RE: Detecting Network scans

http://www.xs4all.nl/~hgj/ipflt/ sounds like what you are wanting,
but I still have got to give props to snort:

http://www.snort.org/    (free)

There is a Windows version that isn't too hard to get running.

If you have a hub somewhere before your internet router, and place
the snort box on the hub - you'll see all traffic coming and going
to the internet.  

I am guessing you'd want:
http://www.snort.org/snort-db/sid.html?sid=630
and modify $HOME_NET any to match the IP of your AS/400




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