User Mode Linux

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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 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It has been largely superseded by more advanced technologies Kernel-mode Virtual Machine KVM and Linux Containers lxc , and is probably considered to be of only academic interest these days,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.

  1. Connections to UML driver.
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Note for Fatdog64 users : if you are familiar with UML operation in Fatdog64 series, please read on. The way UML is configured has changed. 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. 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 MB 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.

Discovering the versatility of user mode Linux

Any of these settings can be "blanked" by not specifying the value, e. By default start-uml. 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.

User Mode Linux

These variables are ignored if you already have an existing savefile. 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.

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. All of these can be left blank, and start-uml. You can set them manually if you require some special configuration or if the automatic values does not work. It is not really used but must be assigned. Unless you want to use masquerading, this IP address. Unless you want to use masquerading , this address must be on the same subnet as the physical network that the host is attached to.

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. If you need to do this you probably want to write your own UML script wrapper, using start-uml. By default the value is 8. Specifies the command to be automatically executed when UML starts.

Linux Single User Mode - ITzGeek

You can set this to run your own programs directly inside UML; it will be executed in the "root" login context as the last stage of rc. 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 the nice command for use when running the UML kernel.


It is used to lower down the process priority used by the UML kernel, if you're running as root then it can be used to make UML run faster too by giving it more priority. Specifies additional boot options that you want to pass to the UML Linux kernel. Virtual block devices can also be pointed at physical block devices on the host system. It is also possible to specify a tar file on the host system as a block device in the virtual machine. This is one simple way of transferring files from the host to the virtual machine.

Imagine that you have several very similar virtual machines. If you think about it, this is not a good idea, as all of the machines could write to this shared image and cause problems for the other machines. Furthermore, it is probable that the files are very similar to each other and have only minor differences. The basic idea is that you can create a virtual block device from two files: one that is read-only and contains all of the shared data, and another that is read-write and stores all of the private changes.

At first glance, the size of the file looks to be 1. UML provides a few ways for virtual machines to access files on the physical host. The simplest method is to use hostfs. You can download Linux kernel source code for the Linux kernel of your choice from kernel. Note that it is easier to build UML guest kernels using source code from Linux kernel versions 2.

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  • Part I - Creating and configuring an UML kernel.
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  • I recommend going with a more recent version of the kernel if at all possible. The downloaded kernel source code can be unzipped and untarred as usual. This file is large, because it includes debugging symbols. To run your new UML kernel, just execute it with. For example:. Even with early versions of UML, you could run guest kernels without any special support in the host kernel by using something called Tracing Thread TT mode. TT mode, however, has disadvantages that impact the security and performance of UML.

    Part II - Creating and configuring a file system

    Thus, if you were going to be doing serious work with UML, you were best served by building for yourself a host kernel with what is now known as the SKAS3 patch. SKAS0 support is included in Linux kernels from version 2. UML is a rapidly evolving technology and so, unfortunately, some of the documentation on the Web site is a bit outdated.

    Hopefully, by the time you read this, the documentation will be updated. Eventually, though, you may want to create your own root filesystems. With the eth0 interface created in the guest, you will need to assign an IP to the interface or obtain one via DHCP and you will probably want to use the route command to configure a default route through the new interface. Well, this is possible too. So for example, you could do something like this to run a GNOME desktop on the guest that you can see in a window on the host display:. UML allows you to create completely independent virtual machines that run isolated from each other, complete with their own Linux kernels.