net: dns: Make dns_unpack_answer() to check non-compressed answers

Modify dns_unpack_answer() function to check if the answer is
compressed or not, and return correct values regardless.

Fixes #16594

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

subsys/net/lib/dns/dns_pack.c

@@ -74,6 +74,8 @@ static inline void set_dns_msg_response(struct dns_msg_t *dns_msg, int type,
 
 int dns_unpack_answer(struct dns_msg_t *dns_msg, int dname_ptr, u32_t *ttl)
 {
+	int dname_len = DNS_COMMON_UINT_SIZE;
+	int i = 0;
 	u16_t buf_size;
 	u16_t pos;
 	u16_t len;
@@ -82,14 +84,33 @@ int dns_unpack_answer(struct dns_msg_t *dns_msg, int dname_ptr, u32_t *ttl)
 
 	answer = dns_msg->msg + dns_msg->answer_offset;
 
-	if (answer[0] < DNS_LABEL_MAX_SIZE) {
-		return -ENOMEM;
-	}
+	if (answer[i] == 0xc0 && answer[i + 1] == 0x0c) {
+	check_pointer:
+		if (answer[i] < DNS_LABEL_MAX_SIZE) {
+			return -ENOMEM;
+		}
 
-	/* Recovery of the pointer value */
-	ptr = (((answer[0] & DNS_LABEL_MAX_SIZE) << 8) + answer[1]);
-	if (ptr != dname_ptr) {
-		return -ENOMEM;
+		/* Recovery of the pointer value */
+		ptr = ((answer[i] & DNS_LABEL_MAX_SIZE) << 8) + answer[i + 1];
+		if (ptr != dname_ptr) {
+			return -ENOMEM;
+		}
+	} else {
+		dname_len = answer[i++] + 1;
+		while (answer[i]) {
+			if (answer[i] == 0xc0 && answer[i + 1] == 0x0c) {
+				dname_len += DNS_COMMON_UINT_SIZE;
+				goto check_pointer;
+			}
+
+			if (answer[i] < DNS_LABEL_MAX_SIZE) {
+				dname_len += answer[i] + 1;
+			}
+
+			i++;
+		}
+
+		dname_len++;
 	}
 
 	/*
@@ -111,16 +132,20 @@ int dns_unpack_answer(struct dns_msg_t *dns_msg, int dname_ptr, u32_t *ttl)
 	/* Only DNS_CLASS_IN answers
 	 * Here we use 2 as an offset because a ptr uses only 2 bytes.
 	 */
-	if (dns_answer_class(DNS_COMMON_UINT_SIZE, answer) != DNS_CLASS_IN) {
+	if (dns_answer_class(dname_len, answer) != DNS_CLASS_IN) {
 		return -EINVAL;
 	}
 
 	/* TTL value */
-	*ttl = dns_answer_ttl(DNS_COMMON_UINT_SIZE, answer);
-	pos = dns_msg->answer_offset + DNS_ANSWER_MIN_SIZE;
-	len = dns_answer_rdlength(DNS_COMMON_UINT_SIZE, answer);
-
-	switch (dns_answer_type(DNS_COMMON_UINT_SIZE, answer)) {
+	*ttl = dns_answer_ttl(dname_len, answer);
+	len = dns_answer_rdlength(dname_len, answer);
+	pos = dns_msg->answer_offset + dname_len +
+		DNS_COMMON_UINT_SIZE + /* class length */
+		DNS_COMMON_UINT_SIZE + /* type length */
+		DNS_TTL_LEN +
+		DNS_RDLENGTH_LEN;
+
+	switch (dns_answer_type(dname_len, answer)) {
 	case DNS_RR_TYPE_A:
 	case DNS_RR_TYPE_AAAA:
 		set_dns_msg_response(dns_msg, DNS_RESPONSE_IP, pos, len);