In the realm of Linux security, SELinux (Security-Enhanced Linux) stands as a robust, mandatory access control (MAC) system, enforcing security policies that define what processes can access and perform on files and other resources. Developed by the National Security Agency (NSA), SELinux adds an extra layer of security checks to the Linux kernel, significantly enhancing the system’s defense against unauthorized access and malicious attacks. This article aims to demystify SELinux for newcomers, providing a foundational understanding, practical examples, and guidance on configuring and using it effectively.

What is SELinux?

SELinux operates on the principle of least privilege, meaning that by default, everything is denied access unless explicitly permitted. It uses security labels (or contexts) to assign permissions to processes and files, significantly reducing the risk of a compromised application or service affecting the entire system. SELinux policies control the interactions between processes and resources, ensuring that even if an attacker gains access to a part of the system, their ability to cause harm is limited.

SELinux Modes

SELinux operates in three modes:

• Enforcing: SELinux enforces its policies and denies access based on those policies, logging the action.
• Permissive: SELinux does not enforce policies but logs what actions it would have denied in enforcing mode.
• Disabled: SELinux is turned off.

To check the current SELinux mode, use the getenforce command:

getenforce

To switch between modes, the setenforce command is used, like so:

sudo setenforce Enforcing

or

sudo setenforce Permissive

Configuring SELinux

SELinux configuration resides in /etc/selinux/config. You can edit this file to change the SELinux mode permanently (effective after reboot):

sudo nano /etc/selinux/config

In this file, change the SELINUX= line to either enforcing, permissive, or disabled.

Managing SELinux Policies

SELinux policies are rules that define how processes interact with resources. Two main policy types are used:

• Targeted: Most common, focusing on confining specific daemons.
• MLS (Multi-Level Security): More complex, used in highly secure environments.

Installing SELinux Management Tools

To manage SELinux policies and contexts, install the SELinux management tools:

sudo yum install -y policycoreutils-python-utils

or for Debian-based systems:

sudo apt-get install -y policycoreutils-python-utils

Working with SELinux Contexts

Every file, process, and resource in SELinux has a context. Use the ls -Z command to view file contexts:

ls -Z /var/www/html

To change a file or directory’s SELinux context, use the chcon command. For example, to change the context of a web server’s document root:

sudo chcon -t httpd_sys_content_t /var/www/html -R

However, note that using chcon can be temporary. Changes may be lost upon filesystem relabeling or if the file is restored from a backup. To make permanent changes, use the semanage fcontext command to modify the policy itself, and then apply those changes with restorecon.

Creating Custom SELinux Policies

When the default policies don’t meet your needs, you can create custom policies. After identifying denied actions from the audit logs (/var/log/audit/audit.log), use audit2allow to generate a custom policy module:

grep httpd /var/log/audit/audit.log | audit2allow -M myhttpdmodule

Then, install the module:

semodule -i myhttpdmodule.pp

Shape.host and SELinux-Enhanced Cloud VPS

For those seeking a secure and scalable hosting solution, Shape.host offers Cloud VPS services enhanced with SELinux. With Shape.host, users can leverage the power of SELinux to secure their virtual private servers, ensuring that their applications and data are protected against unauthorized access and breaches. Shape.host’s Cloud VPS services provide the flexibility and performance needed to support a wide range of applications, from web hosting to more complex, secure applications requiring the stringent security measures that SELinux offers.

Whether you’re a newcomer to Linux security or a seasoned administrator, Shape.host’s Cloud VPS services, combined with the security benefits of SELinux, provide a solid foundation for deploying secure, high-performance applications. With comprehensive support and a range of configurations, Shape.host ensures that your hosting environment is not only performant but also conforms to the highest security standards, thanks to SELinux.