Effective Tools for Enhancing CentOS Server Security

I’m seeking practical recommendations and tools to improve the security of my Linux server (CentOS) and minimize unauthorized access. I’m interested in both fundamental and advanced solutions that can be easily integrated and configured. Specifically, I need: * Protection Against Unauthorized Access: What tools and practices are most effective for preventing hacking and intrusion into the server? This includes authentication methods (e.g., two-factor authentication) and tools for monitoring activity and detecting suspicious behavior. What I’ve Tried: I experimented with SELinux, but when attempting to set the strictest security level (“military level”), the server failed to boot after rebooting. I likely made a configuration error. I would appreciate detailed guidance on correctly configuring SELinux, or alternative approaches to kernel-level security hardening. What Kind of Answers I’m Looking For: * Recommendations for specific tools, including open-source alternatives. * Detailed instructions on how to configure and use these tools. * Example configurations for different security levels (from basic to advanced). * Alternatives to SELinux for kernel-level security enhancement.