SELinux (Security-Enhanced Linux) is a Linux kernel security module that provides a mechanism for supporting access control security policies in the operating system. It was developed by the US National Security Agency (NSA) and is included in many Linux distributions, including Red Hat, CentOS, and Fedora.
SELinux operates at a lower level than traditional Unix-style permissions and can be used to enforce more fine-grained control over access to resources on the system. It uses a set of rules to define what actions are allowed for each process, based on the process's security context. This allows administrators to define policies that restrict the actions that processes can perform, even if they are running with high privileges.
SELinux can be used in different modes, including targeted, minimum, and mls (multi-level security). In targeted mode, SELinux policies are applied to select processes, while in minimum mode, SELinux policies are applied to all processes. MLS mode is used for systems with a higher level of security requirements.
SELinux can be a powerful tool for securing a Linux system, but it can also be complex to configure and manage. It is important for administrators to understand how SELinux works and to carefully consider whether it is necessary for their particular system.