Blocking a program it the VPN tunnel goes down

I have a small problem.
I know what I want but I have no idea on how to get there.

What I would like to do is to set-up a VPN tunnel from a jail where I would run one program (torrent). The program should not be able to access the net if the tunnel goes down. I only have one NIC so I assume that I would have to use alias.

Would something like that be possible? Is there a better way to do the same thing?

I’ve found some decent tutorials on how to set-up the alias, the jail and the mpd5. But how would I go about making sure that its access to the net is cut off if the tunnel goes down?
 
I don't have a clear understanding of the setup, but my guess is you need to call a script from MPD with its up-script and down-script iface options. The script would kill/run the torrent app, or apply/disable firewall rules.
 
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