[routing-manager] ignore checksum in RA hash calculation (#10230)

This commit modifies the `CalculateHash()` method to use a zero
checksum value for RA (ICMPv6) header for both received and emitted
RA message. In prepared RA messages, the checksum is always set to
zero, and the platform layer is responsible for calculating and
updating it. For a received RA, while platforms typically zero out
the checksum after validation, this behavior isn't explicitly
required by `otPlatInfraIf` APIs. By ignoring the checksum(setting it
to zero) during hash calculation, this change ensures correct
calculation regardless of platform behavior.

This commit also updates `test_routing_manager` to intentionally
modify the checksum field in an emitted RA message before passing it
back to the OT stack. This validates the updated hash calculation
behavior.

src/core/border_router/routing_manager.cpp

@@ -3503,10 +3503,10 @@ void RoutingManager::RaInfo::IncrementTxCountAndSaveHash(const InfraIf::Icmp6Pac
         mLastHashIndex = 0;
     }
 
-    CalculateHash(aRaMessage, mHashes[mLastHashIndex]);
+    CalculateHash(RouterAdvert::RxMessage(aRaMessage), mHashes[mLastHashIndex]);
 }
 
-bool RoutingManager::RaInfo::IsRaFromManager(const Ip6::Nd::RouterAdvert::RxMessage &aRaMessage) const
+bool RoutingManager::RaInfo::IsRaFromManager(const RouterAdvert::RxMessage &aRaMessage) const
 {
     // Determines whether or not a received RA message was prepared by
     // by `RoutingManager` itself (is present in the saved `mHashes`).
@@ -3516,7 +3516,7 @@ bool RoutingManager::RaInfo::IsRaFromManager(const Ip6::Nd::RouterAdvert::RxMess
     uint32_t count         = Min<uint32_t>(mTxCount, kNumHashEntries);
     Hash     hash;
 
-    CalculateHash(aRaMessage.GetAsPacket(), hash);
+    CalculateHash(aRaMessage, hash);
 
     for (; count > 0; count--)
     {
@@ -3541,12 +3541,17 @@ bool RoutingManager::RaInfo::IsRaFromManager(const Ip6::Nd::RouterAdvert::RxMess
     return isFromManager;
 }
 
-void RoutingManager::RaInfo::CalculateHash(const InfraIf::Icmp6Packet &aRaMessage, Hash &aHash)
+void RoutingManager::RaInfo::CalculateHash(const RouterAdvert::RxMessage &aRaMessage, Hash &aHash)
 {
-    Crypto::Sha256 sha256;
+    RouterAdvert::Header header;
+    Crypto::Sha256       sha256;
+
+    header = aRaMessage.GetHeader();
+    header.SetChecksum(0);
 
     sha256.Start();
-    sha256.Update(aRaMessage.GetBytes(), aRaMessage.GetLength());
+    sha256.Update(header);
+    sha256.Update(aRaMessage.GetOptionStart(), aRaMessage.GetOptionLength());
     sha256.Finish(aHash);
 }
 

src/core/border_router/routing_manager.hpp

@@ -1259,8 +1259,8 @@ private:
         }
 
         void        IncrementTxCountAndSaveHash(const InfraIf::Icmp6Packet &aRaMessage);
-        bool        IsRaFromManager(const Ip6::Nd::RouterAdvert::RxMessage &aRaMessage) const;
-        static void CalculateHash(const InfraIf::Icmp6Packet &aRaMessage, Hash &aHash);
+        bool        IsRaFromManager(const RouterAdvert::RxMessage &aRaMessage) const;
+        static void CalculateHash(const RouterAdvert::RxMessage &aRaMessage, Hash &aHash);
 
         RouterAdvert::Header mHeader;
         TimeMilli            mHeaderUpdateTime;

src/core/net/nd6.hpp

@@ -729,6 +729,22 @@ public:
          */
         bool ContainsAnyOptions(void) const { return (mData.GetLength() > sizeof(Header)); }
 
+        /**
+         * Returns pointer to the start of option bytes (after header).
+         *
+         * @returns Pointer to start of options.
+         *
+         */
+        const uint8_t *GetOptionStart(void) const { return (mData.GetBytes() + sizeof(Header)); }
+
+        /**
+         * Gets the length (number of bytes) of options.
+         *
+         * @returns Number of bytes after header specifying RA options.
+         *
+         */
+        uint16_t GetOptionLength(void) const { return ContainsAnyOptions() ? mData.GetLength() - sizeof(Header) : 0; }
+
         // The following methods are intended to support range-based `for`
         // loop iteration over `Option`s in the RA message.
 
@@ -736,7 +752,6 @@ public:
         Option::Iterator end(void) const { return Option::Iterator(); }
 
     private:
-        const uint8_t *GetOptionStart(void) const { return (mData.GetBytes() + sizeof(Header)); }
         const uint8_t *GetDataEnd(void) const { return mData.GetBytes() + mData.GetLength(); }
 
         Data<kWithUint16Length> mData;

tests/unit/test_routing_manager.cpp

@@ -295,7 +295,8 @@ otError otPlatInfraIfSendIcmp6Nd(uint32_t            aInfraIfIndex,
                                  const uint8_t      *aBuffer,
                                  uint16_t            aBufferLength)
 {
-    Icmp6Packet packet;
+    Icmp6Packet        packet;
+    Ip6::Icmp::Header *header;
 
     Log("otPlatInfraIfSendIcmp6Nd(aDestAddr: %s, aBufferLength:%u)", AsCoreType(aDestAddress).ToString().AsCString(),
         aBufferLength);
@@ -306,7 +307,9 @@ otError otPlatInfraIfSendIcmp6Nd(uint32_t            aInfraIfIndex,
 
     VerifyOrQuit(aBufferLength >= sizeof(Ip6::Icmp::Header));
 
-    switch (reinterpret_cast<const Ip6::Icmp::Header *>(aBuffer)->GetType())
+    header = reinterpret_cast<Ip6::Icmp::Header *>(const_cast<uint8_t *>(aBuffer));
+
+    switch (header->GetType())
     {
     case Ip6::Icmp::Header::kTypeRouterSolicit:
         Log("  Router Solicit message");
@@ -317,6 +320,9 @@ otError otPlatInfraIfSendIcmp6Nd(uint32_t            aInfraIfIndex,
         Log("  Router Advertisement message");
         LogRouterAdvert(packet);
         ValidateRouterAdvert(packet);
+        // Intentionally modify the checksum field in RA Header
+        // before passing it back to the OT stack.
+        header->SetChecksum(0x1234);
         otPlatInfraIfRecvIcmp6Nd(sInstance, kInfraIfIndex, &sInfraIfAddress, aBuffer, aBufferLength);
         break;