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.
27 MAX_HOST_LEN = 16, // longest host name allowed is 15
28 IP_STRING_LEN = 18, // .xxx.xxx.xxx.xxx\0
30 //must be strlen('.in-addr.arpa') larger than IP_STRING_LEN
31 MAX_NAME_LEN = (IP_STRING_LEN + 13),
33 /* Cannot get bigger packets than 512 per RFC1035
34 In practice this can be set considerably smaller:
35 Length of response packet is header (12B) + 2*type(4B) + 2*class(4B) +
36 ttl(4B) + rlen(2B) + r (MAX_NAME_LEN =21B) +
37 2*querystring (2 MAX_NAME_LEN= 42B), all together 90 Byte
41 DEFAULT_TTL = 30, // increase this when not testing?
47 struct dns_head { // the message from client and first part of response mag
50 uint16_t nquer; // accepts 0
51 uint16_t nansw; // 1 in response
59 struct dns_entry { // element of known name, ip address and reversed ip address
60 struct dns_entry *next;
61 char ip[IP_STRING_LEN]; // dotted decimal IP
62 char rip[IP_STRING_LEN]; // length decimal reversed IP
63 char name[MAX_HOST_LEN];
66 static struct dns_entry *dnsentry;
67 static uint32_t ttl = DEFAULT_TTL;
69 static const char *fileconf = "/etc/dnsd.conf";
71 // Must match getopt32 call
72 #define OPT_daemon (option_mask32 & 0x10)
73 #define OPT_verbose (option_mask32 & 0x20)
77 * Convert host name from C-string to dns length/string.
79 static void convname(char *a, uint8_t *q)
81 int i = (q[0] == '.') ? 0 : 1;
82 for (; i < MAX_HOST_LEN-1 && *q; i++, q++)
89 * Insert length of substrings instead of dots
91 static void undot(uint8_t * rip)
96 for (--i; i >= 0; i--) {
105 * Read one line of hostname/IP from file
106 * Returns 0 for each valid entry read, -1 at EOF
107 * Assumes all host names are lower case only
108 * Hostnames with more than one label are not handled correctly.
109 * Presently the dot is copied into name without
110 * converting to a length/string substring for that label.
112 static int getfileentry(FILE * fp, struct dns_entry *s)
114 unsigned int a,b,c,d;
115 char *line, *r, *name;
118 line = r = xmalloc_fgets(fp);
121 while (*r == ' ' || *r == '\t') {
123 if (!*r || *r == '#' || *r == '\n') {
125 goto restart; /* skipping empty/blank and commented lines */
129 while (*r != ' ' && *r != '\t')
132 if (sscanf(r, ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4) {
134 goto restart; /* skipping wrong lines */
137 sprintf(s->ip, ".%u.%u.%u.%u"+1, a, b, c, d);
138 sprintf(s->rip, ".%u.%u.%u.%u", d, c, b, a);
139 undot((uint8_t*)s->rip);
140 convname(s->name, (uint8_t*)name);
143 fprintf(stderr, "\tname:%s, ip:%s\n", &(s->name[1]),s->ip);
150 * Read hostname/IP records from file
152 static void dnsentryinit(void)
155 struct dns_entry *m, *prev;
157 prev = dnsentry = NULL;
158 fp = xfopen(fileconf, "r");
161 m = xzalloc(sizeof(*m));
163 if (getfileentry(fp, m))
176 * Look query up in dns records and return answer if found
177 * qs is the query string, first byte the string length
179 static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs)
182 struct dns_entry *d = dnsentry;
187 q = (char *)&(qs[1]);
189 fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d",
190 __FUNCTION__, (int)strlen(p), (int)(d->name[0]),
191 p, q, (int)strlen(q));
193 if (type == REQ_A) { /* search by host name */
194 for (i = 1; i <= (int)(d->name[0]); i++)
195 if (tolower(qs[i]) != d->name[i])
197 if (i > (int)(d->name[0])) {
198 strcpy((char *)as, d->ip);
200 fprintf(stderr, " OK as:%s\n", as);
204 } else if (type == REQ_PTR) { /* search by IP-address */
205 if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) {
206 strcpy((char *)as, d->name);
216 * Decode message and generate answer
218 static int process_packet(uint8_t *buf)
220 uint8_t answstr[MAX_NAME_LEN + 1];
221 struct dns_head *head;
222 struct dns_prop *qprop;
223 uint8_t *from, *answb;
227 int lookup_result, type, packet_len;
232 head = (struct dns_head *)buf;
233 if (head->nquer == 0) {
234 bb_error_msg("no queries");
238 if (head->flags & 0x8000) {
239 bb_error_msg("ignoring response packet");
243 from = (void *)&head[1]; // start of query string
244 //FIXME: strlen of untrusted data??!
245 querystr_len = strlen((char *)from) + 1 + sizeof(struct dns_prop);
246 answb = from + querystr_len; // where to append answer block
251 qprop = (struct dns_prop *)(answb - 4);
252 type = ntohs(qprop->type);
254 // only let REQ_A and REQ_PTR pass
255 if (!(type == REQ_A || type == REQ_PTR)) {
256 goto empty_packet; /* we can't handle the query type */
259 if (ntohs(qprop->class) != 1 /* class INET */ ) {
260 outr_flags = 4; /* not supported */
263 /* we only support standard queries */
265 if ((ntohs(head->flags) & 0x7800) != 0)
268 // We have a standard query
269 bb_info_msg("%s", (char *)from);
270 lookup_result = table_lookup(type, answstr, from);
271 if (lookup_result != 0) {
272 outr_flags = 3 | 0x0400; // name do not exist and auth
275 if (type == REQ_A) { // return an address
276 struct in_addr a; // NB! its "struct { unsigned __long__ s_addr; }"
278 if (!inet_aton((char*)answstr, &a)) { //dotted dec to long conv
279 outr_flags = 1; /* Frmt err */
282 v32 = a.s_addr; /* in case long != int */
283 memcpy(answstr, &v32, 4);
284 outr_rlen = 4; // uint32_t IP
286 outr_rlen = strlen((char *)answstr) + 1; // a host name
287 outr_flags |= 0x0400; /* authority-bit */
289 head->nansw = htons(1);
291 // copy query block to answer block
292 memcpy(answb, from, querystr_len);
293 answb += querystr_len;
295 // and append answer rr
296 // FIXME: unaligned accesses??
297 *(uint32_t *) answb = htonl(ttl);
299 *(uint16_t *) answb = htons(outr_rlen);
301 memcpy(answb, answstr, outr_rlen);
306 flags = ntohs(head->flags);
307 // clear rcode and RA, set responsebit and our new flags
308 flags |= (outr_flags & 0xff80) | 0x8000;
309 head->flags = htons(flags);
310 head->nauth = head->nadd = 0;
311 head->nquer = htons(1);
313 packet_len = answb - buf;
320 static void interrupt(int sig)
322 /* unlink("/var/run/dnsd.lock"); */
323 bb_error_msg("interrupt, exiting\n");
324 kill_myself_with_sig(sig);
327 int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
328 int dnsd_main(int argc ATTRIBUTE_UNUSED, char **argv)
330 const char *listen_interface = "0.0.0.0";
332 len_and_sockaddr *lsa, *from, *to;
336 /* Paranoid sizing: querystring x2 + ttl + outr_rlen + answstr */
337 /* I'd rather see process_packet() fixed instead... */
338 uint8_t buf[MAX_PACK_LEN * 2 + 4 + 2 + (MAX_NAME_LEN+1)];
340 getopt32(argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport);
341 //if (option_mask32 & 0x1) // -i
342 //if (option_mask32 & 0x2) // -c
343 if (option_mask32 & 0x4) // -t
344 ttl = xatou_range(sttl, 1, 0xffffffff);
345 if (option_mask32 & 0x8) // -p
346 port = xatou_range(sport, 1, 0xffff);
349 bb_info_msg("listen_interface: %s", listen_interface);
350 bb_info_msg("ttl: %d, port: %d", ttl, port);
351 bb_info_msg("fileconf: %s", fileconf);
355 bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
356 openlog(applet_name, LOG_PID, LOG_DAEMON);
357 logmode = LOGMODE_SYSLOG;
362 signal(SIGINT, interrupt);
364 /* why? + (1 << SIGPIPE) */
374 lsa = xdotted2sockaddr(listen_interface, port);
375 udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
376 xbind(udps, &lsa->u.sa, lsa->len);
377 socket_want_pktinfo(udps); /* needed for recv_from_to to work */
378 lsa_size = LSA_LEN_SIZE + lsa->len;
379 from = xzalloc(lsa_size);
380 to = xzalloc(lsa_size);
382 bb_info_msg("Accepting UDP packets on %s",
383 xmalloc_sockaddr2dotted(&lsa->u.sa));
387 /* Try to get *DEST* address (to which of our addresses
388 * this query was directed), and reply from the same address.
389 * Or else we can exhibit usual UDP ugliness:
390 * [ip1.multihomed.ip2] <= query to ip1 <= peer
391 * [ip1.multihomed.ip2] => reply from ip2 => peer (confused) */
392 memcpy(to, lsa, lsa_size);
393 r = recv_from_to(udps, buf, MAX_PACK_LEN + 1, 0, &from->u.sa, &to->u.sa, lsa->len);
394 if (r < 12 || r > MAX_PACK_LEN) {
395 bb_error_msg("invalid packet size");
399 bb_info_msg("Got UDP packet");
400 buf[r] = '\0'; /* paranoia */
401 r = process_packet(buf);
404 send_to_from(udps, buf, r, 0, &to->u.sa, &from->u.sa, lsa->len);