Advanced Policies Firewall (APF)
APF is an Open Source project created and maintained by RF-X Networks and is one of the most common Linux software Firewalls among Entry and Mid-Range servers. APF uses IPTables to set advanced rules based on ports and IPs. Single IPs and entire IP blocks can be allowed or banned with advanced rule sets for maximum performance and security.
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Brute Force Detection (BFD)
BFD is also an Open Source project created and maintained by RF-X Networks. Brute Force Attacks are attacks over common services such as SSH, FTP, and Mail to gain entry into a server. To “Brute Force” is to continuously try to login with different user names and passwords until one works. Often, small programs are written to perform this task multiple of times per second which not only can be a security risk (if you use weak passwords), but can affect overall sever performance. To compensate this, BFD logs failed logins on these common servers, and if too many are reached, the offending IP is banned by the APF firewall.
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RKHunter
RKHunter is a useful tool to check for rootkits on your server. This program isn't so much preventative as much as it is identifying a compromised server. If a server is compromised with any rootkit, it is HIGHLY advisable to backup your data, reformat the server, and check your backed up data for the signs of a rootkit before moving back to your server.
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CHKROOTKIT
CHKROOTKIT is a program similar to RKHunter to detect a compromised server of rootkits.
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Libsafe
Libsafe is a Linux layer built to provide extra security for installed Libraries from overflows. This not only protects you from many already known but unpatched vulnerabilities, but it protects you from unknown and future vulnerabilities. Libsafe is only available for i386 architecture servers.
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mod_security
mod_security is an Apache Module which protects you from many known website exploits. Dynamic web pages built in languages like CGI/Perl, PHP, and so on can have many vulnerabilities. When hackers discover these vulnerabilities, they take advantage of them and can exploit your server over your website. mod_security prevents this with custom rules by filtering the request string and if something checks against one of those filters, the hacker gets an error page and the exploit is stopped before it even reaches the vulnerable file.
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eAccelerator
eAccelerator is the rebirth of Turck's MMCache extension for PHP. eAccelerator caches already compiled PHP scripts so PHP applications load faster. It is also bundled with a simple encoder so you can make sure your PHP files are safe. To run these encoded PHP files, they must be used on a server with the eAccelerator extension installed to decode the PHP scripts.
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Zend Optimizer
Zend Optimizer is a PHP extension created by the Zend Corporation. It optimizes PHP code at the PHP Engine level at runtime to get maximum performance out of the scripts. Additionally, many closed source PHP programs require Zend Optimizer to run as they are encoded by Zend.
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MyTOP
MyTOP is a top-like program that shows the status of current MySQL queries in real-time. It is useful in discovering resource-hogging queries.
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