According to User Mode Linux website, User Mode Linux (UML for short) is:
User-Mode Linux is a safe, secure way of running Linux versions and Linux processes. Run buggy software, experiment with new Linux kernels or distributions, and poke around in the internals of Linux, all without risking your main Linux setup.
User-Mode Linux gives you a virtual machine that may have more hardware and software virtual resources than your actual, physical computer. Disk storage for the virtual machine is entirely contained inside a single file on your physical machine. You can assign your virtual machine only the hardware access you want it to have. With properly limited access, nothing you do on the virtual machine can change or damage your real computer, or its software.
UML is one of the earliest attempt at Linux virtualisation (the other one being lguest, which is even lighter than UML but unfortunately only runs on 32-bit kernel). It has been largely superseded by more advanced technologies Kernel-mode Virtual Machine (KVM) and Linux Containers (lxc), and is probably considered to be of only academic interest these days, however it still has life left in it. For example it can be used to provide relatively easy and safe, secure sandboxing of untrusted applications.
Fatdog (since version 620) comes with User Mode Linux (UML) environment. All you need to do to use it, is to install linux_uml and uml_utilities packages, from the Package Manager.
You can shutdown the UML VM by:
It is "throwaway" because as soon as you stop the UML VM, all the information in it is gone - there will be no traces left. In this way, UML is very useful for testing purposes.
To run a persistent UML session (i.e., to be able to the states between UML shutdown and restart), do the following:
Once UML has run for the first time in this way, it will create a "config" file in that session directory; which you can edit later. A 128MB savefile is also automatically created for you; if you need anything bigger, just delete that savefile and edit the config to specify a larger value - the savefile will be re-created when you restart UML next time. See below for the details of the config file.
By default, UML will start with a graphical desktop. If you don't want the graphical desktop, start it like this:
The Xephyr window will still open (so that you can type "xwin" to launch a graphical desktop later). If you don't need it, just close it after it opens.
You can put a file named config in the session directory to configure UML's behaviour. This file is automatically created (with default values) if it doesn't exist, or you can create it yourself before you start UML for the first time.
The file contains configuration variables, as follows. Any of these settings can be "blanked" by not specifying the value, e.g. you can blank RAM_SIZE by putting RAM_SIZE= inside the config file.
Determines the size of the RAM allocated to the VM. By default start-uml.sh will assign 256 MB of RAM to the VM. This is a reasonable size for most graphical applications. If you don't run any GUI or other memory intensive applications, you can lower this number. UML itself will happily start with 32MB of RAM (although what you probably can't do much with it).
Note: the letter 'M' is required, it specifies "megabytes".
SAVEFILE_SIZE determines the size (in megabytes) of the savefile to be created if it doesn't exist. SAVEFILE_FS determines the filesystem used when the savefile is created (you can specify ext2, ext3 or ext4). If you leave SAVEFILE_SIZE blank, no savefile will be created if it doesn't exist. These variables are ignored if you already have an existing savefile.
Note: Do not append 'M' for SAVEFILE_SIZE, just specify the numbers.
The location of the savefile, relative to the location of the session directory. Set to blank if you don't want to use a savefile at all (e.g, run a "throwaway" session). Unlike the real Fatdog, if you run without savefile, you will not be prompted to create savefile at the end of the UML session - the session is simply gone after you shut it down.
HOST_IP sets the IP address to be created on the host and the UML. The UML IP address will be one less than the one specified here - if you set HOST_IP=10.0.0.100, the IP address of the UML VM will be 10.0.0.99. This host IP address isn't really used, but it have to be different from any existing IP addresses on the host.
If you leave this parameter blank, it will be set to 100 + IP address from the HOST_ADAPTER will be used. If HOST_ADAPTER is blank, then the first non-local IP address will be used. E.g. if you have both eth0 (IP: 10.0.0.50) and wlan0 (192.168.1.5) and both variables are set to blank, the HOST_IP will be set to 10.0.0.150 and the UML IP will be set to 10.0.0.149.
Note: UML supports sophisticated routing, if you wish you can create network bridges with iptables masquerade etc but these are complex things beyond the purpose of start-uml.sh. If you need to do this you probably want to start writing your own UML script wrapper.
The DNS server which will be used by UML VM (this is the value that get set inside /etc/resolv.conf) to resolve host names. By default the value is 126.96.36.199, which is Google's DNS server.
Specifies the command to be automatically executed when UML starts. By default, it will execute /etc/X11/xinitrc, which starts the graphical desktop. Set this to blank if you don't want to launch the graphical desktop. You can also set this to your own script.
Sets the window size of graphical desktop of the virtual X server (Xephyr). If you leave it blank, no virtual X server will be started and you cannot launch graphical applications.
Specifies additional boot options that you want to pass to the UML Linux kernel. For example if you want to load additional SFS or additional disk images, you need to pass the ubdxxx parameters; specify them here. For the full set of available boot options, run /usr/lib64/uml/vmlinux from the terminal.
If you have custom startup script, you can also pass parameters here which can be read by your startup script by inspecting /proc/cmdline.
Note: A few of the variables you define in the config file can be overridden by putting them into the command line when you start UML, for example when starting UML without graphical desktop as above.
Question: Why UML, why not KVM or even VirtualBox?
Answer: UML is much smaller, use lesses resources, makes use of your existing Fatdog installation (no need for separate ISO files), and easier to setup. You can run UML with only 32MB (for text-mode applications) - you can't do that with KVM or VirtualBox.
Question: Fatdog already has sandbox, so what is this UML for?
Answer: Fatdog's sandbox isn't meant for security. It was originally devised for testing foreign packages, so while it can be used for other purposes, there is information leakages to/from the sandbox; and this is intentional - otherwise you'll face problems when testing. For example sandbox and the host shares the same "/tmp" directory. Fatdog's UML however, doesn't suffer from such problem - host and UML guest is completely separate. E.g. If you choose "shutdown" from sandbox's desktop, you will shutdown your entire system (not only the sandbox). Not so with UML. Choosing "shutdown" from UML's desktop will just shutdown UML VM.
Question: What can the UML VM do?
Answer: Almost everything the host system can do. Browse internet, run network applications (servers), etc. Probably not good enough to watch videos, though - there is no video acceleration in UML.
Question: I understand that the UML is isolated, but is there a way to pass data to/from the the UML?
Otherwise how can I ever do anything worthwhile with UML?
Answer: Of course there is. The point is, these data sharing is completely under your control.
Question: UML only loads Fatdog's base sfs. Is there a way to load additional SFS too?
Answer: Yes. Specify additional SFS like this (assuming you want to use a throwaway session):
start-uml.sh "" ubd1rc=/path/to/your/sfs ubd2rc=/path/to/your/sfs, etc
where udb1rc will show up as /dev/udbb inside the UML VM, udb2rc will show up as /dev/udbc, and so on.
Then to load these SFS, once you're inside the UML VM, open terminal and type:
and so on. If you're using persistence then replace "" with path to your session directory.
Question: UML desktop doesn't show any drive icons.
Answer: Yes, on purpose. Drive icons needs "udev daemon" and daemons takes up additional resources, so they are not run unless they are absolutely necessary. If you need drive icons do this from terminal; type this in terminal inside the UML VM: udevd --daemon; udevadm trigger --action=add
Question: Does running UML require root access?
Answer: No. You can run UML even if you're not root. Inside UML, you're still root :)
Question: The UML VM starts with a desktop. I don't need no stinking desktop, give me the console!
Answer: If you use saved session, edit your config file and empty out the START_CMD option. If you use throwaway session, do it like this:
start-uml.sh "" START_CMD=
You still still get an empty Xephyr window (so that you can type xwin later and start a desktop if you wish), if you don't need it just close that Xephyr window.
Question: OK, now that I can use console only stuff, I need more consoles. How do I get that?
Answer: Run this: getty 38400 tty1 & inside the UML VM.
Replace tty1 with tty2, tty3, etc as many as you need.
Warning: Do not close the xterm windows that are opened automatically, otherwise your UML VM may hang. They will be closed automatically when you close the UML VM.
Question: UML is slow!
Answer: Unlike other virtualisation solutions, UML does not create demand on CPU - it will simply use whatever idle power which has been given. The default CPU frequency scaling governor in Fatdog is "ondemand", which means that the CPU power will be controlled by "demand" - if the CPU power is not needed then the frequency of the CPU will lowered to save power and lower down the temperature too. If it detects that there is a "demand" or need for it, it will then increase the frequency to make the CPU more powerful so that it can do its job faster.
UML does not create this demand (even though in reality it is running slow), so the "ondemand" governor does not think it necessary to make the CPU go faster. To fix this, temporarily change the default CPU frequency scaling governor from the default "ondemand" to "performance" (which means run the CPU at full power at all times) while running UML. There is a commented script of how to do so in /etc/rc.d/rc.local