SELinux (Security-Enhanced Linux) is a security architecture that was originally developed by the NSA and later integrated into various Linux distributions. It implements what's called Mandatory Access Control (MAC), which is a more sophisticated and stringent approach to security than traditional Linux permissions.
I'm using Ubuntu as my primary operating system, so for those who might be interested:
Ubuntu actually doesn't use SELinux by default. Instead, Ubuntu uses AppArmor, which is a different but similar security system. AppArmor was chosen for Ubuntu because it's generally considered easier to configure and maintain, though some argue it's less powerful than SELinux.
Enforcing mode is the strictest and most secure option. When SELinux operates in enforcing mode, it actively enforces all security policies defined in the system. This means it will deny any actions that violate the security policy and log these denial events. Think of enforcing mode like a strict security guard who checks everyone's credentials and won't let anyone pass without proper authorization.
Permissive mode is particularly useful for testing and troubleshooting. In this mode, SELinux doesn't actually enforce the security policies, but it does log what it would have blocked if it were in enforcing mode. This is similar to having a security guard who notes down policy violations but doesn't actually stop anyone. System administrators often use permissive mode when setting up new applications or diagnosing SELinux-related issues, as it helps identify what policies need to be adjusted without disrupting system operation.
Disabled mode, as the name suggests, completely turns off SELinux. The security policies are neither enforced nor logged. While this might seem convenient, it's generally not recommended for production systems as it removes an important layer of security. It's like removing the security guard entirely from the building.