4 Guest Additions Generic host/guest communication channels The Guest Additions enable you to control and monitor guest execution in ways other than those mentioned above. The so-called “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 chapter 4.6, Guest properties, page 66 for details. Additionally, applications can be started in a guest from the host see chapter 4.7, Guest control, page 68. Time synchronization With the Guest Additions installed, VirtualBox 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 via 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 (e.g., a VM paused for hours or restored from saved state), the guest time is changed immediately, without a gradual adjustment. The Guest Additions will re-synchronize the time regularly. See chapter 9.12.3, Tuning the Guest Additions time synchronization parameters, page 149 for how to configure the parameters of the time synchronization mechanism. Shared clipboard With the Guest Additions installed, the clipboard of the guest operating sys- tem can optionally be shared with your host operating system see chapter 3.3, General settings, page 42. Automated logons (credentials passing) For details, please see chapter 9.2, Automated guest logons, page 136. Each version of VirtualBox, even minor releases, ship with their own version of the Guest Additions. While the interfaces through which the VirtualBox core communicates with the Guest Additions are kept stable so that Guest Additions already installed in a VM should continue to work when VirtualBox is upgraded on the host, for best results, it is recommended to keep the Guest Additions at the same version. Starting with VirtualBox 3.1, the Windows and Linux Guest Additions therefore check auto- matically whether they have to be updated. If the host is running a newer VirtualBox 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 chapter 4.6, Guest properties, page 66 for details. 4.2 Installing and Maintaining Guest Additions Guest Additions are available for virtual machines running Windows, Linux, Solaris or OS/2. The following sections describe the specifics of each variant in detail. 4.2.1 Guest Additions for Windows The VirtualBox Windows Guest Additions are designed to be installed in a virtual machine run- ning a Windows operating system. The following versions of Windows guests are supported: • Microsoft Windows NT 4.0 (any service pack) • Microsoft Windows 2000 (any service pack) 54