[posix] DNS resolver to handle link-local server address (openthread#11548)

When `sendto` is used with a Link-Local IPv6 destination, the index of
the outgoing network interface must be included.

Author: yangsong-cnyn

src/posix/platform/resolver.cpp

@@ -231,8 +231,8 @@ otError Resolver::SendQueryToServer(Transaction        *aTxn,
 {
     otError      error = OT_ERROR_NONE;
     otIp4Address ip4Addr;
-    sockaddr_in  serverAddr4;
-    sockaddr_in6 serverAddr6;
+    sockaddr_in  serverAddr4 = {};
+    sockaddr_in6 serverAddr6 = {};
 
     if (otIp4FromIp4MappedIp6Address(&aServerAddress, &ip4Addr) == OT_ERROR_NONE)
     {
@@ -249,6 +249,11 @@ otError Resolver::SendQueryToServer(Transaction        *aTxn,
         memcpy(&serverAddr6.sin6_addr, &aServerAddress, sizeof(otIp6Address));
         serverAddr6.sin6_family = AF_INET6;
         serverAddr6.sin6_port   = htons(53);
+        if (IsIp6AddressLinkLocal(aServerAddress))
+        {
+            // Network interface index is required for link local destinations
+            serverAddr6.sin6_scope_id = otSysGetInfraNetifIndex();
+        }
 
         VerifyOrExit(sendto(aTxn->mUdpFd6, aPacket, aLength, MSG_DONTWAIT, reinterpret_cast<sockaddr *>(&serverAddr6),
                             sizeof(serverAddr6)) > 0,