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Disposable VMs (DispVMs)
A Disposable VM (DispVM) is a lightweight VM that can be created quickly and will disappear when closed. Disposable VMs are usually created in order to host a single application, like a viewer, editor, or web browser. Changes made to a file opened in a Disposable VM are passed back to the originating VM. This means that you can safely work with untrusted files without risk of compromising your other VMs. DispVMs can be created either directly from Dom0 or from within AppVMs. Once a DispVM has been created it will appear in Qubes VM Manager with the name "dispX".
See this article for more on why one would want to use a Disposable VM.
Disposable VMs and Networking
NetVM and firewall rules for Disposable VMs can be set as they can for a normal VM. By default a DispVM will inherit the NetVM and firewall settings of the VM from which it is launched. Thus if an AppVM uses sys-net as its NetVM, any DispVM launched from this AppVM will also have sys-net as its NetVM. You can change this behaviour for individual VMs: in Qubes VM Manager open VM Settings for the VM in question and go to the "Advanced" tab. Here you can edit the "NetVM for DispVM" setting to change the NetVM of any DispVM launched from that VM.
A Disposable VM launched from the Start Menu inherits the NetVM of the DVM Template.
By default the DVM template is called fedora-XX-dvm (where XX is the Fedora version of the default TemplateVM).
As an "internal" VM it is hidden in Qubes VM Manager, but can be shown by selecting "Show/Hide internal VMs".
Note that changing the "NetVM for DispVM" setting for the DVM Template does not affect the NetVM of DispVMs launched from the Start Menu; only changing the DVM Template's own NetVM does.
Opening a file in a Disposable VM via GUI
In an AppVM's file manager, right click on the file you wish to open in a Disposable VM, then choose "Open in Disposable VM". Wait a few seconds and the default application for this file type should appear displaying the file content. This app is running in its own dedicated VM -- a Disposable VM created for the purpose of viewing or editing this very file. Once you close the viewing application the whole Disposable VM will be destroyed. If you have edited the file and saved the changes, the changed file will be saved back to the original AppVM, overwriting the original.
Opening a fresh web browser instance in a new Disposable VM
Sometimes it is desirable to open an instance of Firefox within a new fresh Disposable VM. This can be done easily using the Start Menu: just go to Start -> System Tools -> DispVM:Firefox web browser. Wait a few seconds until a web browser starts. Once you close the viewing application the whole Disposable VM will be destroyed.
Opening a file in a Disposable VM via command line (from AppVM)
Use the qvm-open-in-dvm command from a terminal in your AppVM:
[user@work-pub ~]$ qvm-open-in-dvm Downloads/apple-sandbox.pdf
Note that the qvm-open-in-dvm process will not exit until you close the application in the Disposable VM.
Starting an arbitrary program in a Disposable VM from an AppVM
Sometimes it can be useful to start an arbitrary program in a DispVM. This can be done from an AppVM by running
[user@vault ~]$ qvm-run '$dispvm' xterm
The created Disposable VM can be accessed via other tools (such as qvm-copy-to-vm) using its "dispX" name as shown in the Qubes Manager or qvm-ls.
Starting an arbitrary application in a Disposable VM via command line (from Dom0)
The Start Menu has shortcuts for opening a terminal and a web browser in dedicated DispVMs, since these are very common tasks. However, it is possible to start an arbitrary application in a DispVM directly from Dom0 by running
[joanna@dom0 ~]$ echo xterm | /usr/lib/qubes/qfile-daemon-dvm qubes.VMShell dom0 DEFAULT red
(The Disposable VM appmenu used for starting Firefox runs a very similar command to the one above.)
Customizing Disposable VMs
You can change the template used to generate the Disposable VM, and change settings used in the Disposable VM savefile. These changes will be reflected in every new Disposable VM. Full instructions can be found here.
Disposable VMs and Local Forensics
At this time, DispVMs should not be relied upon to circumvent local forensics, as they do not run entirely in RAM. For details, see this thread.
When it is essential to avoid leaving any trace, consider using Tails.