Is there any free software that will detect the status of internet and shutdown the system when the internet disconnects?

Or (more generally) can something run a task when the internet disconnects, and I can route it to shutdown the machine?

I am downloading things overnight. But my problem is that sometimes the internet connection will be disconnected in the middle of the night, so there is no point in keeping the system running the rest of the night.

So I need to avoid this problem by detecting the net connectivity.