Virtualization technologies have been compared to the movie, The Matrix. In this, Neo and other humans, are captured in a virtual world. Neo is offered a blue pill or a red pill. The blue pill will return him to his normal unreal world in the matrix. The red will set his mind free by exposing the matrix. When it comes to virtualization technologies, the red pill and blue pill have similar meanings.
The red pill is a small piece of code that, when run on a virtual machine, is able to determine if it is running in a virtual system or a real, physical system. It does this by detecting if the operating system is under the control of a hypervisor, the monitoring process that enables virtualization.
To learn more and to read the entire article, please refer to the rest of the article at its source: Virtualization: Red Pill or Blue? - Steven McElwee, CISSP