arpalert uses ARP address monitoring to help
prevent unauthorized connections on the local
network. If an illegal connection is detected, a
program or script is launched, which could be used
to send an alert message, for example.
With Snort2Pf, you can turn your local Snort
installation easily into an "intrusion detection
and prevention system", rather than simply using
it for post-mortem analysis. Such a system also
blocks the recognized violations.