These days many dedicated servers are offered with automation installation of operating systems and it have never been so easy and fast to pay a server and to get it up in minutes! Yes, we are talking for dedicated machines not virtual servers or cloud ones, but many cloud ones are in the same situation, when the host uses full virtualization.
It is fast, you can choose from many different linux distros and the installation is unattended and happens immediately, but in most cases the selinux is disabled, because is more easy for the support, for the user, for the admin, and for the offered preinstalled software…If you do not have some strange software in most cases it is advisable to enable the selinux, because it is of great security enhancement for your server and for the software in general. It is very simple to enable selinux, but there is an additional step, which if you omit, the server will probably get unusable (probably you won’t be able even to ssh it or login). These steps are tested under CentOS7, but probably works in all other distros, which support selinux!
So here are the steps:

STEP 1) Enable selinux in configuration

Edit the configuration file in

/etc/selinux/config

SELINUX=enforcing

STEP 2) relabel the file system

When using the selinux, there are labels (extended attributes of the file system), which are additional layer of security. Every system comes with prebuild rules instructing what label is set in which file or directory, so when you enable the selinux you must relabel the entire file system (or at least the root partition, to be able to boot normally). This is done with just a line of code below:

touch /.autorelabel

STEP 3) reboot

To take effect the changes made above the server must be rebooted.

reboot

The init process will find the file from step 2 “/.autorelabel” and will initiate a proper relabel according the current selinux rules file, then the server will be rebooted automatically again, the relabel could take time and it depends on the number of files you have in your server, just keep patient.
After the second reboot (which is automatically after the relabeling)

4) Recommendations

USE SELINUX, do not disable it! In most cases it is really simple to configure it in minutes for the need of your special software and for the generic one bet it there are rules offered in the distro’s packet system.

4) Post install check

You can check if the selinux is enabled with

[root@srv ~]# getenforce 
Enforcing
[root@srv ~]#