To enable the full functionality of VirtualBox, you often have to install Guest Additions, which enable features such as shared drives and better video support, such as being able to auto size the Guest VM display to the current dimensions of the VirtualBox gui, or to view the Guest VM in full screen.


“Guest Additions are designed to be installed inside a virtual machine after the guest operating system has been installed. They consist of device drivers and system applications that optimize the guest operating system for better performance and usability.”

VirtualBox Manual


When you upgrade the OS within a Guest VM, it’s kernel may change requiring you to re-install Guest Additions. You will know the Guest Additions need to be re-installed when basic features such as viewing the Guest VM in full screen no longer work.


To install VirtualBox you can follow this guide How to Install VirtualBox

To install a Ubuntu VM you can follow this guide How to Install a Ubuntu Virtual Machine


To re-install VirtualBox Guest Additions

Click Devices and choose Insert Guest Additions CD Image


Most likely you will be able to Run the Guest Additions and then reboot and be back to a fully functional VirtualBox Guest VM.

But, sometimes the Guest Additions CD does not run ie does not mount


Some possible solutions are:


• Simply reboot to ensure the current OS upgrade has been applied

  The try to re-install the Guest Additions again.

  If that works, your done!

• If not, try to manually mount the Guest Additions CD

  From a terminal window (right click on Ubuntu desktop -> Terminal)

  sudo mkdir --p /media/cdrom
  sudo mount -t auto /dev/cdrom /media/cdrom/

  ls -l /media/cdrom/
  sudo sh

  If that works, your done!


  To check the device location of the CD ie /dev/cdrom,

     vi /var/log/kernel.log

  Look for CD (type forward slash CD and press enter ie /CD)

  You should see a reference to /dev/cdrom or /dev/sr0

  More details at askubuntu unable-to-mount-virtualbox-guest-additions


• If when you run mount you receive an error

  sudo mount -t auto /dev/cdrom /media/cdrom/
  mount: unknown filesystem type 'iso9660'

  This means a basic kernel module was not loaded,

  so re-install the kernel and reboot.

  sudo apt-get install --reinstall linux-image-$(uname -r)

  (uname -r)

  Displays the kernel version, making the --reinstall command above generic



After rebooting you may notice that the Guest VM resizes and runs as full screen, as expected.

It is still a good idea to re-install the Guest Addition CD at this point to ensure the kernel has been updated with VirutalBox additions.


To re-install VirtualBox Guest Additions

Click Devices and choose Insert Guest Additions CD Image

Then choose Run


The Guest Additions may prompt you to confirm that you are re-installing

Type yes and press enter

Press enter to close the window

And once more reboot


You should have a fully functional VirtualBox again.

If you are still having issues, check the askubuntu links for more ideas