[dns] allow partial read of TXT data and add config for max size in CLI (#8232)

This commit updates `Dns::Client` so that when reading service info
`otDnsServiceInfo` if the received TXT data does not fit in the given
buffer, it is read partially. A new field `mTxtDataTruncated` is added
in `otDnsServiceInfo` to indicate that TXT data was not fully read.

This commit also updates `dns` related commands in CLI so to output
the partially read TXT data and add a new config specifying the max
TXT data size used by CLI.

include/openthread/dns_client.h

@@ -323,7 +323,8 @@ typedef struct otDnsServiceInfo
     otIp6Address mHostAddress;        ///< The host IPv6 address. Set to all zero if not available.
     uint32_t     mHostAddressTtl;     ///< The host address TTL.
     uint8_t *    mTxtData;            ///< Buffer to output TXT data (can be NULL if not needed).
-    uint16_t     mTxtDataSize;        ///< On input, size of `mTxtData` buffer. On output `mTxtData` length.
+    uint16_t     mTxtDataSize;        ///< On input, size of `mTxtData` buffer. On output number bytes written.
+    bool         mTxtDataTruncated;   ///< Indicates if TXT data could not fit in `mTxtDataSize` and was truncated.
     uint32_t     mTxtDataTtl;         ///< The TXT data TTL.
 } otDnsServiceInfo;
 
@@ -408,6 +409,7 @@ otError otDnsBrowseResponseGetServiceInstance(const otDnsBrowseResponse *aRespon
  * - If no matching SRV record is found in @p aResponse, `OT_ERROR_NOT_FOUND` is returned.
  * - If a matching SRV record is found in @p aResponse, @p aServiceInfo is updated and `OT_ERROR_NONE` is returned.
  * - If no matching TXT record is found in @p aResponse, `mTxtDataSize` in @p aServiceInfo is set to zero.
+ * - If TXT data length is greater than `mTxtDataSize`, it is read partially and `mTxtDataTruncated` is set to true.
  * - If no matching AAAA record is found in @p aResponse, `mHostAddress is set to all zero or unspecified address.
  * - If there are multiple AAAA records for the host name in @p aResponse, `mHostAddress` is set to the first one. The
  *   other addresses can be retrieved using `otDnsBrowseResponseGetHostAddress()`.
@@ -537,6 +539,7 @@ otError otDnsServiceResponseGetServiceName(const otDnsServiceResponse *aResponse
  * - If no matching SRV record is found in @p aResponse, `OT_ERROR_NOT_FOUND` is returned.
  * - If a matching SRV record is found in @p aResponse, @p aServiceInfo is updated and `OT_ERROR_NONE` is returned.
  * - If no matching TXT record is found in @p aResponse, `mTxtDataSize` in @p aServiceInfo is set to zero.
+ * - If TXT data length is greater than `mTxtDataSize`, it is read partially and `mTxtDataTruncated` is set to true.
  * - If no matching AAAA record is found in @p aResponse, `mHostAddress is set to all zero or unspecified address.
  * - If there are multiple AAAA records for the host name in @p aResponse, `mHostAddress` is set to the first one. The
  *   other addresses can be retrieved using `otDnsServiceResponseGetHostAddress()`.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (247)
+#define OPENTHREAD_API_VERSION (248)
 
 /**
  * @addtogroup api-instance

src/cli/cli.cpp

@@ -3023,7 +3023,18 @@ void Interpreter::OutputDnsServiceInfo(uint8_t aIndentSize, const otDnsServiceIn
     OutputIp6Address(aServiceInfo.mHostAddress);
     OutputLine(" TTL:%u", aServiceInfo.mHostAddressTtl);
     OutputFormat(aIndentSize, "TXT:");
-    OutputDnsTxtData(aServiceInfo.mTxtData, aServiceInfo.mTxtDataSize);
+
+    if (!aServiceInfo.mTxtDataTruncated)
+    {
+        OutputDnsTxtData(aServiceInfo.mTxtData, aServiceInfo.mTxtDataSize);
+    }
+    else
+    {
+        OutputFormat("[");
+        OutputBytes(aServiceInfo.mTxtData, aServiceInfo.mTxtDataSize);
+        OutputFormat("...]");
+    }
+
     OutputLine(" TTL:%u", aServiceInfo.mTxtDataTtl);
 }
 
@@ -3036,7 +3047,7 @@ void Interpreter::HandleDnsBrowseResponse(otError aError, const otDnsBrowseRespo
 {
     char             name[OT_DNS_MAX_NAME_SIZE];
     char             label[OT_DNS_MAX_LABEL_SIZE];
-    uint8_t          txtBuffer[255];
+    uint8_t          txtBuffer[kMaxTxtDataSize];
     otDnsServiceInfo serviceInfo;
 
     IgnoreError(otDnsBrowseResponseGetServiceName(aResponse, name, sizeof(name)));
@@ -3078,7 +3089,7 @@ void Interpreter::HandleDnsServiceResponse(otError aError, const otDnsServiceRes
 {
     char             name[OT_DNS_MAX_NAME_SIZE];
     char             label[OT_DNS_MAX_LABEL_SIZE];
-    uint8_t          txtBuffer[255];
+    uint8_t          txtBuffer[kMaxTxtDataSize];
     otDnsServiceInfo serviceInfo;
 
     IgnoreError(otDnsServiceResponseGetServiceName(aResponse, label, sizeof(label), name, sizeof(name)));

src/cli/cli.hpp

@@ -271,6 +271,8 @@ private:
     static constexpr uint32_t kNetworkDiagnosticTimeoutMsecs = 5000;
     static constexpr uint32_t kLocateTimeoutMsecs            = 2500;
 
+    static constexpr uint16_t kMaxTxtDataSize = OPENTHREAD_CONFIG_CLI_TXT_RECORD_MAX_SIZE;
+
     using Command = CommandEntry<Interpreter>;
 
     template <typename ValueType> using GetHandler         = ValueType (&)(otInstance *);

src/cli/cli_config.h

@@ -126,4 +126,17 @@
 #define OPENTHREAD_CONFIG_CLI_PROMPT_ENABLE 1
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_CLI_TXT_RECORD_MAX_SIZE
+ *
+ * Specifies the max TXT record data length to use when performing DNS queries.
+ *
+ * If the service TXT record data length is greater than the specified value, it will be read partially (up to the given
+ * size) and output as a sequence of raw hex bytes `[{hex-bytes}...]`
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_CLI_TXT_RECORD_MAX_SIZE
+#define OPENTHREAD_CONFIG_CLI_TXT_RECORD_MAX_SIZE 512
+#endif
+
 #endif // CONFIG_CLI_H_

src/core/net/dns_client.cpp

@@ -269,11 +269,10 @@ Error Client::Response::FindServiceInfo(Section aSection, const Name &aName, Ser
     {
         AsCoreType(&aServiceInfo.mHostAddress).Clear();
         aServiceInfo.mHostAddressTtl = 0;
+        error                        = kErrorNone;
     }
-    else
-    {
-        SuccessOrExit(error);
-    }
+
+    SuccessOrExit(error);
 
     // A null `mTxtData` indicates that caller does not want to retrieve TXT data.
     VerifyOrExit(aServiceInfo.mTxtData != nullptr);
@@ -282,19 +281,30 @@ Error Client::Response::FindServiceInfo(Section aSection, const Name &aName, Ser
     // setting `aServiceInfo.mTxtDataSize` to zero.
 
     SelectSection(aSection, offset, numRecords);
+
+    aServiceInfo.mTxtDataTruncated = false;
+
     error = ResourceRecord::FindRecord(*mMessage, offset, numRecords, /* aIndex */ 0, aName, txtRecord);
 
     switch (error)
     {
     case kErrorNone:
-        SuccessOrExit(error =
-                          txtRecord.ReadTxtData(*mMessage, offset, aServiceInfo.mTxtData, aServiceInfo.mTxtDataSize));
+        error = txtRecord.ReadTxtData(*mMessage, offset, aServiceInfo.mTxtData, aServiceInfo.mTxtDataSize);
+
+        if (error == kErrorNoBufs)
+        {
+            error                          = kErrorNone;
+            aServiceInfo.mTxtDataTruncated = true;
+        }
+
+        SuccessOrExit(error);
         aServiceInfo.mTxtDataTtl = txtRecord.GetTtl();
         break;
 
     case kErrorNotFound:
         aServiceInfo.mTxtDataSize = 0;
         aServiceInfo.mTxtDataTtl  = 0;
+        error                     = kErrorNone;
         break;
 
     default:

src/core/net/dns_types.cpp

@@ -1136,12 +1136,13 @@ Error TxtRecord::ReadTxtData(const Message &aMessage,
 {
     Error error = kErrorNone;
 
-    VerifyOrExit(GetLength() <= aTxtBufferSize, error = kErrorNoBufs);
-    SuccessOrExit(error = aMessage.Read(aOffset, aTxtBuffer, GetLength()));
-    VerifyOrExit(VerifyTxtData(aTxtBuffer, GetLength(), /* aAllowEmpty */ true), error = kErrorParse);
-    aTxtBufferSize = GetLength();
+    SuccessOrExit(error = aMessage.Read(aOffset, aTxtBuffer, Min(GetLength(), aTxtBufferSize)));
     aOffset += GetLength();
 
+    VerifyOrExit(GetLength() <= aTxtBufferSize, error = kErrorNoBufs);
+    aTxtBufferSize = GetLength();
+    VerifyOrExit(VerifyTxtData(aTxtBuffer, aTxtBufferSize, /* aAllowEmpty */ true), error = kErrorParse);
+
 exit:
     return error;
 }

src/core/net/dns_types.hpp

@@ -1692,7 +1692,8 @@ public:
     /**
      * This method parses and reads the TXT record data from a message.
      *
-     * This method also checks if the TXT data is well-formed by calling `VerifyTxtData()`.
+     * This method also checks if the TXT data is well-formed by calling `VerifyTxtData()` when it is successfully
+     * read.
      *
      * @param[in]      aMessage         The message to read from.
      * @param[in,out]  aOffset          On input, the offset in @p aMessage to start of TXT record data.
@@ -1705,7 +1706,9 @@ public:
      * @retval kErrorNone           The TXT data was read successfully. @p aOffset, @p aTxtBuffer and @p aTxtBufferSize
      *                              are updated.
      * @retval kErrorParse          The TXT record in @p aMessage could not be parsed (invalid format).
-     * @retval kErrorNoBufs         TXT data could not fit in @p aTxtBufferSize bytes.
+     * @retval kErrorNoBufs         TXT data could not fit in @p aTxtBufferSize bytes. TXT data is still partially read
+     *                              into @p aTxtBuffer up to its size and @p aOffset is updated to skip over the full
+     *                              TXT record.
      *
      */
     Error ReadTxtData(const Message &aMessage, uint16_t &aOffset, uint8_t *aTxtBuffer, uint16_t &aTxtBufferSize) const;

tests/unit/test_dns.cpp

@@ -1021,6 +1021,8 @@ void TestHeaderAndResourceRecords(void)
 
     for (const char *instanceName : kInstanceNames)
     {
+        uint16_t savedOffset;
+
         // SRV record
         SuccessOrQuit(Dns::Name::CompareName(*message, offset, instanceName));
         SuccessOrQuit(Dns::ResourceRecord::ReadRecord(*message, offset, srvRecord));
@@ -1037,12 +1039,21 @@ void TestHeaderAndResourceRecords(void)
         SuccessOrQuit(Dns::Name::CompareName(*message, offset, instanceName));
         SuccessOrQuit(Dns::ResourceRecord::ReadRecord(*message, offset, txtRecord));
         VerifyOrQuit(txtRecord.GetTtl() == kTxtTtl);
-        len = sizeof(buffer);
+        savedOffset = offset;
+        len         = sizeof(buffer);
         SuccessOrQuit(txtRecord.ReadTxtData(*message, offset, buffer, len));
         VerifyOrQuit(len == sizeof(kTxtData));
         VerifyOrQuit(memcmp(buffer, kTxtData, len) == 0);
         printf("    \"%s\" TXT %u %d \"%s\"\n", instanceName, txtRecord.GetTtl(), txtRecord.GetLength(),
                reinterpret_cast<const char *>(buffer));
+
+        // Partial read of TXT data
+        len = sizeof(kTxtData) - 1;
+        memset(buffer, 0, sizeof(buffer));
+        VerifyOrQuit(txtRecord.ReadTxtData(*message, savedOffset, buffer, len) == kErrorNoBufs);
+        VerifyOrQuit(len == sizeof(kTxtData) - 1);
+        VerifyOrQuit(memcmp(buffer, kTxtData, len) == 0);
+        VerifyOrQuit(savedOffset == offset);
     }
 
     SuccessOrQuit(Dns::Name::CompareName(*message, offset, kHostName));