It runs on 2.2.15 or later, and all 2.4 and 2.6 kernels.
Booting UML is straightforward. Simply run 'linux': it will try to mount the file `root_fs' in the current directory. You do not need to run it as root. If your root filesystem is not named `root_fs', then you need to put a `ubd0=root_fs_whatever' switch on the linux command line.
You will need a filesystem to boot UML from. There are a number available for download from here . There are also several tools which can be used to generate UML-compatible filesystem images from media.
The kernel will boot up and present you with a login prompt.
Note: If the host is configured with a 2G/2G address space split rather than the usual 3G/1G split, then the packaged UML binaries will not run. They will immediately segfault. See UML on 2G/2G hosts for the scoop on running UML on your system.
The prepackaged filesystems have a root account with password 'root' and a user account with password 'user'. The login banner will generally tell you how to log in. So, you log in and you will find yourself inside a little virtual machine. Our filesystems have a variety of commands and utilities installed (and it is fairly easy to add more), so you will have a lot of tools with which to poke around the system.
There are a couple of other ways to log in:
Each virtual console that is configured (i.e. the device exists in /dev and /etc/inittab runs a getty on it) will come up in its own xterm. If you get tired of the xterms, read Setting up serial lines and consoles to see how to attach the consoles to something else, like host ptys.
In the boot output, find a line that looks like:
serial line 0 assigned pty /dev/ptyp1
Attach your favorite terminal program to the corresponding tty. I.e. for minicom, the command would be
host% minicom -o -p /dev/ttyp1
If the network is running, then you can telnet to the virtual machine and log in to it. See Setting up the network to learn about setting up a virtual network.
When you're done using it, run halt, and the kernel will bring itself down and the process will exit.
Here are some examples of UML in action: