How to start external script when ipfilter rule matches

I'd like to start Squid when packets arrive on port 3128 on my router. When the router disconnects from my ISP, Squid is shutdown again. Ideally the packets would be kept on hold until Squid is running and then delivered to the daemon. Another question is to shutdown squid after a certain time of inactivity on port 3128. Is there a way to achive this with ipfilter?

Thanks in advance.

Regards,

Sören Lorenz