ref centos

• http://www.tecmint.com/install-virtualbox-on-redhat-centos-fedora/
• https://tecadmin.net/install-oracle-virtualbox-on-centos-redhat-and-fedora/
• https://wiki.centos.org/HowTos/Virtualization/VirtualBox
• http://resources.infosecinstitute.com/installing-configuring-centos-7-virtualbox/