User Mode Linux (UML) is a way to compile a Linux kernel such that it can run as a process in another Linux system (potentially as a *BSD or Windows process). See http://user-mode-linux.sourceforge.net for more information about UML.
UML is a good platform for testing and experimenting with Openswan. It allows several network nodes to be simulated on a single machine. Creating, configuring, installing, monitoring, and controlling these nodes is generally simpler and easier to script with UML than with real hardware. There are other virtual machine implementations, such as Xen and VMware, but these are not supported by the UML test suite.
You will need about 2 Gigabytes of free disk space for a full setup of seven to nine UMLs. You can possibly get this down by 130Mb if you remove the Sunrise/Sunset kernel build. If you just want to play around, then you can even remove the East/West kernel build.
Nothing needs be done as superuser, and we encourage people to run the...