Red Hat Enterprise Linux / CentOS Linux Enable EPEL (Extra Packages for Enterprise Linux) Repository

How do I enable EPEL (Extra Packages for Enterprise Linux) repo and install the packages under RHEL / CentOS Linux server systems?

EPEL (Extra Packages for Enterprise Linux) is a volunteer-based community effort from the Fedora project to create a repository of high-quality add-on packages that complement the Fedora-based Red Hat Enterprise Linux (RHEL) and its compatible spinoffs, such as CentOS and Scientific Linux.
EPEL provide lots of packages for CentOS / RHEL, It is not part of RedHat or CentOS but is designed to work with these major distributions. Please note that EPEL only provides free and open source software unencumbered by patents or any legal issues. In short you will not find mp3, dvd and music / media player under EPEL. However, you will find many programs related to networking, monitoring, sys admin, programming and so on.
Packages are supplied in RPM format and in most cases are ready to use. Beware that some packages may break something and you should not blindly install those packages.

How Do I Enable EPEL Repo under CentOS or RHEL Servers?

You need to type the following command as per your distro version / release.

RHEL 5.x / CentOS 5.x Users

Type the following command as root user to install repo:
# rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-5.noarch.rpm

RHEL 6.x / CentOS 6.x Users

Type the following command as root user to install repo:
# rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-5.noarch.rpm

Protect Base Packages

You need to to protect base and core packages from EPEL and other repositories. Type the following command under RHEL 6/CentOS 6:
# yum install yum-plugin-protectbase.noarch
Sample outputs:

Loaded plugins: product-id, rhnplugin, subscription-manager
Updating certificate-based repositories.
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package yum-plugin-protectbase.noarch 0:1.1.30-10.el6 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
====================================================================================================
 Package                    Arch       Version              Repository                         Size
====================================================================================================
Installing:
 yum-plugin-protectbase     noarch     1.1.30-10.el6        rhel-x86_64-server-optional-6      20 k
Transaction Summary
====================================================================================================
Install       1 Package(s)
Total download size: 20 k
Installed size: 0
Is this ok [y/N]: y
Downloading Packages:
yum-plugin-protectbase-1.1.30-10.el6.noarch.rpm                              |  20 kB     00:00
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : yum-plugin-protectbase-1.1.30-10.el6.noarch                                      1/1
Installed products updated.
Installed:
  yum-plugin-protectbase.noarch 0:1.1.30-10.el6
Complete!

RHEL 5/ CentOS 5 user type the following command to protect packages:
# yum install yum-protectbase.noarch
This plugin allows certain repositories to be protected. Packages in the protected repositories can't be overridden by packages in non-protected repositories even if the non-protected repo has a later version.

How Do I Use EPEL Repo?

To list newly installed repo, enter:
# yum repolist
Sample output:

Loading "skip-broken" plugin
Loading "fastestmirror" plugin
repo id              repo name                                 status
addons               CentOS-5 - Addons                         enabled
base                 CentOS-5 - Base                           enabled
epel                 Extra Packages for Enterprise Linux 5 -   enabled
extras               CentOS-5 - Extras                         enabled
updates              CentOS-5 - Updates                        enabled

Now, use the yum command to list, view and install the packages:
# yum search package-name
# yum install package-name
See how to use yum command for installing and searching packages using various repos.