/* 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 "busybox.h" static char *fileconf = "/etc/dnsd.conf"; #define LOCK_FILE "/var/run/dnsd.lock" #define LOG_FILE "/var/log/dnsd.log" 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 = NULL; static int daemonmode = 0; static uint32_t ttl = DEFAULT_TTL; /* * 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++; } } /* * Append message to log file */ static void log_message(char *filename, char *message) { FILE *logfile; if (!daemonmode) return; logfile = fopen(filename, "a"); if (!logfile) return; fprintf(logfile, "%s\n", message); fclose(logfile); } /* * 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 is 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, int verb) { unsigned int a,b,c,d; char *r, *name; restart: if(!(r = bb_get_line_from_file(fp))) return -1; while(*r == ' ' || *r == '\t') { r++; if(!*r || *r == '#' || *r == '\n') goto restart; /* skipping empty/blank and commented lines */ } name = r; while(*r != ' ' && *r != '\t') r++; *r++ = 0; if(sscanf(r,"%u.%u.%u.%u",&a,&b,&c,&d) != 4) goto restart; /* skipping wrong lines */ sprintf(s->ip,"%u.%u.%u.%u",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(verb) fprintf(stderr,"\tname:%s, ip:%s\n",&(s->name[1]),s->ip); return 0; /* warningkiller */ } /* * Read hostname/IP records from file */ static void dnsentryinit(int verb) { FILE *fp; struct dns_entry *m, *prev; prev = dnsentry = NULL; fp = xfopen(fileconf, "r"); while (1) { m = xmalloc(sizeof(struct dns_entry)); m->next = NULL; if (getfileentry(fp, m, verb)) break; if (prev == NULL) dnsentry = m; else prev->next = m; prev = m; } fclose(fp); } /* * Set up UDP socket */ static int listen_socket(char *iface_addr, int listen_port) { struct sockaddr_in a; char msg[100]; int s; int yes = 1; s = xsocket(PF_INET, SOCK_DGRAM, 0); #ifdef SO_REUSEADDR if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *)&yes, sizeof(yes)) < 0) bb_perror_msg_and_die("setsockopt() failed"); #endif memset(&a, 0, sizeof(a)); a.sin_port = htons(listen_port); a.sin_family = AF_INET; if (!inet_aton(iface_addr, &a.sin_addr)) bb_perror_msg_and_die("bad iface address"); xbind(s, (struct sockaddr *)&a, sizeof(a)); xlisten(s, 50); /* xlisten? */ sprintf(msg, "accepting UDP packets on addr:port %s:%d\n", iface_addr, (int)listen_port); log_message(LOG_FILE, msg); return s; } /* * 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 { #ifdef DEBUG char *p,*q; q = (char *)&(qs[1]); p = &(d->name[1]); fprintf(stderr, "\ntest: %d <%s> <%s> %d", strlen(p), p, q, 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]) continue; #ifdef DEBUG fprintf(stderr, " OK"); #endif strcpy((char *)as, d->ip); #ifdef DEBUG fprintf(stderr, " %s ", 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; } } } while ((d = d->next) != NULL); return -1; } /* * Decode message and generate answer */ #define eret(s) do { fprintf (stderr, "%s\n", s); return -1; } while (0) 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) eret("no queries"); if ((head->flags & 0x8000)) eret("ignoring response packet"); from = (void *)&head[1]; // start of query string next = answb = from + strlen((char *)&head[1]) + 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 log_message(LOG_FILE, (char *)head); lookup_result = table_lookup(type, answstr, (uint8_t*)(&head[1])); 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 = next - (void *)buf; return packet_len; } /* * Exit on signal */ static void interrupt(int x) { unlink(LOCK_FILE); write(2, "interrupt exiting\n", 18); exit(2); } #define is_daemon() (flags&16) #define is_verbose() (flags&32) //#define DEBUG 1 int dnsd_main(int argc, char **argv) { int udps; uint16_t port = 53; uint8_t buf[MAX_PACK_LEN]; unsigned long flags = 0; char *listen_interface = "0.0.0.0"; char *sttl=NULL, *sport=NULL; if(argc > 1) flags = bb_getopt_ulflags(argc, argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport); if(sttl) if(!(ttl = atol(sttl))) bb_show_usage(); if(sport) if(!(port = atol(sport))) bb_show_usage(); if(is_verbose()) { fprintf(stderr,"listen_interface: %s\n", listen_interface); fprintf(stderr,"ttl: %d, port: %d\n", ttl, port); fprintf(stderr,"fileconf: %s\n", fileconf); } if(is_daemon()) #ifdef BB_NOMMU /* reexec for vfork() do continue parent */ vfork_daemon_rexec(1, 0, argc, argv, "-d"); #else xdaemon(1, 0); #endif dnsentryinit(is_verbose()); signal(SIGINT, interrupt); signal(SIGPIPE, SIG_IGN); signal(SIGHUP, SIG_IGN); #ifdef SIGTSTP signal(SIGTSTP, SIG_IGN); #endif #ifdef SIGURG signal(SIGURG, SIG_IGN); #endif udps = listen_socket(listen_interface, port); if (udps < 0) exit(1); while (1) { fd_set fdset; int r; FD_ZERO(&fdset); FD_SET(udps, &fdset); // Block until a message arrives if((r = select(udps + 1, &fdset, NULL, NULL, NULL)) < 0) bb_perror_msg_and_die("select error"); else if(r == 0) bb_perror_msg_and_die("select spurious return"); /* Can this test ever be false? */ if (FD_ISSET(udps, &fdset)) { struct sockaddr_in from; int fromlen = sizeof(from); r = recvfrom(udps, buf, sizeof(buf), 0, (struct sockaddr *)&from, (void *)&fromlen); if(is_verbose()) fprintf(stderr, "\n--- Got UDP "); log_message(LOG_FILE, "\n--- Got UDP "); if (r < 12 || r > 512) { bb_error_msg("invalid packet size"); continue; } if (r > 0) { r = process_packet(buf); if (r > 0) sendto(udps, buf, r, 0, (struct sockaddr *)&from, fromlen); } } // end if } // end while return 0; }