I downloaded virtualbox-4.2_4.2.12-84980~Ubuntu~raring_i386.deb (63.9 MB) and Oracle_VM_VirtualBox_Extension_Pack-4.2.12-84980.vbox-extpack (11.6 MB) from https://www.virtualbox.org/wiki/Linux_Downloads.

I installed VBox on my laptop (Dell 1545 Core2Duo T6400; 4 GB RAM) running Lubuntu 13.04 giving the guest OS (also Lubuntu 13.04) 1536 MB RAM and 8 GB fixed disk space.

Performance is okay but I'm having trouble with the screen size. The desktop doesn't occupy the full width of the screen as the image shows (red ellipses).

VBox screen size

I can use "scale mode" but that looks ugly as though things have been stretched horizontally.

Some reading indicates that installing Guest Addditions would help. But I can't figure out how to install it. I have /usr/share/virtualbox/VBoxGuestAdditions.iso.

I've tried to mount VBoxGuestAdditions.iso:


But when I click on Devices, Install Guest Additions, I see this window:

Force Unmount

Also puzzling is that there's an option to "force unmount" when the text above says "unable to mount".

  • 1
    Did you try to run virtualbox as sudo? For similar problems usually such thing helped me: In machine settings unmount the guest auditions and set host cd as IDE. Then try to install GA via menu of the running virtualbox machine.
    – Misery
    Commented Jun 21, 2013 at 18:09
  • But sudo VirtualBox brings up the screen for a new VM. It doesn't see the VM existing already.
    – user25656
    Commented Jun 21, 2013 at 18:16
  • @vasa1 In settings for graphics in virtualbox (before you start the machine for Linux) - you can set the size of memory for graphics - with this you have enfluence on size of the virtual-display ?! - or not ?! Commented Jun 21, 2013 at 19:03
  • 1
    @ vasa1 @Misery VirtualBox never needs sudo, and should be never run as sudo. And this problem is independent of sudo.
    – ignis
    Commented Jun 21, 2013 at 19:46

1 Answer 1


Clicking the "Install Guest Additions" menu item just adds the iso to the virtual machine. You don't need to click it, because, in your case, the VirtualBoxGuestAdditions.iso is already attached to the virtual machine (as per the Storage tab of the virtual machine settings). It's named like that because the guest is supposed to automount the CD and autorun the installer, but I've never seen this behavior on Ubuntu, so the name is misleading in this case.

From the VirtualBox manual, Chapter 4 (Guest Additions), section "Installing the Linux Guest Additions":


sudo apt-get update
sudo apt-get upgrade
sudo apt-get install dkms

Reboot your guest system in order to activate the updates and then proceed as described above:

  • Insert the VBoxGuestAdditions.iso CD file into your Linux guest's virtual CD-ROM drive [Don't do this, if the Storage tab shows it to be already attached to the virtual machine]
  • [open a terminal, ] Change to the directory where your CD-ROM drive is mounted, and execute:
  • sudo sh ./VBoxLinuxAdditions.run
  • 5
    Installing virtualbox additions in Ubuntu to enable "Auto-Size Guest Display" - worked out of the box. Not so with lubuntu. However, installing the extra dkms package as you suggested, did the trick.
    – Greg Woods
    Commented Dec 20, 2013 at 11:32
  • 1
    This answer was very good. Once point id like to add is that its important to reboot after you install dkms. I didnt first time round and ran into problems.
    – Robbo_UK
    Commented Jun 17, 2014 at 10:32
  • 2
    Another tidbit - You might try sudo sh ./VBoxLinuxAdditions.run on the last step. I had to do this in Lubuntu. Thanks for all the other input!
    – user314931
    Commented Aug 12, 2014 at 14:03
  • 1
    @LearningUbuntu Yes, that's what "execute as root" is intended to mean (although they actually mean "execute with root permissions").
    – ignis
    Commented Aug 12, 2014 at 20:09
  • How would you go about that next to last step, logging in as root exactly? I am in lubuntu 15. I never got to set a root password. Commented Apr 2, 2016 at 19:59

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