Getting Started with SELinux
Updated by Angel
SELinux is a Mandatory Access Control (MAC) system, developed by the NSA. SELinux was developed as a replacement for Discretionary Access Control (DAC) that ships with most Linux distributions.
The difference between Discretionary Access Controls and Mandatory Access Controls is the means by which users and applications gain access to machines. Traditionally, the command
sudo gives a user the ability to heighten permissions to
root-level. Root access on a DAC system gives the person or program with root access permission to perform as desired on a machine.
Ideally, the person with root access should be trusted with it. But if security has been compromised, so too has the system. SELinux and MACs resolve this issue by both confining privleged proccesses and automating security policy creation.
SELinux defaults to denying anything that is not explicitly allowed. SELinux has global modes,
Permissive mode allows the system to function like a Discretionary Access Control system, while logging every violation to SELinux. The
enforcing mode enforces a strict denial of access to anything that isn’t explicitly allowed. To explicitly allow certain behavior on a machine, you, as the system administrator, have to write policies that allow it.
We do not recommend you disable SELinux. But if you wish to disable SELinux, please read our quick-answer guide on disabling SELinux
Before You Begin
- This guide requires you to OWN the box you are going to use. SELinux is a security-control system; a small misconfiguration could cause your system to be compromised.
- Linode uses a custom kernel by default. This kernel does not support SELinux. If you are using a Linode, switch to a distribution-supplied kernel by using this guide: Run a non-custom kernel
Update your system:
On CentOS 7 most of the SELinux packages are installed by default. Look to see what packages are installed:
rpm -aq | grep selinux
If you are dealing with a freshly installed CentOS 7 Linode, your output should be:
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[root@centos ~]# rpm -aq | grep selinux libselinux-utils-2.5-6.el7.x86_64 libselinux-2.5-6.el7.x86_64 libselinux-python-2.5-6.el7.x86_64 selinux-policy-3.13.1-102.el7_3.16.noarch selinux-policy-targeted-3.13.1-102.el7_3.16.noarch
Install the following packages and their associated dependencies:
yum install policycoreutils policycoreutils-python selinux-policy selinux-policy-targeted libselinux-utils setools setools-console
setroubleshoot-server allows, among many other things, for email notifications to be sent from the server to notify you of any policy violations. The
mctrans daemon translates the output of SELinux to human readable text.
SELinux has two modes:
Enforcingmode, SELinux enforces strict policies on the system. Things that are not allowed, will not be allowed to run under any circumstance.
Permissivemode, your system is not protected by SELinux; instead, SELinux just records any violation to a log file.
You can check what mode your system is in by running the
[root@centos ~ ]# getenforce Enforcing
You can also retrieve even more information using
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[root@centos ~]# sestatus SELinux status: enabled SELinuxfs mount: /sys/fs/selinux SELinux root directory: /etc/selinux Loaded policy name: targeted Current mode: enforcing Mode from config file: permissive Policy MLS status: enabled Policy deny_unknown status: allowed Max kernel policy version: 28
You have to set SELinux to
permissive, so that you can create policies on your system for SELinux to enforce. After changing SELinux’s mode, you have to reboot your system.
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[root@centos ~]# setenforce 0 [root@centos ~]# getenforce Permissive [root@centos ~]# reboot
Now that SELinux is set to
Permissive, you can see the log of privacy violations by using:
grep "selinux" /var/log/messages
The output will look very similar to this:
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[root@centos ~]# grep "selinux" /var/log/messages Jun 26 12:27:16 li482-93 yum: Updated: selinux-policy-3.13.1-102.el7_3.16.noarch Jun 26 12:27:38 li482-93 yum: Updated: selinux-policy-targeted-3.13.1-102.el7_3.16.noarch Jun 26 16:38:15 li482-93 systemd: Removed slice system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice. Jun 26 16:38:15 li482-93 systemd: Stopping system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice. Jun 26 16:54:46 li482-93 systemd: Removed slice system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice. Jun 26 16:54:46 li482-93 systemd: Stopping system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice. Jun 26 16:55:45 li482-93 kernel: EVM: security.selinux Jun 26 17:33:43 li482-93 kernel: EVM: security.selinux Jun 26 17:36:21 li482-93 kernel: EVM: security.selinux
The file that contains the security states of the system is located at
/etc/selinux/config, you can edit that file to change the state of the system.
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vi /etc/selinux/config # This file controls the state of SELinux on the system. # SELINUX= can take one of these three values: # enforcing - SELinux security policy is enforced. # permissive - SELinux prints warnings instead of enforcing. # disabled - No SELinux policy is loaded. SELINUX=permissive # SELINUXTYPE= can take one of three two values: # targeted - Targeted processes are protected, # minimum - Modification of targeted policy. Only selected processes are protected. # mls - Multi Level Security protection. SELINUXTYPE=targeted
The uncommented lines can be changed to any state. After changing the state of SELinux,
reboot the machine for the changes to take effect.
Before switching to the
enforce state in SELinux, you have to understand contexts, as they pertain to SELinux.
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[root@centos ~]# useradd user [root@centos ~]# su user [user@centos ~]$ cd ~/ && mkdir test [user@centos ~]$ ls -Z drwxrwxr-x. user user unconfined_u:object_r:user_home_t:s0 test
The output of
ls -Z may look familiar, but the
-Z context flag prints out the SELinux security context of any file.
SELinux marks every single object on a machine with a context. That means every file, daemon, and process has a context, according to SELinux. The context is broken into three parts: user, role and type. In SELinux, a policy controls which users can get which roles. Each specific role places a constraint on what
type of files that user can enter. When a user logs in to a system, a role is assigned, which can be seen in the
ls -Z example above: the output
unconfined_u is a user role.
An SELinux Boolean is a variable that can be toggled on and off without needing to reload or recompile an SELinux polcy. You can view the list of boolean variables using the
getsebool -a command. It’s a long list, so you can pipe it through
grep to narrow down the results:
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[root@centos ~]# getsebool -a | grep xdm xdm_bind_vnc_tcp_port --> off xdm_exec_bootloader --> off xdm_sysadm_login --> off xdm_write_home --> off
You can change the value of any variable using the
setsebool command. If you set the
-P flag, the setting will persist through reboots. If you want to permit a service like openVPN to run unconfined in your system, you have to edit the policies boolean variable:
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[root@centos ~]# getsebool -a | grep "vpn" openvpn_can_network_connect --> on openvpn_enable_homedirs --> on openvpn_run_unconfined --> off [root@centos ~]# setsebool -P openvpn_run_unconfined ON [root@centos ~]# getsebool -a | grep "vpn" openvpn_can_network_connect --> on openvpn_enable_homedirs --> on openvpn_run_unconfined --> on
Now, you are able to use OpenVPN unconfined or in permissive mode on your system, even if it is actively in enforcing mode. Set your system to
enforce, and let SELinux protect your system.
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[root@centos ~]# setenforce 1 [root@centos ~]# getenforce Enforcing
SELinux is complicated. Please see the links under More Information to gather a deeper understanding of the subject. SELinux can play a critical role in system administration and security, especially once it is mastered.
You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.
This guide is published under a CC BY-ND 4.0 license.