dnsd: fix a number of bugs. Ideas by Ming-Ching Tiew (mctiew AT yahoo.com)

function                                             old     new   delta
undot                                                  -      42     +42
dnsd_main                                           1394    1213    -181
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 42/-181)          Total: -139 bytes

1 files changed, 325 insertions, 163 deletions

diff --git a/networking/dnsd.c b/networking/dnsd.c
index f95ba6b..4f980dc 100644
--- a/networking/dnsd.c
+++ b/networking/dnsd.c
@@ -24,73 +24,63 @@
 #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
-*/
+	/* Can tweak this */
+	DEFAULT_TTL = 120,
+
+/* 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 Bytes
+ */
 	MAX_PACK_LEN = 512,
-
-	DEFAULT_TTL = 30,       // increase this when not testing?
-
+	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
+	REQ_PTR = 12,
 };
 
-struct dns_head {		// the message from client and first part of response mag
+/* the message from client and first part of response msg */
+struct dns_head {
 	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
+	uint16_t nquer;
+	uint16_t nansw;
+	uint16_t nauth;
+	uint16_t nadd;
 };
 struct dns_prop {
 	uint16_t type;
 	uint16_t class;
 };
-struct dns_entry {		// element of known name, ip address and reversed ip address
+/* element of known name, ip address and reversed ip address */
+struct dns_entry {
 	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];
+	uint32_t ip;
+	char rip[IP_STRING_LEN]; /* length decimal reversed IP */
+	char name[1];
 };
 
 #define OPT_verbose (option_mask32)
 
 
 /*
- * 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)
+static void undot(char *rip)
 {
-	int i = 0, s = 0;
+	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++;
+		} else {
+			s++;
+		}
 	}
 }
 
@@ -101,199 +91,369 @@ static struct dns_entry *parse_conf_file(const char *fileconf)
 {
 	char *token[2];
 	parser_t *parser;
-	struct dns_entry *m, *prev, *conf_data;
+	struct dns_entry *m, *conf_data;
+	struct dns_entry **nextp;
+
+	conf_data = NULL;
+	nextp = &conf_data;
 
-	prev = conf_data = NULL;
 	parser = config_open(fileconf);
 	while (config_read(parser, token, 2, 2, "# \t", PARSE_NORMAL)) {
-		unsigned a, b, c, d;
-		/*
-		 * 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.
-		 */
-//		if (!token[1] || sscanf(token[1], ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4)
-		if (sscanf(token[1], ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4)
-			continue;
+		struct in_addr ip;
+		uint32_t v32;
 
-		m = xzalloc(sizeof(*m));
-		/*m->next = NULL;*/
-		sprintf(m->ip, ".%u.%u.%u.%u"+1, a, b, c, d);
-		sprintf(m->rip, ".%u.%u.%u.%u", d, c, b, a);
-		undot((uint8_t*)m->rip);
-		convname(m->name, (uint8_t*)token[0]);
+		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", &(m->name[1]), m->ip);
+			bb_error_msg("name:%s, ip:%s", token[0], token[1]);
 
-		if (prev == NULL)
-			conf_data = m;
-		else
-			prev->next = m;
-		prev = m;
+		/* 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
- * qs is the query string, first byte the string length
+ * Look query up in dns records and return answer if found.
+ * qs is the query string.
  */
-static int table_lookup(struct dns_entry *d, uint16_t type, uint8_t *as, uint8_t *qs)
+static int table_lookup(uint8_t *as, struct dns_entry *d, uint16_t type, uint8_t *qs)
 {
-	int i;
-
-	do {
+	while (d) {
+		unsigned len = d->name[0];
+		/* d->name[len] is the last (non NUL) char */
 #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));
+		fprintf(stderr, "%d/%d p:%s q:%s %d\n",
+			(int)strlen(p), len,
+			p, q, (int)strlen(q)
+		);
 #endif
-		if (type == REQ_A) {
+		if (type == htons(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])
-			 || (d->name[0] == 1 && d->name[1] == '*')
-			) {
-				strcpy((char *)as, d->ip);
+			if (len != 1 || d->name[1] != '*') {
+				if (strcasecmp(d->name, (char*)qs) != 0)
+					goto next;
+			}
+			move_to_unaligned32((uint32_t *)as, d->ip);
 #if DEBUG
-				fprintf(stderr, " OK as:%s\n", as);
+			fprintf(stderr, "OK as:%x\n", (int)d->ip);
 #endif
-				return 0;
-			}
-		} else if (type == REQ_PTR) {
-			/* search by IP-address */
-			if ((d->name[0] != 1 || d->name[1] != '*')
-			 && !strncmp(d->rip + 1, (char*)qs + 1, strlen(d->rip)-1)
-			) {
-				strcpy((char *)as, d->name);
-				return 0;
-			}
+			return 0;
 		}
+		/* search by IP-address */
+		if ((len != 1 || d->name[1] != '*')
+		/* assume (do not check) that qs ends in ".in-addr.arpa" */
+		 && strncmp(d->rip, (char*)qs, strlen(d->rip)) == 0
+		) {
+			strcpy((char *)as, d->name);
+#if DEBUG
+			fprintf(stderr, "OK as:%s\n", as);
+#endif
+			return 0;
+		}
+ next:
 		d = d->next;
-	} while (d);
+	}
+
 	return -1;
 }
 
 /*
  * 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
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      ID                       |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |QR|   OPCODE  |AA|TC|RD|RA|   Z    |   RCODE   |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    QDCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ANCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    NSCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ARCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ID      16 bit random identifier assigned by query.
+        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,
+        and specifies that the responding name server is an
+        authority for the domain name in question section.
+        Note that the contents of the 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 - specifies that 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.
+Z       Reserved for future use.  Must be zero.
+RCODE   Response code.
+        0   No error condition
+        1   Format error
+        2   Server failure - The name server was
+            unable to process this query due to a
+            problem with the name server.
+        3   Name Error - Meaningful only for
+            responses from an authoritative name
+            server, this code signifies that the
+            domain name referenced in the query does
+            not exist.
+        4   Not Implemented.
+        5   Refused.
+QDCOUNT number of entries in the question section.
+ANCOUNT number of resource records in the answer section.
+NSCOUNT number of name server resource records in the authority records section.
+ARCOUNT number of resource records in the additional records section.
+
+4.1.2. Question section format
+
+The section contains QDCOUNT (usually 1) entries, each of the following format:
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                     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 the following format:
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                                               /
+    /                      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 resource record may be
+        cached before it should be discarded.  Zero values are
+        interpreted to mean that the RR can only be used for the
+        transaction in progress, and should not be cached.
+RDLENGTH an unsigned 16 bit integer that specifies the 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.
+        For example, if the TYPE is A and the CLASS is IN,
+        the RDATA field is a 4 octet ARPA Internet address.
+
+4.1.4. Message compression
+
+In order to reduce the size of messages, the domain system utilizes a
+compression scheme which eliminates the repetition of domain names in a
+message.  In this scheme, 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.
+
+The compression scheme allows a domain name in a message to 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)
 {
 	uint8_t answstr[MAX_NAME_LEN + 1];
 	struct dns_head *head;
-	struct dns_prop *qprop;
+	struct dns_prop *unaligned_qprop;
 	uint8_t *from, *answb;
 	uint16_t outr_rlen;
 	uint16_t outr_flags;
-	uint16_t flags;
-	int lookup_result, type, packet_len;
+	uint16_t type;
+	uint16_t class;
 	int querystr_len;
 
 	answstr[0] = '\0';
 
 	head = (struct dns_head *)buf;
 	if (head->nquer == 0) {
-		bb_error_msg("no queries");
+		bb_error_msg("packet has 0 queries, ignored");
 		return -1;
 	}
 
-	if (head->flags & 0x8000) {
-		bb_error_msg("ignoring response packet");
+	if (head->flags & htons(0x8000)) { /* QR bit */
+		bb_error_msg("response packet, ignored");
 		return -1;
 	}
 
-	from = (void *)&head[1];	//  start of query string
-//FIXME: strlen of untrusted data??!
-	querystr_len = strlen((char *)from) + 1 + sizeof(struct dns_prop);
-	answb = from + querystr_len;   // where to append answer block
+	/* start of query string */
+	from = (void *)(head + 1);
+	/* caller guarantees strlen is <= MAX_PACK_LEN */
+	querystr_len = strlen((char *)from) + 1;
+	/* may be unaligned! */
+	unaligned_qprop = (void *)(from + querystr_len);
+	/* where to append answer block */
+	answb = (void *)(unaligned_qprop + 1);
 
 	outr_rlen = 0;
-	outr_flags = 0;
-
-	qprop = (struct dns_prop *)(answb - 4);
-	type = ntohs(qprop->type);
+	/* QR = 1 "response", RCODE = 4 "Not Implemented" */
+	outr_flags = htons(0x8000 | 4);
 
-	// 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 */
+	move_from_unaligned16(type, &unaligned_qprop->type);
+	if (type != htons(REQ_A) && type != htons(REQ_PTR)) {
+		/* we can't handle the query type */
+		goto empty_packet;
 	}
-
-	if (ntohs(qprop->class) != 1 /* class INET */ ) {
-		outr_flags = 4; /* not supported */
+	move_from_unaligned16(class, &unaligned_qprop->class);
+	if (class != htons(1)) { /* not class INET? */
 		goto empty_packet;
 	}
-	/* we only support standard queries */
-
-	if ((ntohs(head->flags) & 0x7800) != 0)
+	/* OPCODE != 0 "standard query" ? */
+	if ((head->flags & htons(0x7800)) != 0) {
 		goto empty_packet;
+	}
 
-	// We have a standard query
 	bb_info_msg("%s", (char *)from);
-	lookup_result = table_lookup(conf_data, type, answstr, from);
-	if (lookup_result != 0) {
-		outr_flags = 3 | 0x0400;	// name do not exist and auth
+	if (table_lookup(answstr, conf_data, type, from) != 0) {
+		/* QR = 1 "response"
+		 * AA = 1 "Authoritative Answer"
+		 * RCODE = 3 "Name Error" */
+		outr_flags = htons(0x8000 | 0x0400 | 3);
 		goto empty_packet;
 	}
-	if (type == REQ_A) {    // return an address
-		struct in_addr a; // NB! its "struct { unsigned __long__ s_addr; }"
-		uint32_t v32;
-		if (!inet_aton((char*)answstr, &a)) { //dotted dec to long conv
-			outr_flags = 1; /* Frmt err */
-			goto empty_packet;
-		}
-		v32 = a.s_addr; /* in case long != int */
-		move_to_unaligned32(answstr, v32);
-		outr_rlen = 4;			// uint32_t IP
-	} else
-		outr_rlen = strlen((char *)answstr) + 1;	// a host name
-	outr_flags |= 0x0400;			/* authority-bit */
-	// we have an answer
+	/* return an address */
+	outr_rlen = 4;
+	if (type == htons(REQ_PTR)) {
+		/* return a host name */
+		outr_rlen = strlen((char *)answstr) + 1;
+	}
+	/* QR = 1 "response",
+	 * AA = 1 "Authoritative Answer",
+	 * RCODE = 0 "success" */
+	outr_flags = htons(0x8000 | 0x0400 | 0);
+	/* we have one answer */
 	head->nansw = htons(1);
-
-	// copy query block to answer block
+	/* copy query block to answer block */
+	querystr_len += sizeof(unaligned_qprop);
 	memcpy(answb, from, querystr_len);
 	answb += querystr_len;
-
-	// and append answer rr
-// FIXME: unaligned accesses??
-	*(uint32_t *) answb = htonl(conf_ttl);
+	/* append answer Resource Record */
+	move_to_unaligned32((uint32_t *)answb, htonl(conf_ttl));
 	answb += 4;
-	*(uint16_t *) answb = htons(outr_rlen);
+	move_to_unaligned32((uint16_t *)answb, htons(outr_rlen));
 	answb += 2;
 	memcpy(answb, answstr, outr_rlen);
 	answb += 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->flags |= outr_flags;
 	head->nauth = head->nadd = 0;
-	head->nquer = htons(1);
+	head->nquer = htons(1); // why???
 
-	packet_len = answb - buf;
-	return packet_len;
+	return answb - buf;
 }
 
-/*
- * Exit on signal
- */
-//static void interrupt(int sig)
-//{
-//	/* unlink("/var/run/dnsd.lock"); */
-//	bb_error_msg("interrupt, exiting\n");
-//	kill_myself_with_sig(sig);
-//}
-
 int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dnsd_main(int argc UNUSED_PARAM, char **argv)
 {
@@ -328,7 +488,6 @@ int dnsd_main(int argc UNUSED_PARAM, char **argv)
 
 	conf_data = parse_conf_file(fileconf);
 
-//	signal(SIGINT, interrupt); - just for one message?
 	bb_signals(0
 		/* why? + (1 << SIGPIPE) */
 		+ (1 << SIGHUP)
@@ -348,8 +507,11 @@ int dnsd_main(int argc UNUSED_PARAM, char **argv)
 	from = xzalloc(lsa_size);
 	to = xzalloc(lsa_size);
 
-	bb_info_msg("Accepting UDP packets on %s",
-			xmalloc_sockaddr2dotted(&lsa->u.sa));
+	{
+		char *p = xmalloc_sockaddr2dotted(&lsa->u.sa);
+		bb_info_msg("Accepting UDP packets on %s", p);
+		free(p);
+	}
 
 	while (1) {
 		int r;
@@ -361,7 +523,7 @@ int dnsd_main(int argc UNUSED_PARAM, char **argv)
 		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("invalid packet size");
+			bb_error_msg("packet size %d, ignored", r);
 			continue;
 		}
 		if (OPT_verbose)