NSA Security-Enhanced Linux (SELinux) is an implementation of a flexible mandatory access control architecture in the Linux operating system. The SELinux architecture provides general support for the enforcement of many kinds of mandatory access control policies, including those based on the concepts of Type Enforcement®, Role- Based Access Control, and Multi-Level Security. Background information and technical documentation about SELinux can be found at http://www.nsa.gov/selinux.
The /etc/selinux/config configuration file controls whether SELinux is enabled or disabled, and if enabled, whether SELinux operates in permissive mode or enforcing mode. The SELINUX variable may be set to any one of disabled, permissive, or enforcing to select one of these options. The disabled option completely disables the SELinux kernel and application code, leaving the system running without any SELinux protection. The permissive option enables the SELinux code, but causes it to operate in a mode where accesses that would be denied by policy are permitted but audited. The enforcing option enables the SELinux code and causes it to enforce access denials as well as auditing them. Permissive mode may yield a different set of denials than enforcing mode, both because enforcing mode will prevent an operation from proceeding past the first denial and because some application code will fall back to a less privileged mode of operation if denied access.
The /etc/selinux/config configuration file also controls what policy is active on the system. SELinux allows for multiple policies to be installed on the system, but only one policy may be active at any given time. At present, two kinds of SELinux policy exist: targeted and strict. The targeted policy is designed as a policy where most processes operate without restrictions, and only specific services are placed into distinct security domains that are confined by the policy. For example, the user would run in a completely unconfined domain while the named daemon or apache daemon would run in a specific domain tailored to its operation. The strict policy is designed as a policy where all processes are partitioned into fine-grained security domains and confined by policy. It is anticipated in the future that other policies will be created (Multi-Level Security for example). You can define which policy you will run by setting the SELINUXTYPE environment variable within /etc/selinux/config. The corresponding policy configuration for each such policy must be installed in the /etc/selinux/SELINUXTYPE/ directories.
A given SELinux policy can be customized further based on a set of compile-time tunable options and a set of runtime policy booleans. system-config-selinux allows customization of these booleans and tunables.
Many domains that are protected by SELinux also include selinux man pages explainging how to customize their policy.
All files, directories, devices ... have a security context/label associated with them. These context are stored in the extended attributes of the file system. Problems with SELinux often arise from the file system being mislabeled. This can be caused by booting the machine with a non selinux kernel. If you see an error message containing file_t, that is usually a good indicator that you have a serious problem with file system labeling.
The best way to relabel the file system is to create the flag file /.autorelabel and reboot. system-config-selinux, also has this capability. The restorcon/fixfiles commands are also available for relabeling files.