Run a userspace script upon rule matching?

[tobi <tobster@brain-force.ch>]

Hello list

my first post here so I hope I do not ask something which is so clear
that I doesn't have to be asked ;-)
As mentioned in subject I want to run an external script upon matching
of a certain rule. but I have no idea how or even if it's possible at
all. I could achieve the "same" by loging those rules to the syslog and
regularly running a cron script that checks for those loglines. But it
would be much easier if I could tell netfilter to call an external
script upon matching.

thanks for any hints and tipps

tobi