Understanding SELinux and Its Importance in CentOS 7
Security-Enhanced Linux (SELinux) is a mandatory access control (MAC) security mechanism implemented in the kernel. SELinux offers a means of enforcing some security policies which would otherwise not be effectively implemented by a System Administrator. When working with CentOS 7, understanding how to check the status of SELinux is crucial for maintaining system security.
What is SELinux?
SELinux is an advanced security infrastructure integrated into several Linux distributions, including CentOS. It was originally developed by the United States National Security Agency (NSA) to protect computer systems from unauthorized access. SELinux extends the existing permissions model in Linux to offer finer-grained control over resources and processes.
Why is SELinux Important?
The importance of SELinux lies in its ability to restrict programs to the minimum level of access they require to function properly. This principle of least privilege reduces the potential damage that can be caused by malicious software or user error. For administrators, SELinux provides a robust framework to manage access controls beyond traditional Unix permissions.
Checking SELinux Status on CentOS 7
To ensure your CentOS 7 system is protected by SELinux, you need to verify whether it is enabled and what mode it is operating in. There are multiple ways to check the status of SELinux, each providing different levels of detail.
Using getenforce Command
The simplest method to check the SELinux status is by using the getenforce command. This command returns the current mode SELinux is running in, which can be either Enforcing, Permissive, or Disabled.
getenforce
- Enforcing: SELinux security policy is enforced.
- Permissive: SELinux prints warnings instead of enforcing.
- Disabled: SELinux is turned off.
Inspecting /etc/selinux/config File
Another way to check SELinux status is by inspecting the configuration file located at /etc/selinux/config. This file contains the persistent configuration that will be applied on system boot.
cat /etc/selinux/config
Look for the SELINUX= line to determine the default SELinux mode:
- SELINUX=enforcing – SELinux is set to enforce the policy rules.
- SELINUX=permissive – SELinux is set to only warn about policy violations.
- SELINUX=disabled – SELinux is completely disabled.
Using sestatus Command
For a more detailed status report, use the sestatus command. This command provides comprehensive information about the current SELinux status, including the loaded policy and the mode of operation.
sestatus
The output includes:
- SELinux status: whether SELinux is enabled or disabled.
- Current mode: enforcing, permissive, or disabled.
- Loaded policy name: targeted, mls, etc.
- Policy MLS status: enabled or disabled.
- Policy deny_unknown status: allowed or denied.
- Max kernel policy version: shows the highest policy version supported.
Managing SELinux Modes and Policies
Switching Between SELinux Modes
Changing the operational mode of SELinux on-the-fly can be done without rebooting the system. To switch between enforcing and permissive modes, use the setenforce command.
setenforce Enforcing
setenforce Permissive
Note that disabling SELinux without a reboot is not recommended as it may leave your system in an unpredictable state. To disable SELinux, you must edit the /etc/selinux/config file and then reboot the system.
Understanding and Managing SELinux Policies
SELinux policies are the actual rules that govern the access controls enforced by SELinux. The most commonly used policy type on CentOS 7 is the ‘targeted’ policy, which focuses on confining specific services known to be vulnerable to attack while leaving other services unconfined.
Managing these policies involves creating, modifying, and applying policy modules. Tools such as audit2allow, semodule, and semanage are used for this purpose. These tools help translate audit logs into policy language, manage active policy modules, and adjust policy booleans and file contexts, respectively.
Troubleshooting SELinux Issues
Interpreting SELinux Log Messages
When SELinux denies access, it logs messages to the audit log, typically found at /var/log/audit/audit.log. Understanding these log entries is essential for troubleshooting SELinux issues.
ausearch -m avc -ts recent
This command searches for AVC (Access Vector Cache) denials that have occurred recently. Each entry provides details about the denied operation, including the source context, target context, and the class of object being accessed.
Addressing Common SELinux Denials
Common SELinux denials can often be resolved by adjusting file contexts, setting appropriate booleans, or generating and installing custom policy modules based on audit logs.
- To adjust file contexts, use the chcon or restorecon commands.
- To toggle SELinux booleans, use the setsebool command.
- To generate a custom policy module, use the audit2allow utility.
FAQ Section
How do I temporarily disable SELinux?
You can temporarily set SELinux to permissive mode using setenforce 0, which will stop enforcing the policy but still log denials. Disabling it entirely requires a system reboot after changing the /etc/selinux/config file.
Can I run CentOS 7 without SELinux?
Yes, you can run CentOS 7 with SELinux disabled; however, it is not recommended due to the additional security layer that SELinux provides.
What should I do if SELinux is blocking a legitimate process?
If SELinux is blocking a legitimate process, you can create a custom policy module to allow the necessary access or adjust the relevant file contexts or booleans to accommodate the process.
Is it safe to set SELinux to permissive mode?
Setting SELinux to permissive mode is generally safe for debugging purposes as it does not enforce policies but still logs actions that would have been denied. However, for maximum security, it is best to keep SELinux in enforcing mode.
How do I know if a denial is caused by SELinux?
Denials caused by SELinux are logged in the audit log with an AVC message. You can use tools like ausearch or sealert to search for these messages and interpret them.
