Changes

SELinux stands for ''Security Enhanced Linux'' and is based on research performed at NSA and other locations. Where the normal Unix/Linux security system, based upon file permissions, is a ''discretionary access control'' system (DAC), SELinux is a ''mandatory access control'' system (MAC). This means that it attempts to enforce a consistent policy across the entire system, regardless of settings that any user has configured.

SELinux is decisions are based upon on the ''security context'' of system resources such as files and processes. The security context consists of a user, role, type, and sensitivity component; you can see the security context of files and processes by adding the <code>-Z</code> option to the <code>ls</code> and <code>ps</code> commands:

The SELinux policy controls the interactions between security contexts. For example, it the policy may be configured so specify that the Apache httpd webserver cannot read files in <code>/etc</code>, so if an attacker finds a way to make httpd (or a script run by httpd) read a file in <code>/etc</code>, SELinux will recognize that this should is not be permitted normal and will deny the access. Since this is done at the kernel level, httpd will get a "file not found" error, even though the file is present, and there is no way for httpd to work around that error.