The RPMForge repository has been dis-continued. Please use other yum repositories. Use the below link to find useful yum repositories
Top 5 Yum Repositories
RPMforge is a collaboration of Dag and other packages. It provides a large number of packages for CentOS and RHEL systems.
This article will help you to how to add rpmforge repository on your CentOS/RHEL 7/6/5 systems. To install more yum repositories visit article top 5 yum repositories.