SELinux (Security-Enhanced Linux) is a powerful security framework that enforces...
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SELinux (Security-Enhanced Linux) is a powerful security framework that enforces fine-grained access controls on Linux systems. While its default policies offer strong protection, modern administrators often encounter situations where applications need unique permissions not covered by stock policies. Writing custom SELinux policies is the best way to grant the minimum required access—without disabling SELinux entirely or compromising security.
In this guide, you’ll learn what custom SELinux policies are, why custom SELinux policies matter, and exactly how to write, compile, and load your own policy modules to address real-world needs.
Why Write Custom SELinux Policies?
Default SELinux policies are strict by design, which sometimes leads to legitimate apps being blocked. For example, a web server may need to write to an unconventional log directory, or a custom script might require network access. Instead of disabling SELinux (a major security risk), you can craft a custom SELinux policy that grants just enough access for your application to function no more, no less. This approach maintains security while ensuring smooth operations.
Step-by-Step: Creating and Loading a Custom SELinux Policy
Identify What Needs Permission
First, determine what is being blocked. Use SELinux audit logs to find denied actions:
ausearch -m avc -ts recent
# Or, if not installed:
grep "denied" /var/log/audit/audit.log
Look for lines referencing your application or service. Note the type of access being denied (e.g., read, write, connect).
Write a Custom Policy Module (.te File)
A simple custom policy (Type Enforcement file) grants the specific permission your service needs. For example, if Apache needs to write to /custom/logs:
echo 'module myapache 1.0;
gen_require(` type httpd_t; type var_log_t; ');
allow httpd_t var_log_t:dir { write add_name };
allow httpd_t var_log_t:file { create write append }
;' > myapache.te
Compile and Package the Policy Module
Install the SELinux development toolls if you don’t have them:
On RHEL/CentOS/Fedora
sudo dnf install selinux-policy-devel
On Debia/Ubuntu
sudo apt install policycoreutils selinux-utils
selinux-policy-dev
Compile your .te file into a .pp module:
checkmodule -M -m -o myapache.mod myapache.te
semodule_package -o myapache.pp -m myapache.mod
Load the Policy Module
Load your custom policy:
sudo semodule -i myapache.pp
Test the Changes
Retry your app’s operation. If issues persist, check the audit log again and update your policy as needed. It may take several iterations to get all necessary permissions covered.
Custom SELinux policies - Troubleshooting and Testing Tips
- Use
audit2allowto automate policy suggestions:
grep httpd_t /var/log/audit/audit.log | audit2allow
-m myapache > myapache.te
List installed modules:
sudo semodule -lRemove a faulty policy:
sudo semodule -r myapacheAlways reload/restart the affected service after applying new policies.
Custom SELinux policies - Best Practices for Custom SELinux Policies
Principle of Least Privilege: Only allow the minimum access required.
Modular Approach: Write small, targeted modules for each app/service.
Documentation: Comment your policy files for future reference.
Testing: Always test thoroughly in a staging environment before deploying to production.
Keep SELinux Enforcing: Never switch to permissive/disabled just to “fix” a blocked action; use custom policies instead.
Custom SELinux policies - Call to Action
Are you ready to take your Linux security to the next level with custom SELinux policies?
Try writing a simple policy in your lab environment and share your experience or questions below.
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