Indeed that's an idea but unfortunately there's no notion of "connection" for UDP so it's not possible to know if a port is filtered or if a service is simply not responding because it waits for a specific message for example. There is just one obvious case when the port is "officially" closed and so a response is sent to tell you that. Which kind of service would you like to monitor? would it require to send a specific packet? would it need to check for a response? Is just knowing that no response is received (not knowing if it's filtered or open) enough for your use-case?