SELinux security feature of the Linux kernel. To manage the security enhanced Linux behavior of a system to keep it secure in case of a network service compromise.

SELinux is an additional layer of system security. It protects user data from your system services that have been compromised. Linux administrators are known with the standard user/group/other(u/g/o) permissions security model.

As a example if you see above Picture 1 Whenever outside client request for a data to access from Linux Server, SELinux will verify requested data port is allowed from SELinux, It will verify process SELinux context is enabled and File security context enabled. Three layer security system. This security will work only when SELinux is in enforcing mode.

SELinux is a set of security rules that determine which process can access which directories, files and ports. Every file, process, port and directory has a special label called a SELinux context.

SELinux label context are user, role, type and sensitivity. The type context names end with “_t“

To display or set SELinux contexts with option “Z”:

[root@server ~]# ls -lZ
-rw-------. root root system_u:object_r:admin_home_t:s0 anaconda-ks.cfg
drwxr-xr-x. root root unconfined_u:object_r:admin_home_t:s0 Desktop
drwxr-xr-x. root root unconfined_u:object_r:admin_home_t:s0 Documents
drwxr-xr-x. root root unconfined_u:object_r:admin_home_t:s0 Downloads

[root@server ~]# ls -ldZ /etc/
drwxr-xr-x. root root system_u:object_r:etc_t:s0 /etc/

[root@server ~]# ls -ldZ /var/
drwxr-xr-x. root root system_u:object_r:var_t:s0 /var/

[root@server ~]# ls -ldZ /
dr-xr-xr-x. root root system_u:object_r:root_t:s0 /

[root@server ~]# ls -ldZ /var/log/
drwxr-xr-x. root root system_u:object_r:var_log_t:s0 /var/log/

SELinux Modes:

SELinux modes are three types :-

1. Enforcing Mode
2. Permissive Mode
3. Disabled Mode

Enforcing Mode: Default mode which will enforce and enabled the SELinux security on your system. In this mode SELinux logs and protects.

Permissive Mode: This mode can be used to temporarily allow access to content that SELinux is restricting. No reboot required to go from enforcing to permissive vice versa. This mode is useful for troubleshooting SELinux security issues. When SELinux is in permissive mode it will not deny the access it will only log.

Disabled Mode: Completely disables SELinux your system. Your system reboot is required to disable SELinux entirely or to get disabled mode to enforcing. Until unless you reboot your machine after disable it will not effect.

For the first time when you change SELinux from disable mode to enforcing mode SELinux will relabel all the files and processes from context rules

Change SELinux modes

To check SELinux security status

[root@server ~]# sestatus
SELinux status: disabled

[root@server ~]# getenforce 
Disabled

Enable / Disable SELinux Security mode. Edit configuration file and change SELINUX=’enforcing/disabled’ 

[root@server ~]# vim /etc/selinux/config 
SELINUX=enforcing
SELINUXTYPE=targeted

Enforced mode 

[root@server ~]# 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: enforcing
Policy MLS status: enabled
Policy deny_unknown status: allowed
Max kernel policy version: 28

[root@server ~]# getenforce 
Enforcing

To keep in permissive mode

[root@server ~]# setenforce 0
[root@server ~]# getenforce 
Permissive

What is the default context for newly created files / Directories

When we create an file / directory under / (slash) it will assign default_t context.

But if we create an file / directory under /etc/, /var/, /var/www/html/ it will apply different SELinux security context let see the examples below

[root@server ~]# mkdir /var/test
[root@server ~]# ls -ldZ /var/test/
drwxr-xr-x. root root unconfined_u:object_r:var_t:s0 /var/test/

[root@server ~]# mkdir /etc/test
[root@server ~]# ls -ldZ /etc/test
drwxr-xr-x. root root unconfined_u:object_r:etc_t:s0 /etc/test

[root@server ~]# mkdir /test
[root@server ~]# ls -ldZ /test
drwxr-xr-x. root root unconfined_u:object_r:default_t:s0 /test

How to assign SELinux Security Context

To assign a security context to file

[root@server ~]# semanage fcontext -a -t samba_share_t "/test(/.*)?"

[root@server ~]# ls -ldZ /test/
drwxr-xr-x. root root unconfined_u:object_r:default_t:s0 /test/

[root@server ~]# restorecon -vRF /test/
restorecon reset /test context unconfined_u:object_r:default_t:s0->system_u:object_r:samba_share_t:s0

[root@server ~]# ls -ldZ /test/
drwxr-xr-x. root root system_u:object_r:samba_share_t:s0 /test/

To enable port

[root@server ~]# semanage port -l |grep http_port
http_port_t tcp 80, 81, 443, 488, 8008, 8009, 8443, 9000
pegasus_http_port_t tcp 5988

[root@server ~]# semanage port -a -t http_port_t -p tcp 15000

[root@server ~]# semanage port -l |grep http_port
http_port_t tcp 15000, 80, 81, 443, 488, 8008, 8009, 8443, 9000
pegasus_http_port_t tcp 5988

Too See SELinux Boolean values. Enable / Disable sebool parameters

[root@server ~]# getsebool -a |grep ftp
ftp_home_dir --> off
ftpd_anon_write --> off
ftpd_connect_all_unreserved --> off
ftpd_connect_db --> off
ftpd_full_access --> off
ftpd_use_cifs --> off
ftpd_use_fusefs --> off
ftpd_use_nfs --> off
ftpd_use_passive_mode --> off
httpd_can_connect_ftp --> off
httpd_enable_ftp_server --> off
sftpd_anon_write --> off
sftpd_enable_homedirs --> off
sftpd_full_access --> off
sftpd_write_ssh_home --> off
tftp_anon_write --> off
tftp_home_dir --> off

[root@server ~]# setsebool -P ftpd_anon_write on

[root@server ~]# getsebool -a |grep ftpd_anon_write
ftpd_anon_write --> on
sftpd_anon_write --> off

Conclusion

SELinux security context is highly improved in the newer version of Linux RHEL 7 / Centos 7 / Fedora 24. 

That’s it.

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