"There isn't an easy and automated way to monitor open ports in
Red Hat Enterprise Linux 5 (RHEL 5), says the author of Hardening
Linux, James Turnbull. However, you can scan with some handy open
source tools. Turnbull recommends using hardscan, a Netstat-based
tool, if you think you have been compromised.
"In this tip, SearchEnterpriseLinux.com's security expert
describes how to configure Nagios with BMC Patrol,
non-platform-specific tips for using Secure Shell (SSH) and