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 tarball for details. 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 20#include <syslog.h> 21#include "libbb.h" 22 23//#define DEBUG 1 24#define DEBUG 0 25 26enum { 27 MAX_HOST_LEN = 16, // longest host name allowed is 15 28 IP_STRING_LEN = 18, 29 30//must be strlen('.in-addr.arpa') larger than IP_STRING_LEN 31 MAX_NAME_LEN = (IP_STRING_LEN + 13), 32 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 38*/ 39 MAX_PACK_LEN = 512 + 1, 40 41 DEFAULT_TTL = 30, // increase this when not testing? 42 43 REQ_A = 1, 44 REQ_PTR = 12 45}; 46 47struct dns_repl { // resource record, add 0 or 1 to accepted dns_msg in resp 48 uint16_t rlen; 49 uint8_t *r; // resource 50 uint16_t flags; 51}; 52 53struct dns_head { // the message from client and first part of response mag 54 uint16_t id; 55 uint16_t flags; 56 uint16_t nquer; // accepts 0 57 uint16_t nansw; // 1 in response 58 uint16_t nauth; // 0 59 uint16_t nadd; // 0 60}; 61struct dns_prop { 62 uint16_t type; 63 uint16_t class; 64}; 65struct dns_entry { // element of known name, ip address and reversed ip address 66 struct dns_entry *next; 67 char ip[IP_STRING_LEN]; // dotted decimal IP 68 char rip[IP_STRING_LEN]; // length decimal reversed IP 69 char name[MAX_HOST_LEN]; 70}; 71 72static struct dns_entry *dnsentry; 73static uint32_t ttl = DEFAULT_TTL; 74 75static const char *fileconf = "/etc/dnsd.conf"; 76 77// Must match getopt32 call 78#define OPT_daemon (option_mask32 & 0x10) 79#define OPT_verbose (option_mask32 & 0x20) 80 81 82/* 83 * Convert host name from C-string to dns length/string. 84 */ 85static void convname(char *a, uint8_t *q) 86{ 87 int i = (q[0] == '.') ? 0 : 1; 88 for (; i < MAX_HOST_LEN-1 && *q; i++, q++) 89 a[i] = tolower(*q); 90 a[0] = i - 1; 91 a[i] = 0; 92} 93 94/* 95 * Insert length of substrings instead of dots 96 */ 97static void undot(uint8_t * rip) 98{ 99 int i = 0, s = 0; 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 s++; 107 } 108} 109 110/* 111 * Read one line of hostname/IP from file 112 * Returns 0 for each valid entry read, -1 at EOF 113 * Assumes all host names are lower case only 114 * Hostnames with more than one label are not handled correctly. 115 * Presently the dot is copied into name without 116 * converting to a length/string substring for that label. 117 */ 118static int getfileentry(FILE * fp, struct dns_entry *s) 119{ 120 unsigned int a,b,c,d; 121 char *line, *r, *name; 122 123 restart: 124 line = r = xmalloc_fgets(fp); 125 if (!r) 126 return -1; 127 while (*r == ' ' || *r == '\t') { 128 r++; 129 if (!*r || *r == '#' || *r == '\n') { 130 free(line); 131 goto restart; /* skipping empty/blank and commented lines */ 132 } 133 } 134 name = r; 135 while (*r != ' ' && *r != '\t') 136 r++; 137 *r++ = '\0'; 138 if (sscanf(r, ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4) { 139 free(line); 140 goto restart; /* skipping wrong lines */ 141 } 142 143 sprintf(s->ip, ".%u.%u.%u.%u"+1, a, b, c, d); 144 sprintf(s->rip, ".%u.%u.%u.%u", d, c, b, a); 145 undot((uint8_t*)s->rip); 146 convname(s->name, (uint8_t*)name); 147 148 if (OPT_verbose) 149 fprintf(stderr, "\tname:%s, ip:%s\n", &(s->name[1]),s->ip); 150 151 free(line); 152 return 0; 153} 154 155/* 156 * Read hostname/IP records from file 157 */ 158static void dnsentryinit(void) 159{ 160 FILE *fp; 161 struct dns_entry *m, *prev; 162 163 prev = dnsentry = NULL; 164 fp = xfopen(fileconf, "r"); 165 166 while (1) { 167 m = xzalloc(sizeof(*m)); 168 /*m->next = NULL;*/ 169 if (getfileentry(fp, m)) 170 break; 171 172 if (prev == NULL) 173 dnsentry = m; 174 else 175 prev->next = m; 176 prev = m; 177 } 178 fclose(fp); 179} 180 181/* 182 * Look query up in dns records and return answer if found 183 * qs is the query string, first byte the string length 184 */ 185static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs) 186{ 187 int i; 188 struct dns_entry *d = dnsentry; 189 190 do { 191#if DEBUG 192 char *p,*q; 193 q = (char *)&(qs[1]); 194 p = &(d->name[1]); 195 fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d", 196 __FUNCTION__, (int)strlen(p), (int)(d->name[0]), 197 p, q, (int)strlen(q)); 198#endif 199 if (type == REQ_A) { /* search by host name */ 200 for (i = 1; i <= (int)(d->name[0]); i++) 201 if (tolower(qs[i]) != d->name[i]) 202 break; 203 if (i > (int)(d->name[0])) { 204 strcpy((char *)as, d->ip); 205#if DEBUG 206 fprintf(stderr, " OK as:%s\n", as); 207#endif 208 return 0; 209 } 210 } else if (type == REQ_PTR) { /* search by IP-address */ 211 if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) { 212 strcpy((char *)as, d->name); 213 return 0; 214 } 215 } 216 d = d->next; 217 } while (d); 218 return -1; 219} 220 221 222/* 223 * Decode message and generate answer 224 */ 225static int process_packet(uint8_t * buf) 226{ 227 struct dns_head *head; 228 struct dns_prop *qprop; 229 struct dns_repl outr; 230 void *next, *from, *answb; 231 232 uint8_t answstr[MAX_NAME_LEN + 1]; 233 int lookup_result, type, len, packet_len; 234 uint16_t flags; 235 236 answstr[0] = '\0'; 237 238 head = (struct dns_head *)buf; 239 if (head->nquer == 0) { 240 bb_error_msg("no queries"); 241 return -1; 242 } 243 244 if (head->flags & 0x8000) { 245 bb_error_msg("ignoring response packet"); 246 return -1; 247 } 248 249 from = (void *)&head[1]; // start of query string 250 next = answb = from + strlen((char *)from) + 1 + sizeof(struct dns_prop); // where to append answer block 251 252 outr.rlen = 0; // may change later 253 outr.r = NULL; 254 outr.flags = 0; 255 256 qprop = (struct dns_prop *)(answb - 4); 257 type = ntohs(qprop->type); 258 259 // only let REQ_A and REQ_PTR pass 260 if (!(type == REQ_A || type == REQ_PTR)) { 261 goto empty_packet; /* we can't handle the query type */ 262 } 263 264 if (ntohs(qprop->class) != 1 /* class INET */ ) { 265 outr.flags = 4; /* not supported */ 266 goto empty_packet; 267 } 268 /* we only support standard queries */ 269 270 if ((ntohs(head->flags) & 0x7800) != 0) 271 goto empty_packet; 272 273 // We have a standard query 274 bb_info_msg("%s", (char *)from); 275 lookup_result = table_lookup(type, answstr, (uint8_t*)from); 276 if (lookup_result != 0) { 277 outr.flags = 3 | 0x0400; //name do not exist and auth 278 goto empty_packet; 279 } 280 if (type == REQ_A) { // return an address 281 struct in_addr a; 282 if (!inet_aton((char*)answstr, &a)) {//dotted dec to long conv 283 outr.flags = 1; /* Frmt err */ 284 goto empty_packet; 285 } 286 memcpy(answstr, &a.s_addr, 4); // save before a disappears 287 outr.rlen = 4; // uint32_t IP 288 } else 289 outr.rlen = strlen((char *)answstr) + 1; // a host name 290 outr.r = answstr; // 32 bit ip or a host name 291 outr.flags |= 0x0400; /* authority-bit */ 292 // we have an answer 293 head->nansw = htons(1); 294 295 // copy query block to answer block 296 len = answb - from; 297 memcpy(answb, from, len); 298 next += len; 299 300 // and append answer rr 301 *(uint32_t *) next = htonl(ttl); 302 next += 4; 303 *(uint16_t *) next = htons(outr.rlen); 304 next += 2; 305 memcpy(next, (void *)answstr, outr.rlen); 306 next += outr.rlen; 307 308 empty_packet: 309 310 flags = ntohs(head->flags); 311 // clear rcode and RA, set responsebit and our new flags 312 flags |= (outr.flags & 0xff80) | 0x8000; 313 head->flags = htons(flags); 314 head->nauth = head->nadd = htons(0); 315 head->nquer = htons(1); 316 317 packet_len = (uint8_t *)next - buf; 318 return packet_len; 319} 320 321/* 322 * Exit on signal 323 */ 324static void interrupt(int x) 325{ 326 /* unlink("/var/run/dnsd.lock"); */ 327 bb_error_msg("interrupt, exiting\n"); 328 exit(2); 329} 330 331int dnsd_main(int argc, char **argv); 332int dnsd_main(int argc, char **argv) 333{ 334 const char *listen_interface = "0.0.0.0"; 335 char *sttl, *sport; 336 len_and_sockaddr *lsa; 337 int udps; 338 uint16_t port = 53; 339 uint8_t buf[MAX_PACK_LEN]; 340 341 getopt32(argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport); 342 //if (option_mask32 & 0x1) // -i 343 //if (option_mask32 & 0x2) // -c 344 if (option_mask32 & 0x4) // -t 345 ttl = xatou_range(sttl, 1, 0xffffffff); 346 if (option_mask32 & 0x8) // -p 347 port = xatou_range(sport, 1, 0xffff); 348 349 if (OPT_verbose) { 350 bb_info_msg("listen_interface: %s", listen_interface); 351 bb_info_msg("ttl: %d, port: %d", ttl, port); 352 bb_info_msg("fileconf: %s", fileconf); 353 } 354 355 if (OPT_daemon) { 356 bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv); 357 openlog(applet_name, LOG_PID, LOG_DAEMON); 358 logmode = LOGMODE_SYSLOG; 359 } 360 361 dnsentryinit(); 362 363 signal(SIGINT, interrupt); 364 /* why? signal(SIGPIPE, SIG_IGN); */ 365 signal(SIGHUP, SIG_IGN); 366#ifdef SIGTSTP 367 signal(SIGTSTP, SIG_IGN); 368#endif 369#ifdef SIGURG 370 signal(SIGURG, SIG_IGN); 371#endif 372 373 lsa = xdotted2sockaddr(listen_interface, port); 374 udps = xsocket(lsa->sa.sa_family, SOCK_DGRAM, 0); 375 xbind(udps, &lsa->sa, lsa->len); 376 /* xlisten(udps, 50); - ?!! DGRAM sockets are never listened on I think? */ 377 bb_info_msg("Accepting UDP packets on %s", 378 xmalloc_sockaddr2dotted(&lsa->sa)); 379 380 while (1) { 381 int r; 382 socklen_t fromlen = lsa->len; 383// this query was directed), and reply from the same address. 384// Or else we can exhibit usual UDP ugliness: 385// [ip1.multihomed.ip2] <= query to ip1 <= peer 386// [ip1.multihomed.ip2] => reply from ip2 => peer (confused) 387 r = recvfrom(udps, buf, sizeof(buf), 0, &lsa->sa, &fromlen); 388 if (OPT_verbose) 389 bb_info_msg("Got UDP packet"); 390 if (r < 12 || r > 512) { 391 bb_error_msg("invalid packet size"); 392 continue; 393 } 394 r = process_packet(buf); 395 if (r <= 0) 396 continue; 397 sendto(udps, buf, r, 0, &lsa->sa, fromlen); 398 } 399 return 0; 400} 401