[ip6] add multicast forwarding restrictions to host (#9863)

On Android, the platform doesn't restrict link-local/mesh-local source
addresses when forwarding multicast packets. For multicast packets sent
from link-local/mesh-local address to scope larger than realm-local,
set the hoplimit to 1 so the packet can be delivered to host, and will
not be forwarded to infrastructure link.

The feature is guarded by OPENTHREAD_CONFIG_IP6_RESTRICT_FORWARDING_LARGER_SCOPE_MCAST_WITH_LOCAL_SRC,
set as 1 to enable.

src/core/config/ip6.h

@@ -227,6 +227,16 @@
 #define OPENTHREAD_CONFIG_IP6_BR_COUNTERS_ENABLE OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_IP6_RESTRICT_FORWARDING_LARGER_SCOPE_MCAST_WITH_LOCAL_SRC
+ *
+ * Define as 1 to restrict multicast forwarding to larger scope from local sources.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_IP6_RESTRICT_FORWARDING_LARGER_SCOPE_MCAST_WITH_LOCAL_SRC
+#define OPENTHREAD_CONFIG_IP6_RESTRICT_FORWARDING_LARGER_SCOPE_MCAST_WITH_LOCAL_SRC 0
+#endif
+
 /**
  * @}
  *

src/core/net/ip6.cpp

@@ -1037,6 +1037,19 @@ Error Ip6::PassToHost(OwnedPtr<Message> &aMessagePtr,
     }
 #endif
 
+#if OPENTHREAD_CONFIG_IP6_RESTRICT_FORWARDING_LARGER_SCOPE_MCAST_WITH_LOCAL_SRC
+    // Some platforms (e.g. Android) currently doesn't restrict link-local/mesh-local source
+    // addresses when forwarding multicast packets.
+    // For a multicast packet sent from link-local/mesh-local address to scope larger
+    // than realm-local, set the hop limit to 1 before sending to host, so this packet
+    // will not be forwarded by host.
+    if (aMessageInfo.GetSockAddr().IsMulticastLargerThanRealmLocal() &&
+        (aMessageInfo.GetPeerAddr().IsLinkLocal() || (Get<Mle::Mle>().IsMeshLocalAddress(aMessageInfo.GetPeerAddr()))))
+    {
+        messagePtr->Write<uint8_t>(Header::kHopLimitFieldOffset, 1);
+    }
+#endif
+
     // Pass message to callback transferring its ownership.
     mReceiveIp6DatagramCallback.Invoke(messagePtr.Release());