networking/dnsd.c - codeaurora/busybox - Gitiles

 /* 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 <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;
 };
 struct type_and_class {
 	uint16_t type;
 	uint16_t class;
 };
 /* 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_error_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" */
 		 && strncmp(d->rip, query_string, strlen(d->rip)) == 0
 		) {
 #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 occurance 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_error_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_error_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;
 	uint8_t buf[MAX_PACK_LEN + 1];

 	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_error_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_error_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;
 }
	/* 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 <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;
	};
	struct type_and_class {
	uint16_t type;
	uint16_t class;
	};
	/* 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_error_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" */
	&& strncmp(d->rip, query_string, strlen(d->rip)) == 0
	) {
	#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 occurance 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_error_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_error_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;
	uint8_t buf[MAX_PACK_LEN + 1];

	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_error_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_error_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;
	}