As mentioned in , the 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. See for details on what guest operating systems are fully supported with Guest Additions by .

The Guest Additions for all supported guest operating systems are provided as a single CD-ROM image file which is called VBoxGuestAdditions.iso. This image file is located in the installation directory of . To install the Guest Additions for a particular VM, you mount this ISO file in your VM as a virtual CD-ROM and install from there.

The Guest Additions offer the following features:

• Mouse pointer integration. To overcome the limitations for mouse support described in , this feature provides you with seamless mouse support. You will only have one mouse pointer and pressing the Host key is no longer required to free the mouse from being captured by the guest OS. To make this work, a special mouse driver is installed in the guest that communicates with the physical mouse driver on your host and moves the guest mouse pointer accordingly.

• Shared folders. These provide an easy way to exchange files between the host and the guest. Much like ordinary Windows network shares, you can tell to treat a certain host directory as a shared folder, and will make it available to the guest operating system as a network share, irrespective of whether the guest actually has a network. See .

• Better video support. While the virtual graphics card which emulates for any guest operating system provides all the basic features, the custom video drivers that are installed with the Guest Additions provide you with extra high and nonstandard video modes, as well as accelerated video performance.

  In addition, with Windows, Linux, and Oracle Solaris guests, you can resize the virtual machine's window if the Guest Additions are installed. The video resolution in the guest will be automatically adjusted, as if you had manually entered an arbitrary resolution in the guest's Display settings. See .

  If the Guest Additions are installed, 3D graphics and 2D video for guest applications can be accelerated. See .

• Seamless windows. With this feature, the individual windows that are displayed on the desktop of the virtual machine can be mapped on the host's desktop, as if the underlying application was actually running on the host. See .

• Generic host/guest communication channels. The Guest Additions enable you to control and monitor guest execution. The guest properties provide a generic string-based mechanism to exchange data bits between a guest and a host, some of which have special meanings for controlling and monitoring the guest. See .

  Additionally, applications can be started in a guest from the host. See .

• Time synchronization. With the Guest Additions installed, can ensure that the guest's system time is better synchronized with that of the host.

  For various reasons, the time in the guest might run at a slightly different rate than the time on the host. The host could be receiving updates through NTP and its own time might not run linearly. A VM could also be paused, which stops the flow of time in the guest for a shorter or longer period of time. When the wall clock time between the guest and host only differs slightly, the time synchronization service attempts to gradually and smoothly adjust the guest time in small increments to either catch up or lose time. When the difference is too great, for example if a VM paused for hours or restored from saved state, the guest time is changed immediately, without a gradual adjustment.

  The Guest Additions will resynchronize the time regularly. See Tuning the Guest Additions Time Synchronization Parameters for how to configure the parameters of the time synchronization mechanism.

• Shared clipboard. With the Guest Additions installed, the clipboard of the guest operating system can optionally be shared with your host operating system. See .

• Automated logins. Also called credentials passing. See Automated Guest Logins.

Each version of , even minor releases, ship with their own version of the Guest Additions. While the interfaces through which the core communicates with the Guest Additions are kept stable so that Guest Additions already installed in a VM should continue to work when is upgraded on the host, for best results, it is recommended to keep the Guest Additions at the same version.

The Windows and Linux Guest Additions therefore check automatically whether they have to be updated. If the host is running a newer version than the Guest Additions, a notification with further instructions is displayed in the guest.

To disable this update check for the Guest Additions of a given virtual machine, set the value of its /VirtualBox/GuestAdd/CheckHostVersion guest property to 0. See .