Linux Server Security (2nd Edition( [Electronic resources] نسخه متنی

10.5. Layers of Defense

Test your setup with a vulnerability scanner. The best open source
tool is
nessus
(http://www.nessus.org), which
includes tests for buffer overflows, bad Apache configurations, buggy
CGI scripts, and many other problems. It includes tests from
nikto (http://www.cirt.net/code/nikto.shtml) and
libwhisker (http://www.wiretrip.net/rfp/p/doc.asp/i2/d21),
which can also be run on their own.

When you're ready for production, use multiple
levels of protection:

Firewall (Chapter 2) Intrusion detection and logging, such as
Snort/ACID (Chapter 13) Log monitoring (Chapter 12)