[dns] support longer key length in otDnsTxtEntryIterator (#9616)

This commit introduces `OT_DNS_TXT_KEY_ITER_MAX_LENGTH` with a value
of `64` to represent the maximum TXT data key length supported by
`otDnsTxtEntryIterator`. We intentionally set this value higher than
the recommended maximum key length of `9` (as specified in RFC 6763
section 6.4) to enable the parsing of TXT data when longer keys are
employed.

include/openthread/dns.h

@@ -62,6 +62,8 @@ extern "C" {
 
 #define OT_DNS_TXT_KEY_MAX_LENGTH 9 ///< Recommended maximum length of TXT record key string (RFC 6763 - section 6.4).
 
+#define OT_DNS_TXT_KEY_ITER_MAX_LENGTH 64 ///< Maximum length of TXT key string supported by `otDnsTxtEntryIterator`.
+
 /**
  * Represents a TXT record entry representing a key/value pair (RFC 6763 - section 6.3).
  *
@@ -104,7 +106,7 @@ typedef struct otDnsTxtEntryIterator
 {
     const void *mPtr;
     uint16_t    mData[2];
-    char        mChar[OT_DNS_TXT_KEY_MAX_LENGTH + 1];
+    char        mChar[OT_DNS_TXT_KEY_ITER_MAX_LENGTH + 1];
 } otDnsTxtEntryIterator;
 
 /**
@@ -127,9 +129,9 @@ void otDnsInitTxtEntryIterator(otDnsTxtEntryIterator *aIterator, const uint8_t *
  * data buffer used to initialize the iterator MUST persist and remain unchanged. Otherwise the behavior of this
  * function is undefined.
  *
- * If the parsed key string length is smaller than or equal to `OT_DNS_TXT_KEY_MAX_LENGTH` (recommended max key length)
- * the key string is returned in `mKey` in @p aEntry. But if the key is longer, then `mKey` is set to NULL and the
- * entire encoded TXT entry string is returned in `mValue` and `mValueLength`.
+ * If the parsed key string length is smaller than or equal to `OT_DNS_TXT_KEY_ITER_MAX_LENGTH` the key string is
+ * returned in `mKey` in @p aEntry. But if the key is longer, then `mKey` is set to NULL and the entire encoded TXT
+ * entry string is returned in `mValue` and `mValueLength`.
  *
  * @param[in]  aIterator   A pointer to the iterator (MUST NOT be NULL).
  * @param[out] aEntry      A pointer to a `otDnsTxtEntry` structure to output the parsed/read entry (MUST NOT be NULL).

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (371)
+#define OPENTHREAD_API_VERSION (373)
 
 /**
  * @addtogroup api-instance

src/core/net/dns_types.cpp

@@ -953,7 +953,7 @@ Error TxtEntry::Iterator::GetNextEntry(TxtEntry &aEntry)
     const char *cur;
     char       *keyBuffer = GetKeyBuffer();
 
-    static_assert(sizeof(mChar) == TxtEntry::kMaxKeyLength + 1, "KeyBuffer cannot fit the max key length");
+    static_assert(sizeof(mChar) >= TxtEntry::kMaxKeyLength + 1, "KeyBuffer cannot fit the max key length");
 
     VerifyOrExit(GetTxtData() != nullptr, error = kErrorParse);
 
@@ -985,9 +985,9 @@ Error TxtEntry::Iterator::GetNextEntry(TxtEntry &aEntry)
                 ExitNow();
             }
 
-            if (index >= kMaxKeyLength)
+            if (index >= sizeof(mChar) - 1)
             {
-                // The key is larger than recommended max key length.
+                // The key is larger than supported key string length.
                 // In this case, we return the full encoded string in
                 // `mValue` and `mValueLength` and set `mKey` to
                 // `nullptr`.

src/core/net/dns_types.hpp

@@ -1056,6 +1056,14 @@ public:
      */
     static constexpr uint8_t kMaxKeyLength = OT_DNS_TXT_KEY_MAX_LENGTH;
 
+    /**
+     * Maximum length of TXT key string supported by `Iterator`.
+     *
+     * This is selected to be longer than recommended `kMaxKeyLength` to handle cases where longer keys are used.
+     *
+     */
+    static constexpr uint8_t kMaxIterKeyLength = OT_DNS_TXT_KEY_ITER_MAX_LENGTH;
+
     /**
      * Represents an iterator for TXT record entries (key/value pairs).
      *
@@ -1083,9 +1091,9 @@ public:
          * The `Iterator` instance MUST be initialized using `Init()` before calling this method and the TXT data
          * buffer used to initialize the iterator MUST persist and remain unchanged.
          *
-         * If the parsed key string length is smaller than or equal to `kMaxKeyLength` (recommended max key length)
-         * the key string is returned in `mKey` in @p aEntry. But if the key is longer, then `mKey` is set to NULL and
-         * the entire encoded TXT entry is returned in `mValue` and `mValueLength`.
+         * If the parsed key string length is smaller than or equal to `kMaxIterKeyLength` the key string is returned
+         * in `mKey` in @p aEntry. But if the key is longer, then `mKey` is set to `nullptr` the entire encoded TXT
+         * entry is returned in `mValue` and `mValueLength`.
          *
          * @param[out] aEntry          A reference to a `TxtEntry` to output the parsed/read entry.
          *

src/core/radio/trel_interface.cpp

@@ -263,7 +263,7 @@ Error Interface::ParsePeerInfoTxtData(const Peer::Info       &aInfo,
     while ((error = iterator.GetNextEntry(entry)) == kErrorNone)
     {
         // If the TXT data happens to have entries with key longer
-        // than `kMaxKeyLength`, `mKey` would be `nullptr` and full
+        // than `kMaxIterKeyLength`, `mKey` would be `nullptr` and full
         // entry would be placed in `mValue`. We skip over such
         // entries.
         if (entry.mKey == nullptr)

tests/unit/test_dns.cpp

@@ -1302,6 +1302,9 @@ void TestDnsTxtEntry(void)
     const char kKey6[] = "boolKey";  // Should be encoded as "boolKey" (without `=`).
     const char kKey7[] = "emptyKey"; // Should be encoded as "emptyKey=".
 
+    const char    kKey8[]   = "1234567890123456789012345678901234567890123456789012345678901234567890";
+    const uint8_t kValue8[] = "abcd";
+
     // Invalid key
     const char kShortKey[] = "";
 
@@ -1312,6 +1315,17 @@ void TestDnsTxtEntry(void)
     const uint8_t kEncodedTxt5[] = {12, '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '=', 'a'};
     const uint8_t kEncodedTxt6[] = {7, 'b', 'o', 'o', 'l', 'K', 'e', 'y'};
     const uint8_t kEncodedTxt7[] = {9, 'e', 'm', 'p', 't', 'y', 'K', 'e', 'y', '='};
+    const uint8_t kEncodedTxt8[] = {
+        75,                                               // length
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 10
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 20
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 30
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 40
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 50
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 60
+        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', // 70
+        '=', 'a', 'b', 'c', 'd',
+    };
 
     const uint8_t kInvalidEncodedTxt1[] = {4, 'a', '=', 'b'}; // Incorrect length
 
@@ -1327,6 +1341,7 @@ void TestDnsTxtEntry(void)
         Dns::TxtEntry(kKey5, kValue5, sizeof(kValue5)),
         Dns::TxtEntry(kKey6, nullptr, 0),
         Dns::TxtEntry(kKey7, kValue1, 0),
+        Dns::TxtEntry(kKey8, kValue8, sizeof(kValue8)),
     };
 
     const EncodedTxtData kEncodedTxtData[] = {
@@ -1381,7 +1396,7 @@ void TestDnsTxtEntry(void)
         SuccessOrQuit(iterator.GetNextEntry(txtEntry), "TxtEntry::GetNextEntry() failed");
         printf("key:\"%s\" valueLen:%d\n", txtEntry.mKey != nullptr ? txtEntry.mKey : "(null)", txtEntry.mValueLength);
 
-        if (expectedKeyLength > Dns::TxtEntry::kMaxKeyLength)
+        if (expectedKeyLength > Dns::TxtEntry::kMaxIterKeyLength)
         {
             // When the key is longer than recommended max key length,
             // the full encoded string is returned in `mValue` and