UML - Linux in another linux

Wich UML, you can run one linux inside another!
There are several things you can do wich UML. It's very usefull for testing upgrades in your servers, kernel development, or just for funny.
To do this, basic steps are:

• Download and install Uml package (uml-utilities user-mode-linux)
• Download kernel sources and apply correspondent Uml kernel patch
• Compile kernel for architecture=uml
• Download any filesystem from UserModeLinux page.
  
• Run!!!

At uml pages you can download many distros (redhat, debian, slackware), but you can make your own filesystem! Many tools are avaliable for that. It's possible run graphicaly X or just command line.
I post few links bellow where you can find good documentation and "how to", and links for other tools necessary for run virtual network.

http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO.html - The main site. It have step-by-step how-to, very recommended for
http://user-mode-linux.sourceforge.net - This is official page. You will find many documentation here.
http://user-mode-linux.sourceforge.net/networking.html - How to running network in virtual machine. I'm using TUN/TAP and it's working fine.