SELinux mechanism in linux environment.

Security-Enhanced Linux (SELinux) is a mandatory access control (MAC) security mechanism implemented in the kernel. SELinux was first introduced in CentOS 4 and significantly enhanced in later CentOS releases. 

SELinux follows the model of least-privilege more closely. By default under a strict enforcing setting, everything is denied and then a series of exceptions policies are written that give each element of the system  only the access required to function. If a service, program or user subsequently tries to access or modify a file or resource not necessary for it to function, then access is denied and the action is logged.

SELinux has three basic modes of operation discussed below:

• Enforcing: The default mode which will enable and enforce the SELinux security policy on the system, denying access and logging actions
• Permissive: In Permissive mode, SELinux is enabled but will not enforce the security policy, only warn and log actions. Permissive mode is useful for troubleshooting SELinux issues
• Disabled: SELinux is turned off

Some useful commands for understanding SELinux;

1.check SELinux status

  sestatus

2.Check port is open

   netstat -tlpn | grep 3306

3.check port is blocked by SELinux

   semanage port -l | grep 3306

4.Open port in SELinux

semanage port -a -t http_port_t -p tcp 3306

5.Drop port in SELinux

semanage port -d -t http_port_t -p tcp 3306

6.check SELinux status

cat /var/log/audit/audit.log|grep -i "mysql"

Hope this bring some basic understanding about SELinux !