Thank you for all your responses. Based on this thread I decided to implement this feature in my firewall tool running in userspace. It is already done now (as it IS not much effort to do it there). Only disadvantage of this is, that when you shutdown my firewall tool, the rules wont be deleted. (I dont want to start a cron/at job - it just would make it more complex, and shouldnt be needed that much.)
So long lonestar Martin Maurer
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