This Post Originally From Snort-users by Frank

Q

: How can i use the snort portscan2 preprocessor with snortsam?

A

:it doesn't matter what version of Snort you are using, 1.9.x, 2.0.x, or 2.1.x. Alerts from the portscan preprocessor (or any other preprocessor such as ARP spoof) are not recognized by Snortsam. It only reacts to alerts generated by packets detected with rules.

That said, port scans can be detected. However, you will have to create your own rules for that. There are two ways (and a combined way) of doing this.

a) Create rules that fire when you can packets going to unused IP