_rlen = 4;
451 if (answstr && type == htons(REQ_PTR)) {
452 /* returning a host name */
453 outr_rlen = strlen(answstr) + 1;
454 }
455 if (!answstr
456 || (unsigned)(answb - buf) + query_len + 4 + 2 + outr_rlen > MAX_PACK_LEN
457 ) {
458 /* QR = 1 "response"
459 * AA = 1 "Authoritative Answer"
460 * RCODE = 3 "Name Error" */
461 err_msg = "name is not found";
462 outr_flags = htons(0x8000 | 0x0400 | 3);
463 goto empty_packet;
464 }
465
466 /* Append answer Resource Record */
467 memcpy(answb, query_string, query_len); /* name, type, class */
468 answb += query_len;
469 move_to_unaligned32((uint32_t *)answb, htonl(conf_ttl));
470 answb += 4;
471 move_to_unaligned16((uint16_t *)answb, htons(outr_rlen));
472 answb += 2;
473 memcpy(answb, answstr, outr_rlen);
474 answb += outr_rlen;
475
476 /* QR = 1 "response",
477 * AA = 1 "Authoritative Answer",
478 * TODO: need to set RA bit 0x80? One user says nslookup complains
479 * "Got recursion not available from SERVER, trying next server"
480 * "** server can't find HOSTNAME"
481 * RCODE = 0 "success"
482 */
483 if (OPT_verbose)
484 bb_simple_info_msg("returning positive reply");
485 outr_flags = htons(0x8000 | 0x0400 | 0);
486 /* we have one answer */
487 head->nansw = htons(1);
488
489 empty_packet:
490 if ((outr_flags & htons(0xf)) != 0) { /* not a positive response */
491 if (OPT_verbose) {
492 bb_error_msg("%s, %s",
493 err_msg,
494 OPT_silent ? "dropping query" : "sending error reply"
495 );
496 }
497 if (OPT_silent)
498 return 0;
499 }
500 head->flags |= outr_flags;
501 head->nauth = head->nadd = 0;
502 head->nquer = htons(1); // why???
503
504 return answb - buf;
505 }
506
507 int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
dnsd_main(int argc UNUSED_PARAM,char ** argv)508 int dnsd_main(int argc UNUSED_PARAM, char **argv)
509 {
510 const char *listen_interface = "0.0.0.0";
511 const char *fileconf = "/etc/dnsd.conf";
512 struct dns_entry *conf_data;
513 uint32_t conf_ttl = DEFAULT_TTL;
514 char *sttl, *sport;
515 len_and_sockaddr *lsa, *from, *to;
516 unsigned lsa_size;
517 int udps, opts;
518 uint16_t port = 53;
519 /* Ensure buf is 32bit aligned (we need 16bit, but 32bit can't hurt) */
520 uint8_t buf[MAX_PACK_LEN + 1] ALIGN4;
521
522 opts = getopt32(argv, "vsi:c:t:p:d", &listen_interface, &fileconf, &sttl, &sport);
523 //if (opts & (1 << 0)) // -v
524 //if (opts & (1 << 1)) // -s
525 //if (opts & (1 << 2)) // -i
526 //if (opts & (1 << 3)) // -c
527 if (opts & (1 << 4)) // -t
528 conf_ttl = xatou_range(sttl, 1, 0xffffffff);
529 if (opts & (1 << 5)) // -p
530 port = xatou_range(sport, 1, 0xffff);
531 if (opts & (1 << 6)) { // -d
532 bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
533 openlog(applet_name, LOG_PID, LOG_DAEMON);
534 logmode = LOGMODE_SYSLOG;
535 }
536
537 conf_data = parse_conf_file(fileconf);
538
539 lsa = xdotted2sockaddr(listen_interface, port);
540 udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
541 xbind(udps, &lsa->u.sa, lsa->len);
542 socket_want_pktinfo(udps); /* needed for recv_from_to to work */
543 lsa_size = LSA_LEN_SIZE + lsa->len;
544 from = xzalloc(lsa_size);
545 to = xzalloc(lsa_size);
546
547 {
548 char *p = xmalloc_sockaddr2dotted(&lsa->u.sa);
549 bb_info_msg("accepting UDP packets on %s", p);
550 free(p);
551 }
552
553 while (1) {
554 int r;
555 /* Try to get *DEST* address (to which of our addresses
556 * this query was directed), and reply from the same address.
557 * Or else we can exhibit usual UDP ugliness:
558 * [ip1.multihomed.ip2] <= query to ip1 <= peer
559 * [ip1.multihomed.ip2] => reply from ip2 => peer (confused) */
560 memcpy(to, lsa, lsa_size);
561 r = recv_from_to(udps, buf, MAX_PACK_LEN + 1, 0, &from->u.sa, &to->u.sa, lsa->len);
562 if (r < 12 || r > MAX_PACK_LEN) {
563 bb_error_msg("packet size %d, ignored", r);
564 continue;
565 }
566 if (OPT_verbose)
567 bb_simple_info_msg("got UDP packet");
568 buf[r] = '\0'; /* paranoia */
569 r = process_packet(conf_data, conf_ttl, buf, r);
570 if (r <= 0)
571 continue;
572 send_to_from(udps, buf, r, 0, &from->u.sa, &to->u.sa, lsa->len);
573 }
574 return 0;
575 }
576