4. Appliances, OVA, snapshots and Guest Additions

Installing an operating system from zero takes a long time and every installation is a little different. A virtual appliance lets you start in minutes from a machine that someone has already prepared. Snapshots let you experiment without fear. Guest Additions make the VM comfortable to use. These three tools are what you will use every week in this module.

How to work with this page

Estimated time: about 70 minutes of reading and 90 of practice.

1. Read sections 1 and 2 and do Activity 3 (import an OVA).
2. Read section 3 and do Activity 4 (snapshots).
3. Read section 4 and do Activity 5 (Guest Additions and a shared folder).

1. Virtual appliances

A virtual appliance is a virtual machine that is already installed, configured and ready to start. Think of it as a “golden image”: the author chose the operating system, the users, the updates and the programs, and tested that everything works. You only have to import it.

Why is it useful?

• Consistency. Everyone starts from the same machine. If something fails, the difference is not in the installation.
• Speed. You skip the long installation.
• Portability. An appliance made on one computer imports on another one, whatever the host operating system.
• Rollback. After importing you can take a snapshot of the clean state.

OVF and OVA

Appliances are distributed in an open standard format, so that different hypervisors can read them.

Format	What it is
OVF (Open Virtualization Format)	A folder with several files: a .ovf file (an XML description of the hardware: CPUs, memory, controllers, network), one or more disk images (usually .vmdk) and, often, a manifest .mf with checksums to verify the files.
OVA (Open Virtual Appliance)	A single file: an archive (TAR format) that contains all the files of an OVF. It is easier to download and copy.

Use the OVA when you want one piece, and the OVF when you want to look inside or replace one of the files. On Linux you can see what an OVA contains without importing it:

tar -tvf appliance.ova

Verify before you import

Download sites often publish a checksum: a short code that identifies the exact content of a file. If you calculate it and it is equal to the published one, the file is complete and was not changed. If it is different, delete the file and download it again.

sha256sum appliance.ova          # Linux
shasum -a 256 appliance.ova      # macOS

Get-FileHash .\appliance.ova -Algorithm SHA256    # Windows PowerShell

Only import appliances you trust

An appliance contains an operating system that somebody else prepared. It can include programs, users and passwords you do not know. Use appliances from your teacher or from the official site of a project, never from a random website.

Importing an OVA (graphical way)

1. Save the OVA in a folder with enough free space. The virtual disk can use about 1.3 to 1.5 times the size of the OVA.
2. Verify the checksum if there is one.
3. Open VirtualBox and choose File → Import Appliance….
4. Select the OVA file and press Next.
5. Review the appliance settings. Change the VM name (for example, SOM-U2-Surname) and adjust the CPUs and RAM to what your computer can give. Leave the first network adapter in NAT.
6. In the MAC address policy option (older versions have a checkbox called Reinitialize the MAC address of all network cards) choose to generate new MAC addresses. This avoids conflicts when many students import the same file.
7. Press Import and wait. The new VM appears in the list.
8. Start the VM. The user and password of an appliance are given by the author (in this unit, by your teacher in Aules).

Checks after the first start

Open a terminal inside the guest and run:

hostnamectl      # name of the machine, operating system and kernel
ip a             # network interfaces and addresses
df -h            # disks and free space

If you see an address like 10.0.2.15 in NAT mode, your VM has a connection.

Command line (useful to automate)

VBoxManage import appliance.ova --vsys 0 --vmname "SOM-U2-Surname"
VBoxManage modifyvm "SOM-U2-Surname" --memory 2048 --cpus 2

And the other way: you can export your own VM to an OVA to share it or to keep a copy (File → Export Appliance…, or VBoxManage export).

And for Windows?

The process is the same for Windows appliances, but they are less common because the Windows licence rules limit their distribution. Microsoft publishes official evaluation versions of Windows that are time limited. You will work with one in Activity 6.

2. Snapshots

A snapshot saves the state of a VM at a moment in time so that you can return to it later. It is like a “save game” point.

Each snapshot keeps:

1. The configuration of the VM (virtual hardware).
2. The state of the disk. VirtualBox does not copy the whole disk. It freezes the current disk file as read-only and creates a new, small file called a differencing image. From then on, all changes go to the new file.
3. The memory state, if the VM was running when you took the snapshot. When you restore it, the guest returns with the same windows and processes open.

You can imagine the disk as a pile of transparent sheets. Each snapshot is a new sheet on top of the previous ones. What you see is the combination of all the sheets, and the new sheet always receives the changes.

The operations

Operation	What happens	In the GUI
Take	A new layer is created. The VM goes on writing in the new layer.	Snapshots view → Take
Restore	The changes made after the snapshot are discarded and the VM returns to that point.	Select it → Restore
Delete	The snapshot is removed and its layer is merged with the neighbouring layer, so no data is lost. The VM does not change.	Select it → Delete

Two important details:

• Delete does not delete your work. It deletes the checkpoint. VirtualBox joins the layers (it consolidates them), and this can take time and need temporary free space.
• Restore does delete your work done after the snapshot. VirtualBox can offer to make a snapshot of the current state first. Accept if you are not sure.

The snapshots form a tree: you can restore an old snapshot and then take a different snapshot from there, which creates a new branch.

From the terminal:

VBoxManage snapshot "SOM-U2-Surname" take "baseline" --description "Clean state after import"
VBoxManage snapshot "SOM-U2-Surname" list
VBoxManage snapshot "SOM-U2-Surname" restore "baseline"
VBoxManage snapshot "SOM-U2-Surname" delete "baseline"

Good practices

• Take one right after the import or the installation (name it baseline) and one before every risky change: an upgrade, a driver, a big configuration change.
• Use meaningful names that explain why and when: before-lamp-install_2026-10-05.
• Keep few snapshots. Long chains use more disk and make the VM a little slower, because a read may need to look through several layers.
• Take them with the VM powered off if you do not need the open programs. They are smaller and faster.
• Delete the ones you do not need when the experiment ends.
• A snapshot is not a backup. It lives on the same disk. To keep a copy, export the VM to an OVA or clone it (Machine → Clone, a full clone also creates a VM with a single disk and no snapshot tree).

3. Guest Additions

A fresh guest only knows generic virtual hardware, so the VM feels clumsy: the mouse gets “trapped” in the window, the screen has fixed sizes and copy-and-paste between host and guest does not work. Guest Additions are drivers and small tools that you install inside the guest to fix this. They come in a CD image called VBoxGuestAdditions.iso that VirtualBox provides.

With them you get:

Feature	What it does
Mouse integration	The pointer moves freely between the VM and the host.
Dynamic resolution	The screen adapts when you resize the window. Better graphics drivers.
Shared clipboard	Copy and paste text between host and guest.
Drag and drop	Move files by dragging them (not always reliable).
Shared folders	A folder of the host that the guest can open.
Time synchronisation	The guest clock follows the host clock.
Seamless mode	Windows of the guest appear on the host desktop.

Do not confuse them

Guest Additions are installed in the guest. The Extension Pack is installed in the host. They are different. Also, try to keep the version of the Guest Additions equal to the version of VirtualBox; after you update VirtualBox, update them in your VMs.

Installing them in a Linux guest

1. Start the VM and log in.

2. Prepare the tools needed to compile the drivers. In Ubuntu or Debian:

   sudo apt update
   sudo apt install build-essential dkms linux-headers-$(uname -r)

3. In the VM menu choose Devices → Insert Guest Additions CD image….

4. Open the virtual CD (the file manager shows it) and run the installer. From a terminal it looks like this (the folder name can change):

   sudo sh /media/$USER/VBox_GAs_*/VBoxLinuxAdditions.run

5. Restart the VM.

Installing them in a Windows guest

1. Start the VM and log in.
2. Choose Devices → Insert Guest Additions CD image….
3. Open the CD in File Explorer and run VBoxWindowsAdditions.exe. Accept the driver installation.
4. Finish the wizard and restart.

Shared clipboard and shared folders

For the clipboard: with the VM running, choose Devices → Shared Clipboard → Bidirectional. (It can also be set in Settings → General → Advanced.)

For a shared folder:

1. Shut the VM down or use Devices → Shared Folders → Shared Folders Settings….

2. Add a folder of the host, give it a name and tick Auto-mount and Make Permanent.

3. In a Windows guest the folder appears as a network drive (for example Z:). In a Linux guest it appears under /media/sf_<name>.

4. In Linux, if you get a permission denied message, add your user to the vboxsf group and log in again:

   sudo usermod -aG vboxsf $USER

Convenience against security

A shared clipboard and shared folders break part of the isolation between guest and host. If you analyse suspicious files in a VM, turn them off. In general, share only the folder you need and use read-only when you can.

Check what you have learned

1. What is the difference between an OVF and an OVA?

   Answer

   An OVF is a folder with several files (an XML description, disk images and a manifest). An OVA is a single TAR file that contains all those files.

2. Why do you calculate a checksum before importing a downloaded appliance?

   Answer

   To verify that the file is complete and has not been changed. If your checksum is different from the one published by the author, you must not use the file.

3. You take a snapshot, install a program, and then restore the snapshot. What happens to the program? And if you delete the snapshot instead of restoring it?

   Answer

   When you restore, the program disappears: the VM returns to the state of the snapshot. When you delete the snapshot, the program stays: you only remove the checkpoint and its changes are merged.

4. Name three problems that Guest Additions solve.

   Answer

   For example: the mouse captured by the window, the fixed screen resolution and the lack of copy and paste between host and guest. They also add shared folders and time synchronisation.

5. Why is it a bad idea to use only snapshots as your backup system?

   Answer

   They are stored on the same disk as the VM, so a disk failure would destroy both. A long chain also makes the VM slower and uses more space. Use exports or copies as backups.

Glossary

English	Castellano
virtual appliance	aplicación virtual
golden image	imagen de referencia
OVF / OVA	OVF / OVA (formatos de aplicación virtual)
checksum	suma de verificación
import / export	importar / exportar
manifest	manifiesto
differencing image	imagen diferencial
take / restore / delete a snapshot	tomar / restaurar / eliminar una instantánea
Guest Additions	Guest Additions (complementos del invitado)
shared folder	carpeta compartida
shared clipboard	portapapeles compartido
clone	clonar

Credits

This page adapts the SOM course notes by Moisés Mas Rocamora, Daniel Álvarez Santamaría and Jana Taboada León, licensed under CC BY-NC-SA 4.0. This page is shared under the same licence.