How to Secure Linux Servers

Basic Linux Server Security

Install Firewall (APF or CSF Firewall with BFD)
ModSecurity (Web application firewall)
ModEvasive (Prevent DDOS attacks)
Harden SSH server
Fix Open DNS Recursion
Install RKhunter
Install ClamAV (Antivirus)
XInet Servers Hardening (Disable Telnet/Finger or unwanted services)
Securing PHP
PortsEntry (tool to detect portscans)
Harden host.conf (against IP spoofing)
Check User Uploaded files
Secure /tmp Folders (noexec, nosuid)

This tutorial guide covers only basic linux server security tips intended for linux learners. I am writing this guide assuming that you are running Centos 5 or later versions.

Full article: see here