/* vi: set sw=4 ts=4: */ /* * Mini DNS server implementation for busybox * * Copyright (C) 2005 Roberto A. Foglietta (me@roberto.foglietta.name) * Copyright (C) 2005 Odd Arild Olsen (oao at fibula dot no) * Copyright (C) 2003 Paul Sheer * * Licensed under GPLv2 or later, see file LICENSE in this tarball for details. * * Odd Arild Olsen started out with the sheerdns [1] of Paul Sheer and rewrote * it into a shape which I believe is both easier to understand and maintain. * I also reused the input buffer for output and removed services he did not * need. [1] http://threading.2038bug.com/sheerdns/ * * Some bugfix and minor changes was applied by Roberto A. Foglietta who made * the first porting of oao' scdns to busybox also. */ #include "libbb.h" #include //#define DEBUG 1 #define DEBUG 0 enum { MAX_HOST_LEN = 16, // longest host name allowed is 15 IP_STRING_LEN = 18, // .xxx.xxx.xxx.xxx\0 //must be strlen('.in-addr.arpa') larger than IP_STRING_LEN MAX_NAME_LEN = (IP_STRING_LEN + 13), /* Cannot get bigger packets than 512 per RFC1035 In practice this can be set considerably smaller: Length of response packet is header (12B) + 2*type(4B) + 2*class(4B) + ttl(4B) + rlen(2B) + r (MAX_NAME_LEN =21B) + 2*querystring (2 MAX_NAME_LEN= 42B), all together 90 Byte */ MAX_PACK_LEN = 512 + 1, DEFAULT_TTL = 30, // increase this when not testing? REQ_A = 1, REQ_PTR = 12 }; struct dns_repl { // resource record, add 0 or 1 to accepted dns_msg in resp uint16_t rlen; uint8_t *r; // resource uint16_t flags; }; struct dns_head { // the message from client and first part of response mag uint16_t id; uint16_t flags; uint16_t nquer; // accepts 0 uint16_t nansw; // 1 in response uint16_t nauth; // 0 uint16_t nadd; // 0 }; struct dns_prop { uint16_t type; uint16_t class; }; struct dns_entry { // element of known name, ip address and reversed ip address struct dns_entry *next; char ip[IP_STRING_LEN]; // dotted decimal IP char rip[IP_STRING_LEN]; // length decimal reversed IP char name[MAX_HOST_LEN]; }; static struct dns_entry *dnsentry; static uint32_t ttl = DEFAULT_TTL; static const char *fileconf = "/etc/dnsd.conf"; // Must match getopt32 call #define OPT_daemon (option_mask32 & 0x10) #define OPT_verbose (option_mask32 & 0x20) /* * Convert host name from C-string to dns length/string. */ static void convname(char *a, uint8_t *q) { int i = (q[0] == '.') ? 0 : 1; for (; i < MAX_HOST_LEN-1 && *q; i++, q++) a[i] = tolower(*q); a[0] = i - 1; a[i] = 0; } /* * Insert length of substrings instead of dots */ static void undot(uint8_t * rip) { int i = 0, s = 0; while (rip[i]) i++; for (--i; i >= 0; i--) { if (rip[i] == '.') { rip[i] = s; s = 0; } else s++; } } /* * Read one line of hostname/IP from file * Returns 0 for each valid entry read, -1 at EOF * Assumes all host names are lower case only * Hostnames with more than one label are not handled correctly. * Presently the dot is copied into name without * converting to a length/string substring for that label. */ static int getfileentry(FILE * fp, struct dns_entry *s) { unsigned int a,b,c,d; char *line, *r, *name; restart: line = r = xmalloc_fgets(fp); if (!r) return -1; while (*r == ' ' || *r == '\t') { r++; if (!*r || *r == '#' || *r == '\n') { free(line); goto restart; /* skipping empty/blank and commented lines */ } } name = r; while (*r != ' ' && *r != '\t') r++; *r++ = '\0'; if (sscanf(r, ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4) { free(line); goto restart; /* skipping wrong lines */ } sprintf(s->ip, ".%u.%u.%u.%u"+1, a, b, c, d); sprintf(s->rip, ".%u.%u.%u.%u", d, c, b, a); undot((uint8_t*)s->rip); convname(s->name, (uint8_t*)name); if (OPT_verbose) fprintf(stderr, "\tname:%s, ip:%s\n", &(s->name[1]),s->ip); free(line); return 0; } /* * Read hostname/IP records from file */ static void dnsentryinit(void) { FILE *fp; struct dns_entry *m, *prev; prev = dnsentry = NULL; fp = xfopen(fileconf, "r"); while (1) { m = xzalloc(sizeof(*m)); /*m->next = NULL;*/ if (getfileentry(fp, m)) break; if (prev == NULL) dnsentry = m; else prev->next = m; prev = m; } fclose(fp); } /* * Look query up in dns records and return answer if found * qs is the query string, first byte the string length */ static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs) { int i; struct dns_entry *d = dnsentry; do { #if DEBUG char *p,*q; q = (char *)&(qs[1]); p = &(d->name[1]); fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d", __FUNCTION__, (int)strlen(p), (int)(d->name[0]), p, q, (int)strlen(q)); #endif if (type == REQ_A) { /* search by host name */ for (i = 1; i <= (int)(d->name[0]); i++) if (tolower(qs[i]) != d->name[i]) break; if (i > (int)(d->name[0])) { strcpy((char *)as, d->ip); #if DEBUG fprintf(stderr, " OK as:%s\n", as); #endif return 0; } } else if (type == REQ_PTR) { /* search by IP-address */ if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) { strcpy((char *)as, d->name); return 0; } } d = d->next; } while (d); return -1; } /* * Decode message and generate answer */ static int process_packet(uint8_t * buf) { struct dns_head *head; struct dns_prop *qprop; struct dns_repl outr; void *next, *from, *answb; uint8_t answstr[MAX_NAME_LEN + 1]; int lookup_result, type, len, packet_len; uint16_t flags; answstr[0] = '\0'; head = (struct dns_head *)buf; if (head->nquer == 0) { bb_error_msg("no queries"); return -1; } if (head->flags & 0x8000) { bb_error_msg("ignoring response packet"); return -1; } from = (void *)&head[1]; // start of query string next = answb = from + strlen((char *)from) + 1 + sizeof(struct dns_prop); // where to append answer block outr.rlen = 0; // may change later outr.r = NULL; outr.flags = 0; qprop = (struct dns_prop *)(answb - 4); type = ntohs(qprop->type); // only let REQ_A and REQ_PTR pass if (!(type == REQ_A || type == REQ_PTR)) { goto empty_packet; /* we can't handle the query type */ } if (ntohs(qprop->class) != 1 /* class INET */ ) { outr.flags = 4; /* not supported */ goto empty_packet; } /* we only support standard queries */ if ((ntohs(head->flags) & 0x7800) != 0) goto empty_packet; // We have a standard query bb_info_msg("%s", (char *)from); lookup_result = table_lookup(type, answstr, (uint8_t*)from); if (lookup_result != 0) { outr.flags = 3 | 0x0400; //name do not exist and auth goto empty_packet; } if (type == REQ_A) { // return an address struct in_addr a; if (!inet_aton((char*)answstr, &a)) {//dotted dec to long conv outr.flags = 1; /* Frmt err */ goto empty_packet; } memcpy(answstr, &a.s_addr, 4); // save before a disappears outr.rlen = 4; // uint32_t IP } else outr.rlen = strlen((char *)answstr) + 1; // a host name outr.r = answstr; // 32 bit ip or a host name outr.flags |= 0x0400; /* authority-bit */ // we have an answer head->nansw = htons(1); // copy query block to answer block len = answb - from; memcpy(answb, from, len); next += len; // and append answer rr *(uint32_t *) next = htonl(ttl); next += 4; *(uint16_t *) next = htons(outr.rlen); next += 2; memcpy(next, (void *)answstr, outr.rlen); next += outr.rlen; empty_packet: flags = ntohs(head->flags); // clear rcode and RA, set responsebit and our new flags flags |= (outr.flags & 0xff80) | 0x8000; head->flags = htons(flags); head->nauth = head->nadd = htons(0); head->nquer = htons(1); packet_len = (uint8_t *)next - buf; return packet_len; } /* * Exit on signal */ static void interrupt(int x) { /* unlink("/var/run/dnsd.lock"); */ bb_error_msg("interrupt, exiting\n"); exit(2); } int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE; int dnsd_main(int argc, char **argv) { const char *listen_interface = "0.0.0.0"; char *sttl, *sport; len_and_sockaddr *lsa; int udps; uint16_t port = 53; uint8_t buf[MAX_PACK_LEN]; getopt32(argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport); //if (option_mask32 & 0x1) // -i //if (option_mask32 & 0x2) // -c if (option_mask32 & 0x4) // -t ttl = xatou_range(sttl, 1, 0xffffffff); if (option_mask32 & 0x8) // -p port = xatou_range(sport, 1, 0xffff); if (OPT_verbose) { bb_info_msg("listen_interface: %s", listen_interface); bb_info_msg("ttl: %d, port: %d", ttl, port); bb_info_msg("fileconf: %s", fileconf); } if (OPT_daemon) { bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv); openlog(applet_name, LOG_PID, LOG_DAEMON); logmode = LOGMODE_SYSLOG; } dnsentryinit(); signal(SIGINT, interrupt); /* why? signal(SIGPIPE, SIG_IGN); */ signal(SIGHUP, SIG_IGN); #ifdef SIGTSTP signal(SIGTSTP, SIG_IGN); #endif #ifdef SIGURG signal(SIGURG, SIG_IGN); #endif lsa = xdotted2sockaddr(listen_interface, port); udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0); xbind(udps, &lsa->u.sa, lsa->len); /* xlisten(udps, 50); - ?!! DGRAM sockets are never listened on I think? */ bb_info_msg("Accepting UDP packets on %s", xmalloc_sockaddr2dotted(&lsa->u.sa)); while (1) { int r; socklen_t fromlen = lsa->len; // FIXME: need to get *DEST* address (to which of our addresses // this query was directed), and reply from the same address. // Or else we can exhibit usual UDP ugliness: // [ip1.multihomed.ip2] <= query to ip1 <= peer // [ip1.multihomed.ip2] => reply from ip2 => peer (confused) r = recvfrom(udps, buf, sizeof(buf), 0, &lsa->u.sa, &fromlen); if (OPT_verbose) bb_info_msg("Got UDP packet"); if (r < 12 || r > 512) { bb_error_msg("invalid packet size"); continue; } r = process_packet(buf); if (r <= 0) continue; sendto(udps, buf, r, 0, &lsa->u.sa, fromlen); } return 0; }