1 /* vi: set sw=4 ts=4: */
3 * Mini DNS server implementation for busybox
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
9 * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
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/
16 * Some bugfix and minor changes was applied by Roberto A. Foglietta who made
17 * the first porting of oao' scdns to busybox also.
30 /* cannot get bigger packets than 512 per RFC1035. */
32 IP_STRING_LEN = sizeof(".xxx.xxx.xxx.xxx"),
33 MAX_NAME_LEN = IP_STRING_LEN - 1 + sizeof(".in-addr.arpa"),
38 /* the message from client and first part of response msg */
47 struct type_and_class {
51 /* element of known name, ip address and reversed ip address */
53 struct dns_entry *next;
55 char rip[IP_STRING_LEN]; /* length decimal reversed IP */
59 #define OPT_verbose (option_mask32 & 1)
60 #define OPT_silent (option_mask32 & 2)
64 * Insert length of substrings instead of dots
66 static void undot(char *rip)
73 for (--i; i >= 0; i--) {
84 * Read hostname/IP records from file
86 static struct dns_entry *parse_conf_file(const char *fileconf)
90 struct dns_entry *m, *conf_data;
91 struct dns_entry **nextp;
96 parser = config_open(fileconf);
97 while (config_read(parser, token, 2, 2, "# \t", PARSE_NORMAL)) {
101 if (inet_aton(token[1], &ip) == 0) {
102 bb_error_msg("error at line %u, skipping", parser->lineno);
107 bb_error_msg("name:%s, ip:%s", token[0], token[1]);
109 /* sizeof(*m) includes 1 byte for m->name[0] */
110 m = xzalloc(sizeof(*m) + strlen(token[0]) + 1);
116 strcpy(m->name + 1, token[0]);
118 m->ip = ip.s_addr; /* in network order */
121 sprintf(m->rip, ".%u.%u.%u.%u",
124 (uint8_t)(v32 >> 16),
129 config_close(parser);
134 * Look query up in dns records and return answer if found.
136 static char *table_lookup(struct dns_entry *d,
141 unsigned len = d->name[0];
142 /* d->name[len] is the last (non NUL) char */
145 q = query_string + 1;
147 fprintf(stderr, "%d/%d p:%s q:%s %d\n",
152 if (type == htons(REQ_A)) {
153 /* search by host name */
154 if (len != 1 || d->name[1] != '*') {
155 /* we are lax, hope no name component is ever >64 so that length
156 * (which will be represented as 'A','B'...) matches a lowercase letter.
157 * Actually, I think false matches are hard to construct.
159 * [31] len is represented as '1', [65] as 'A', [65+32] as 'a'.
160 * [65] <65 same chars>[31]<31 same chars>NUL
161 * [65+32]<65 same chars>1 <31 same chars>NUL
162 * This example seems to be the minimal case when false match occurs.
164 if (strcasecmp(d->name, query_string) != 0)
167 return (char *)&d->ip;
169 fprintf(stderr, "Found IP:%x\n", (int)d->ip);
173 /* search by IP-address */
174 if ((len != 1 || d->name[1] != '*')
175 /* we assume (do not check) that query_string
176 * ends in ".in-addr.arpa" */
177 && strncmp(d->rip, query_string, strlen(d->rip)) == 0
180 fprintf(stderr, "Found name:%s\n", d->name);
192 * Decode message and generate answer
196 Whenever an octet represents a numeric quantity, the left most bit
197 in the diagram is the high order or most significant bit.
198 That is, the bit labeled 0 is the most significant bit.
201 4.1.1. Header section format
202 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
203 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
205 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
206 |QR| OPCODE |AA|TC|RD|RA| 0 0 0| RCODE |
207 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
209 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
211 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
213 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
215 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
216 ID 16 bit random identifier assigned by querying peer.
217 Used to match query/response.
218 QR message is a query (0), or a response (1).
219 OPCODE 0 standard query (QUERY)
220 1 inverse query (IQUERY)
221 2 server status request (STATUS)
222 AA Authoritative Answer - this bit is valid in responses.
223 Responding name server is an authority for the domain name
224 in question section. Answer section may have multiple owner names
225 because of aliases. The AA bit corresponds to the name which matches
226 the query name, or the first owner name in the answer section.
227 TC TrunCation - this message was truncated.
228 RD Recursion Desired - this bit may be set in a query and
229 is copied into the response. If RD is set, it directs
230 the name server to pursue the query recursively.
231 Recursive query support is optional.
232 RA Recursion Available - this be is set or cleared in a
233 response, and denotes whether recursive query support is
234 available in the name server.
238 2 Server failure - server was unable to process the query
239 due to a problem with the name server.
240 3 Name Error - meaningful only for responses from
241 an authoritative name server. The referenced domain name
245 QDCOUNT number of entries in the question section.
246 ANCOUNT number of records in the answer section.
247 NSCOUNT number of records in the authority records section.
248 ARCOUNT number of records in the additional records section.
250 4.1.2. Question section format
252 The section contains QDCOUNT (usually 1) entries, each of this format:
253 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
254 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
257 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
259 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
261 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
262 QNAME a domain name represented as a sequence of labels, where
263 each label consists of a length octet followed by that
264 number of octets. The domain name terminates with the
265 zero length octet for the null label of the root. Note
266 that this field may be an odd number of octets; no
268 QTYPE a two octet type of the query.
269 1 a host address [REQ_A const]
270 2 an authoritative name server
271 3 a mail destination (Obsolete - use MX)
272 4 a mail forwarder (Obsolete - use MX)
273 5 the canonical name for an alias
274 6 marks the start of a zone of authority
275 7 a mailbox domain name (EXPERIMENTAL)
276 8 a mail group member (EXPERIMENTAL)
277 9 a mail rename domain name (EXPERIMENTAL)
278 10 a null RR (EXPERIMENTAL)
279 11 a well known service description
280 12 a domain name pointer [REQ_PTR const]
282 14 mailbox or mail list information
286 252 a request for a transfer of an entire zone
287 253 a request for mailbox-related records (MB, MG or MR)
288 254 a request for mail agent RRs (Obsolete - see MX)
289 255 a request for all records
290 QCLASS a two octet code that specifies the class of the query.
292 (others are historic only)
295 4.1.3. Resource Record format
297 The answer, authority, and additional sections all share the same format:
298 a variable number of resource records, where the number of records
299 is specified in the corresponding count field in the header.
300 Each resource record has this format:
301 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
302 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
305 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
307 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
309 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
312 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
314 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
317 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
318 NAME a domain name to which this resource record pertains.
319 TYPE two octets containing one of the RR type codes. This
320 field specifies the meaning of the data in the RDATA field.
321 CLASS two octets which specify the class of the data in the RDATA field.
322 TTL a 32 bit unsigned integer that specifies the time interval
323 (in seconds) that the record may be cached.
324 RDLENGTH a 16 bit integer, length in octets of the RDATA field.
325 RDATA a variable length string of octets that describes the resource.
326 The format of this information varies according to the TYPE
327 and CLASS of the resource record.
328 If the TYPE is A and the CLASS is IN, it's a 4 octet IP address.
330 4.1.4. Message compression
332 In order to reduce the size of messages, domain names coan be compressed.
333 An entire domain name or a list of labels at the end of a domain name
334 is replaced with a pointer to a prior occurance of the same name.
336 The pointer takes the form of a two octet sequence:
337 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
339 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
340 The first two bits are ones. This allows a pointer to be distinguished
341 from a label, since the label must begin with two zero bits because
342 labels are restricted to 63 octets or less. The OFFSET field specifies
343 an offset from the start of the message (i.e., the first octet
344 of the ID field in the domain header).
345 A zero offset specifies the first byte of the ID field, etc.
346 Domain name in a message can be represented as either:
347 - a sequence of labels ending in a zero octet
349 - a sequence of labels ending with a pointer
351 static int process_packet(struct dns_entry *conf_data,
355 struct dns_head *head;
356 struct type_and_class *unaligned_type_class;
367 head = (struct dns_head *)buf;
368 if (head->nquer == 0) {
369 bb_error_msg("packet has 0 queries, ignored");
370 return 0; /* don't reply */
372 if (head->flags & htons(0x8000)) { /* QR bit */
373 bb_error_msg("response packet, ignored");
374 return 0; /* don't reply */
376 /* QR = 1 "response", RCODE = 4 "Not Implemented" */
377 outr_flags = htons(0x8000 | 4);
380 /* start of query string */
381 query_string = (void *)(head + 1);
382 /* caller guarantees strlen is <= MAX_PACK_LEN */
383 query_len = strlen(query_string) + 1;
384 /* may be unaligned! */
385 unaligned_type_class = (void *)(query_string + query_len);
386 query_len += sizeof(unaligned_type_class);
387 /* where to append answer block */
388 answb = (void *)(unaligned_type_class + 1);
390 /* OPCODE != 0 "standard query"? */
391 if ((head->flags & htons(0x7800)) != 0) {
392 err_msg = "opcode != 0";
395 move_from_unaligned16(class, &unaligned_type_class->class);
396 if (class != htons(1)) { /* not class INET? */
397 err_msg = "class != 1";
400 move_from_unaligned16(type, &unaligned_type_class->type);
401 if (type != htons(REQ_A) && type != htons(REQ_PTR)) {
402 /* we can't handle this query type */
403 //TODO: happens all the time with REQ_AAAA (0x1c) requests - implement those?
404 err_msg = "type is !REQ_A and !REQ_PTR";
408 /* look up the name */
409 answstr = table_lookup(conf_data, type, query_string);
411 /* Shows lengths instead of dots, unusable for !DEBUG */
412 bb_error_msg("'%s'->'%s'", query_string, answstr);
415 if (answstr && type == htons(REQ_PTR)) {
416 /* returning a host name */
417 outr_rlen = strlen(answstr) + 1;
420 || (unsigned)(answb - buf) + query_len + 4 + 2 + outr_rlen > MAX_PACK_LEN
423 * AA = 1 "Authoritative Answer"
424 * RCODE = 3 "Name Error" */
425 err_msg = "name is not found";
426 outr_flags = htons(0x8000 | 0x0400 | 3);
430 /* Append answer Resource Record */
431 memcpy(answb, query_string, query_len); /* name, type, class */
433 move_to_unaligned32((uint32_t *)answb, htonl(conf_ttl));
435 move_to_unaligned16((uint16_t *)answb, htons(outr_rlen));
437 memcpy(answb, answstr, outr_rlen);
440 /* QR = 1 "response",
441 * AA = 1 "Authoritative Answer",
442 * TODO: need to set RA bit 0x80? One user says nslookup complains
443 * "Got recursion not available from SERVER, trying next server"
444 * "** server can't find HOSTNAME"
445 * RCODE = 0 "success"
448 bb_error_msg("returning positive reply");
449 outr_flags = htons(0x8000 | 0x0400 | 0);
450 /* we have one answer */
451 head->nansw = htons(1);
454 if ((outr_flags & htons(0xf)) != 0) { /* not a positive response */
456 bb_error_msg("%s, %s",
458 OPT_silent ? "dropping query" : "sending error reply"
464 head->flags |= outr_flags;
465 head->nauth = head->nadd = 0;
466 head->nquer = htons(1); // why???
471 int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
472 int dnsd_main(int argc UNUSED_PARAM, char **argv)
474 const char *listen_interface = "0.0.0.0";
475 const char *fileconf = "/etc/dnsd.conf";
476 struct dns_entry *conf_data;
477 uint32_t conf_ttl = DEFAULT_TTL;
479 len_and_sockaddr *lsa, *from, *to;
483 /* Ensure buf is 32bit aligned (we need 16bit, but 32bit can't hurt) */
484 uint8_t buf[MAX_PACK_LEN + 1] ALIGN4;
486 opts = getopt32(argv, "vsi:c:t:p:d", &listen_interface, &fileconf, &sttl, &sport);
487 //if (opts & (1 << 0)) // -v
488 //if (opts & (1 << 1)) // -s
489 //if (opts & (1 << 2)) // -i
490 //if (opts & (1 << 3)) // -c
491 if (opts & (1 << 4)) // -t
492 conf_ttl = xatou_range(sttl, 1, 0xffffffff);
493 if (opts & (1 << 5)) // -p
494 port = xatou_range(sport, 1, 0xffff);
495 if (opts & (1 << 6)) { // -d
496 bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
497 openlog(applet_name, LOG_PID, LOG_DAEMON);
498 logmode = LOGMODE_SYSLOG;
501 conf_data = parse_conf_file(fileconf);
503 lsa = xdotted2sockaddr(listen_interface, port);
504 udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
505 xbind(udps, &lsa->u.sa, lsa->len);
506 socket_want_pktinfo(udps); /* needed for recv_from_to to work */
507 lsa_size = LSA_LEN_SIZE + lsa->len;
508 from = xzalloc(lsa_size);
509 to = xzalloc(lsa_size);
512 char *p = xmalloc_sockaddr2dotted(&lsa->u.sa);
513 bb_error_msg("accepting UDP packets on %s", p);
519 /* Try to get *DEST* address (to which of our addresses
520 * this query was directed), and reply from the same address.
521 * Or else we can exhibit usual UDP ugliness:
522 * [ip1.multihomed.ip2] <= query to ip1 <= peer
523 * [ip1.multihomed.ip2] => reply from ip2 => peer (confused) */
524 memcpy(to, lsa, lsa_size);
525 r = recv_from_to(udps, buf, MAX_PACK_LEN + 1, 0, &from->u.sa, &to->u.sa, lsa->len);
526 if (r < 12 || r > MAX_PACK_LEN) {
527 bb_error_msg("packet size %d, ignored", r);
531 bb_error_msg("got UDP packet");
532 buf[r] = '\0'; /* paranoia */
533 r = process_packet(conf_data, conf_ttl, buf);
536 send_to_from(udps, buf, r, 0, &from->u.sa, &to->u.sa, lsa->len);