| /* 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 source tree. |
| * |
| * 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. |
| */ |
| //config:config DNSD |
| //config: bool "dnsd (9.8 kb)" |
| //config: default y |
| //config: help |
| //config: Small and static DNS server daemon. |
| |
| //applet:IF_DNSD(APPLET(dnsd, BB_DIR_USR_SBIN, BB_SUID_DROP)) |
| |
| //kbuild:lib-$(CONFIG_DNSD) += dnsd.o |
| |
| //usage:#define dnsd_trivial_usage |
| //usage: "[-dvs] [-c CONFFILE] [-t TTL_SEC] [-p PORT] [-i ADDR]" |
| //usage:#define dnsd_full_usage "\n\n" |
| //usage: "Small static DNS server daemon\n" |
| //usage: "\n -c FILE Config file" |
| //usage: "\n -t SEC TTL" |
| //usage: "\n -p PORT Listen on PORT" |
| //usage: "\n -i ADDR Listen on ADDR" |
| //usage: "\n -d Daemonize" |
| //usage: "\n -v Verbose" |
| //usage: "\n -s Send successful replies only. Use this if you want" |
| //usage: "\n to use /etc/resolv.conf with two nameserver lines:" |
| //usage: "\n nameserver DNSD_SERVER" |
| //usage: "\n nameserver NORMAL_DNS_SERVER" |
| |
| #include "libbb.h" |
| #include <syslog.h> |
| |
| //#define DEBUG 1 |
| #define DEBUG 0 |
| |
| enum { |
| /* can tweak this */ |
| DEFAULT_TTL = 120, |
| |
| /* cannot get bigger packets than 512 per RFC1035. */ |
| MAX_PACK_LEN = 512, |
| IP_STRING_LEN = sizeof(".xxx.xxx.xxx.xxx"), |
| MAX_NAME_LEN = IP_STRING_LEN - 1 + sizeof(".in-addr.arpa"), |
| REQ_A = 1, |
| REQ_PTR = 12, |
| }; |
| |
| /* the message from client and first part of response msg */ |
| struct dns_head { |
| uint16_t id; |
| uint16_t flags; |
| uint16_t nquer; |
| uint16_t nansw; |
| uint16_t nauth; |
| uint16_t nadd; |
| }; |
| /* Structure used to access type and class fields. |
| * They are totally unaligned, but gcc 4.3.4 thinks that pointer of type uint16_t* |
| * is 16-bit aligned and replaces 16-bit memcpy (in move_from_unaligned16 macro) |
| * with aligned halfword access on arm920t! |
| * Oh well. Slapping PACKED everywhere seems to help: */ |
| struct type_and_class { |
| uint16_t type PACKED; |
| uint16_t class PACKED; |
| } PACKED; |
| /* element of known name, ip address and reversed ip address */ |
| struct dns_entry { |
| struct dns_entry *next; |
| uint32_t ip; |
| char rip[IP_STRING_LEN]; /* length decimal reversed IP */ |
| char name[1]; |
| }; |
| |
| #define OPT_verbose (option_mask32 & 1) |
| #define OPT_silent (option_mask32 & 2) |
| |
| |
| /* |
| * Insert length of substrings instead of dots |
| */ |
| static void undot(char *rip) |
| { |
| int i = 0; |
| int s = 0; |
| |
| while (rip[i]) |
| i++; |
| for (--i; i >= 0; i--) { |
| if (rip[i] == '.') { |
| rip[i] = s; |
| s = 0; |
| } else { |
| s++; |
| } |
| } |
| } |
| |
| /* |
| * Read hostname/IP records from file |
| */ |
| static struct dns_entry *parse_conf_file(const char *fileconf) |
| { |
| char *token[2]; |
| parser_t *parser; |
| struct dns_entry *m, *conf_data; |
| struct dns_entry **nextp; |
| |
| conf_data = NULL; |
| nextp = &conf_data; |
| |
| parser = config_open(fileconf); |
| while (config_read(parser, token, 2, 2, "# \t", PARSE_NORMAL)) { |
| struct in_addr ip; |
| uint32_t v32; |
| |
| if (inet_aton(token[1], &ip) == 0) { |
| bb_error_msg("error at line %u, skipping", parser->lineno); |
| continue; |
| } |
| |
| if (OPT_verbose) |
| bb_info_msg("name:%s, ip:%s", token[0], token[1]); |
| |
| /* sizeof(*m) includes 1 byte for m->name[0] */ |
| m = xzalloc(sizeof(*m) + strlen(token[0]) + 1); |
| /*m->next = NULL;*/ |
| *nextp = m; |
| nextp = &m->next; |
| |
| m->name[0] = '.'; |
| strcpy(m->name + 1, token[0]); |
| undot(m->name); |
| m->ip = ip.s_addr; /* in network order */ |
| v32 = ntohl(m->ip); |
| /* inverted order */ |
| sprintf(m->rip, ".%u.%u.%u.%u", |
| (uint8_t)(v32), |
| (uint8_t)(v32 >> 8), |
| (uint8_t)(v32 >> 16), |
| (v32 >> 24) |
| ); |
| undot(m->rip); |
| } |
| config_close(parser); |
| return conf_data; |
| } |
| |
| /* |
| * Look query up in dns records and return answer if found. |
| */ |
| static char *table_lookup(struct dns_entry *d, |
| uint16_t type, |
| char* query_string) |
| { |
| while (d) { |
| unsigned len = d->name[0]; |
| /* d->name[len] is the last (non NUL) char */ |
| #if DEBUG |
| char *p, *q; |
| q = query_string + 1; |
| p = d->name + 1; |
| fprintf(stderr, "%d/%d p:%s q:%s %d\n", |
| (int)strlen(p), len, |
| p, q, (int)strlen(q) |
| ); |
| #endif |
| if (type == htons(REQ_A)) { |
| /* search by host name */ |
| if (len != 1 || d->name[1] != '*') { |
| /* we are lax, hope no name component is ever >64 so that length |
| * (which will be represented as 'A','B'...) matches a lowercase letter. |
| * Actually, I think false matches are hard to construct. |
| * Example. |
| * [31] len is represented as '1', [65] as 'A', [65+32] as 'a'. |
| * [65] <65 same chars>[31]<31 same chars>NUL |
| * [65+32]<65 same chars>1 <31 same chars>NUL |
| * This example seems to be the minimal case when false match occurs. |
| */ |
| if (strcasecmp(d->name, query_string) != 0) |
| goto next; |
| } |
| return (char *)&d->ip; |
| #if DEBUG |
| fprintf(stderr, "Found IP:%x\n", (int)d->ip); |
| #endif |
| return 0; |
| } |
| /* search by IP-address */ |
| if ((len != 1 || d->name[1] != '*') |
| /* we assume (do not check) that query_string |
| * ends in ".in-addr.arpa" */ |
| && is_prefixed_with(query_string, d->rip) |
| ) { |
| #if DEBUG |
| fprintf(stderr, "Found name:%s\n", d->name); |
| #endif |
| return d->name; |
| } |
| next: |
| d = d->next; |
| } |
| |
| return NULL; |
| } |
| |
| /* |
| * Decode message and generate answer |
| */ |
| /* RFC 1035 |
| ... |
| Whenever an octet represents a numeric quantity, the left most bit |
| in the diagram is the high order or most significant bit. |
| That is, the bit labeled 0 is the most significant bit. |
| ... |
| |
| 4.1.1. Header section format |
| 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | ID | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| |QR| OPCODE |AA|TC|RD|RA| 0 0 0| RCODE | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | QDCOUNT | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | ANCOUNT | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | NSCOUNT | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | ARCOUNT | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ID 16 bit random identifier assigned by querying peer. |
| Used to match query/response. |
| QR message is a query (0), or a response (1). |
| OPCODE 0 standard query (QUERY) |
| 1 inverse query (IQUERY) |
| 2 server status request (STATUS) |
| AA Authoritative Answer - this bit is valid in responses. |
| Responding name server is an authority for the domain name |
| in question section. Answer section may have multiple owner names |
| because of aliases. The AA bit corresponds to the name which matches |
| the query name, or the first owner name in the answer section. |
| TC TrunCation - this message was truncated. |
| RD Recursion Desired - this bit may be set in a query and |
| is copied into the response. If RD is set, it directs |
| the name server to pursue the query recursively. |
| Recursive query support is optional. |
| RA Recursion Available - this be is set or cleared in a |
| response, and denotes whether recursive query support is |
| available in the name server. |
| RCODE Response code. |
| 0 No error condition |
| 1 Format error |
| 2 Server failure - server was unable to process the query |
| due to a problem with the name server. |
| 3 Name Error - meaningful only for responses from |
| an authoritative name server. The referenced domain name |
| does not exist. |
| 4 Not Implemented. |
| 5 Refused. |
| QDCOUNT number of entries in the question section. |
| ANCOUNT number of records in the answer section. |
| NSCOUNT number of records in the authority records section. |
| ARCOUNT number of records in the additional records section. |
| |
| 4.1.2. Question section format |
| |
| The section contains QDCOUNT (usually 1) entries, each of this format: |
| 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| / QNAME / |
| / / |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | QTYPE | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | QCLASS | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QNAME a domain name represented as a sequence of labels, where |
| each label consists of a length octet followed by that |
| number of octets. The domain name terminates with the |
| zero length octet for the null label of the root. Note |
| that this field may be an odd number of octets; no |
| padding is used. |
| QTYPE a two octet type of the query. |
| 1 a host address [REQ_A const] |
| 2 an authoritative name server |
| 3 a mail destination (Obsolete - use MX) |
| 4 a mail forwarder (Obsolete - use MX) |
| 5 the canonical name for an alias |
| 6 marks the start of a zone of authority |
| 7 a mailbox domain name (EXPERIMENTAL) |
| 8 a mail group member (EXPERIMENTAL) |
| 9 a mail rename domain name (EXPERIMENTAL) |
| 10 a null RR (EXPERIMENTAL) |
| 11 a well known service description |
| 12 a domain name pointer [REQ_PTR const] |
| 13 host information |
| 14 mailbox or mail list information |
| 15 mail exchange |
| 16 text strings |
| 0x1c IPv6? |
| 252 a request for a transfer of an entire zone |
| 253 a request for mailbox-related records (MB, MG or MR) |
| 254 a request for mail agent RRs (Obsolete - see MX) |
| 255 a request for all records |
| QCLASS a two octet code that specifies the class of the query. |
| 1 the Internet |
| (others are historic only) |
| 255 any class |
| |
| 4.1.3. Resource Record format |
| |
| The answer, authority, and additional sections all share the same format: |
| a variable number of resource records, where the number of records |
| is specified in the corresponding count field in the header. |
| Each resource record has this format: |
| 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| / / |
| / NAME / |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | TYPE | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | CLASS | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | TTL | |
| | | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | RDLENGTH | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--| |
| / RDATA / |
| / / |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NAME a domain name to which this resource record pertains. |
| TYPE two octets containing one of the RR type codes. This |
| field specifies the meaning of the data in the RDATA field. |
| CLASS two octets which specify the class of the data in the RDATA field. |
| TTL a 32 bit unsigned integer that specifies the time interval |
| (in seconds) that the record may be cached. |
| RDLENGTH a 16 bit integer, length in octets of the RDATA field. |
| RDATA a variable length string of octets that describes the resource. |
| The format of this information varies according to the TYPE |
| and CLASS of the resource record. |
| If the TYPE is A and the CLASS is IN, it's a 4 octet IP address. |
| |
| 4.1.4. Message compression |
| |
| In order to reduce the size of messages, domain names coan be compressed. |
| An entire domain name or a list of labels at the end of a domain name |
| is replaced with a pointer to a prior occurrence of the same name. |
| |
| The pointer takes the form of a two octet sequence: |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| | 1 1| OFFSET | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| The first two bits are ones. This allows a pointer to be distinguished |
| from a label, since the label must begin with two zero bits because |
| labels are restricted to 63 octets or less. The OFFSET field specifies |
| an offset from the start of the message (i.e., the first octet |
| of the ID field in the domain header). |
| A zero offset specifies the first byte of the ID field, etc. |
| Domain name in a message can be represented as either: |
| - a sequence of labels ending in a zero octet |
| - a pointer |
| - a sequence of labels ending with a pointer |
| */ |
| static int process_packet(struct dns_entry *conf_data, |
| uint32_t conf_ttl, |
| uint8_t *buf) |
| { |
| struct dns_head *head; |
| struct type_and_class *unaligned_type_class; |
| const char *err_msg; |
| char *query_string; |
| char *answstr; |
| uint8_t *answb; |
| uint16_t outr_rlen; |
| uint16_t outr_flags; |
| uint16_t type; |
| uint16_t class; |
| int query_len; |
| |
| head = (struct dns_head *)buf; |
| if (head->nquer == 0) { |
| bb_error_msg("packet has 0 queries, ignored"); |
| return 0; /* don't reply */ |
| } |
| if (head->flags & htons(0x8000)) { /* QR bit */ |
| bb_error_msg("response packet, ignored"); |
| return 0; /* don't reply */ |
| } |
| /* QR = 1 "response", RCODE = 4 "Not Implemented" */ |
| outr_flags = htons(0x8000 | 4); |
| err_msg = NULL; |
| |
| /* start of query string */ |
| query_string = (void *)(head + 1); |
| /* caller guarantees strlen is <= MAX_PACK_LEN */ |
| query_len = strlen(query_string) + 1; |
| /* may be unaligned! */ |
| unaligned_type_class = (void *)(query_string + query_len); |
| query_len += sizeof(*unaligned_type_class); |
| /* where to append answer block */ |
| answb = (void *)(unaligned_type_class + 1); |
| |
| /* OPCODE != 0 "standard query"? */ |
| if ((head->flags & htons(0x7800)) != 0) { |
| err_msg = "opcode != 0"; |
| goto empty_packet; |
| } |
| move_from_unaligned16(class, &unaligned_type_class->class); |
| if (class != htons(1)) { /* not class INET? */ |
| err_msg = "class != 1"; |
| goto empty_packet; |
| } |
| move_from_unaligned16(type, &unaligned_type_class->type); |
| if (type != htons(REQ_A) && type != htons(REQ_PTR)) { |
| /* we can't handle this query type */ |
| //TODO: happens all the time with REQ_AAAA (0x1c) requests - implement those? |
| err_msg = "type is !REQ_A and !REQ_PTR"; |
| goto empty_packet; |
| } |
| |
| /* look up the name */ |
| answstr = table_lookup(conf_data, type, query_string); |
| #if DEBUG |
| /* Shows lengths instead of dots, unusable for !DEBUG */ |
| bb_info_msg("'%s'->'%s'", query_string, answstr); |
| #endif |
| outr_rlen = 4; |
| if (answstr && type == htons(REQ_PTR)) { |
| /* returning a host name */ |
| outr_rlen = strlen(answstr) + 1; |
| } |
| if (!answstr |
| || (unsigned)(answb - buf) + query_len + 4 + 2 + outr_rlen > MAX_PACK_LEN |
| ) { |
| /* QR = 1 "response" |
| * AA = 1 "Authoritative Answer" |
| * RCODE = 3 "Name Error" */ |
| err_msg = "name is not found"; |
| outr_flags = htons(0x8000 | 0x0400 | 3); |
| goto empty_packet; |
| } |
| |
| /* Append answer Resource Record */ |
| memcpy(answb, query_string, query_len); /* name, type, class */ |
| answb += query_len; |
| move_to_unaligned32((uint32_t *)answb, htonl(conf_ttl)); |
| answb += 4; |
| move_to_unaligned16((uint16_t *)answb, htons(outr_rlen)); |
| answb += 2; |
| memcpy(answb, answstr, outr_rlen); |
| answb += outr_rlen; |
| |
| /* QR = 1 "response", |
| * AA = 1 "Authoritative Answer", |
| * TODO: need to set RA bit 0x80? One user says nslookup complains |
| * "Got recursion not available from SERVER, trying next server" |
| * "** server can't find HOSTNAME" |
| * RCODE = 0 "success" |
| */ |
| if (OPT_verbose) |
| bb_info_msg("returning positive reply"); |
| outr_flags = htons(0x8000 | 0x0400 | 0); |
| /* we have one answer */ |
| head->nansw = htons(1); |
| |
| empty_packet: |
| if ((outr_flags & htons(0xf)) != 0) { /* not a positive response */ |
| if (OPT_verbose) { |
| bb_error_msg("%s, %s", |
| err_msg, |
| OPT_silent ? "dropping query" : "sending error reply" |
| ); |
| } |
| if (OPT_silent) |
| return 0; |
| } |
| head->flags |= outr_flags; |
| head->nauth = head->nadd = 0; |
| head->nquer = htons(1); // why??? |
| |
| return answb - buf; |
| } |
| |
| int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE; |
| int dnsd_main(int argc UNUSED_PARAM, char **argv) |
| { |
| const char *listen_interface = "0.0.0.0"; |
| const char *fileconf = "/etc/dnsd.conf"; |
| struct dns_entry *conf_data; |
| uint32_t conf_ttl = DEFAULT_TTL; |
| char *sttl, *sport; |
| len_and_sockaddr *lsa, *from, *to; |
| unsigned lsa_size; |
| int udps, opts; |
| uint16_t port = 53; |
| /* Ensure buf is 32bit aligned (we need 16bit, but 32bit can't hurt) */ |
| uint8_t buf[MAX_PACK_LEN + 1] ALIGN4; |
| |
| opts = getopt32(argv, "vsi:c:t:p:d", &listen_interface, &fileconf, &sttl, &sport); |
| //if (opts & (1 << 0)) // -v |
| //if (opts & (1 << 1)) // -s |
| //if (opts & (1 << 2)) // -i |
| //if (opts & (1 << 3)) // -c |
| if (opts & (1 << 4)) // -t |
| conf_ttl = xatou_range(sttl, 1, 0xffffffff); |
| if (opts & (1 << 5)) // -p |
| port = xatou_range(sport, 1, 0xffff); |
| if (opts & (1 << 6)) { // -d |
| bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv); |
| openlog(applet_name, LOG_PID, LOG_DAEMON); |
| logmode = LOGMODE_SYSLOG; |
| } |
| |
| conf_data = parse_conf_file(fileconf); |
| |
| lsa = xdotted2sockaddr(listen_interface, port); |
| udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0); |
| xbind(udps, &lsa->u.sa, lsa->len); |
| socket_want_pktinfo(udps); /* needed for recv_from_to to work */ |
| lsa_size = LSA_LEN_SIZE + lsa->len; |
| from = xzalloc(lsa_size); |
| to = xzalloc(lsa_size); |
| |
| { |
| char *p = xmalloc_sockaddr2dotted(&lsa->u.sa); |
| bb_info_msg("accepting UDP packets on %s", p); |
| free(p); |
| } |
| |
| while (1) { |
| int r; |
| /* Try 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) */ |
| memcpy(to, lsa, lsa_size); |
| r = recv_from_to(udps, buf, MAX_PACK_LEN + 1, 0, &from->u.sa, &to->u.sa, lsa->len); |
| if (r < 12 || r > MAX_PACK_LEN) { |
| bb_error_msg("packet size %d, ignored", r); |
| continue; |
| } |
| if (OPT_verbose) |
| bb_info_msg("got UDP packet"); |
| buf[r] = '\0'; /* paranoia */ |
| r = process_packet(conf_data, conf_ttl, buf); |
| if (r <= 0) |
| continue; |
| send_to_from(udps, buf, r, 0, &from->u.sa, &to->u.sa, lsa->len); |
| } |
| return 0; |
| } |