9.2. Automated guest logons

Oracle VM VirtualBox

previous page next page

9.2. Automated guest logons

VirtualBox provides Guest Addition modules for Windows, Linux and Solaris to enable automated logons on the guest.

When a guest operating system is running in a virtual machine, it might be desirable to perform coordinated and automated logons using credentials from a master logon system. (With "credentials", we are referring to logon information consisting of user name, password and domain name, where each value might be empty.)

9.2.1. Automated Windows guest logons

Since Windows NT, Windows has provided a modular system logon subsystem ("Winlogon") which can be customized and extended by means of so-called GINA modules (Graphical Identification and Authentication). With Windows Vista and Windows 7, the GINA modules were replaced with a new mechanism called "credential providers". The VirtualBox Guest Additions for Windows come with both, a GINA and a credential provider module, and therefore enable any Windows guest to perform automated logons.

To activate the VirtualBox GINA or credential provider module, install the Guest Additions with using the command line switch /with_autologon. All the following manual steps required for installing these modules will be then done by the installer.

To manually install the VirtualBox GINA module, extract the Guest Additions (see Section 4.2.1.4, “Manual file extraction”) and copy the file VBoxGINA.dll to the Windows SYSTEM32 directory. Then, in the registry, create the following key: 

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\GinaDLL

with a value of VBoxGINA.dll.

Note

The VirtualBox GINA module is implemented as a wrapper around the standard Windows GINA module (MSGINA.DLL). As a result, it will most likely not work correctly with 3rd party GINA modules.

To manually install the VirtualBox credential provider module, extract the Guest Additions (see Section 4.2.1.4, “Manual file extraction”) and copy the file VBoxCredProv.dll to the Windows SYSTEM32 directory. Then, in the registry, create the following keys:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\
           Authentication\Credential Providers\{275D3BCC-22BB-4948-A7F6-3A3054EBA92B}

HKEY_CLASSES_ROOT\CLSID\{275D3BCC-22BB-4948-A7F6-3A3054EBA92B}

HKEY_CLASSES_ROOT\CLSID\{275D3BCC-22BB-4948-A7F6-3A3054EBA92B}\InprocServer32

with all default values (the key named (Default) in each key) set to VBoxCredProv. After that a new string named 

HKEY_CLASSES_ROOT\CLSID\{275D3BCC-22BB-4948-A7F6-3A3054EBA92B}\InprocServer32\ThreadingModel

with a value of Apartment has to be created.

To set credentials, use the following command on a running VM:

VBoxManage controlvm "Windows XP" setcredentials "John Doe" "secretpassword" "DOMTEST"

While the VM is running, the credentials can be queried by the VirtualBox logon modules (GINA or credential provider) using the VirtualBox Guest Additions device driver. When Windows is in "logged out" mode, the logon modules will constantly poll for credentials and if they are present, a logon will be attempted. After retrieving the credentials, the logon modules will erase them so that the above command will have to be repeated for subsequent logons.

For security reasons, credentials are not stored in any persistent manner and will be lost when the VM is reset. Also, the credentials are "write-only", i.e. there is no way to retrieve the credentials from the host side. Credentials can be reset from the host side by setting empty values.

Depending on the particular variant of the Windows guest, the following restrictions apply:

  1. For Windows XP guests, the logon subsystem needs to be configured to use the classic logon dialog as the VirtualBox GINA module does not support the XP-style welcome dialog.

  2. For Windows Vista, Windows 7 and Windows 8 guests, the logon subsystem does not support the so-called Secure Attention Sequence (CTRL+ALT+DEL). As a result, the guest's group policy settings need to be changed to not use the Secure Attention Sequence. Also, the user name given is only compared to the true user name, not the user friendly name. This means that when you rename a user, you still have to supply the original user name (internally, Windows never renames user accounts).

  3. Auto-logon handling of the built-in Windows Remote Desktop Service (formerly known as Terminal Services) is disabled by default. To enable it, create the registry key 

    HKEY_LOCAL_MACHINE\SOFTWARE\Oracle\VirtualBox Guest Additions\AutoLogon

    with a DWORD value of 1.

The following command forces VirtualBox to keep the credentials after they were read by the guest and on VM reset: 

VBoxManage setextradata "Windows XP" VBoxInternal/Devices/VMMDev/0/Config/KeepCredentials 1

Note that this is a potential security risk as a malicious application running on the guest could request this information using the proper interface.

9.2.2. Automated Linux/Unix guest logons

Starting with version 3.2, VirtualBox provides a custom PAM module (Pluggable Authentication Module) which can be used to perform automated guest logons on platforms which support this framework. Virtually all modern Linux/Unix distributions rely on PAM.

For automated logons on Ubuntu (or Ubuntu-derived) distributions using LightDM as the display manager, please see Section 9.2.2.1, “VirtualBox Greeter for Ubuntu / LightDM”.

The pam_vbox.so module itself does not do an actual verification of the credentials passed to the guest OS; instead it relies on other modules such as pam_unix.so or pam_unix2.so down in the PAM stack to do the actual validation using the credentials retrieved by pam_vbox.so. Therefore pam_vbox.so has to be on top of the authentication PAM service list.

Note

The pam_vbox.so only supports the auth primitive. Other primitives such as account, session or password are not supported.

The pam_vbox.so module is shipped as part of the Guest Additions but it is not installed and/or activated on the guest OS by default. In order to install it, it has to be copied from /opt/VBoxGuestAdditions-<version>/other/ to the security modules directory, usually /lib/security/ on 32-bit guest Linuxes or /lib64/security/ on 64-bit ones. Please refer to your guest OS documentation for the correct PAM module directory.

For example, to use pam_vbox.so with a Ubuntu Linux guest OS and GDM (the GNOME Desktop Manager) to logon users automatically with the credentials passed by the host, the guest OS has to be configured like the following:

  1. The pam_vbox.so module has to be copied to the security modules directory, in this case it is /lib/security.

  2. Edit the PAM configuration file for GDM found at /etc/pam.d/gdm, adding the line auth requisite pam_vbox.so at the top. Additionally, in most Linux distributions there is a file called /etc/pam.d/common-auth. This file is included in many other services (like the GDM file mentioned above). There you also have to add the line auth requisite pam_vbox.so.

  3. If authentication against the shadow database using pam_unix.so or pam_unix2.so is desired, the argument try_first_pass for pam_unix.so or use_first_pass for pam_unix2.so is needed in order to pass the credentials from the VirtualBox module to the shadow database authentication module. For Ubuntu, this needs to be added to /etc/pam.d/common-auth, to the end of the line referencing pam_unix.so. This argument tells the PAM module to use credentials already present in the stack, i.e. the ones provided by the VirtualBox PAM module.

Warning

An incorrectly configured PAM stack can effectively prevent you from logging into your guest system!

To make deployment easier, you can pass the argument debug right after the pam_vbox.so statement. Debug log output will then be recorded using syslog.

Note

By default, pam_vbox will not wait for credentials to arrive from the host, in other words: When a login prompt is shown (for example by GDM/KDM or the text console) and pam_vbox does not yet have credentials it does not wait until they arrive. Instead the next module in the PAM stack (depending on the PAM configuration) will have the chance for authentication.

Starting with VirtualBox 4.1.4 pam_vbox supports various guest property parameters which all reside in /VirtualBox/GuestAdd/PAM/. These parameters allow pam_vbox to wait for credentials to be provided by the host and optionally can show a message while waiting for those. The following guest properties can be set:

  1. CredsWait: Set to "1" if pam_vbox should start waiting until credentials arrive from the host. Until then no other authentication methods such as manually logging in will be available. If this property is empty or get deleted no waiting for credentials will be performed and pam_vbox will act like before (see paragraph above). This property must be set read-only for the guest (RDONLYGUEST).

  2. CredsWaitAbort: Aborts waiting for credentials when set to any value. Can be set from host and the guest.

  3. CredsWaitTimeout: Timeout (in seconds) to let pam_vbox wait for credentials to arrive. When no credentials arrive within this timeout, authentication of pam_vbox will be set to failed and the next PAM module in chain will be asked. If this property is not specified, set to "0" or an invalid value, an infinite timeout will be used. This property must be set read-only for the guest (RDONLYGUEST).

To customize pam_vbox further there are the following guest properties:

  1. CredsMsgWaiting: Custom message showed while pam_vbox is waiting for credentials from the host. This property must be set read-only for the guest (RDONLYGUEST).

  2. CredsMsgWaitTimeout: Custom message showed when waiting for credentials by pam_vbox timed out, e.g. did not arrive within time. This property must be set read-only for the guest (RDONLYGUEST).

Note

If a pam_vbox guest property does not have set the right flags (RDONLYGUEST) this property will be ignored then and - depending on the property - a default value will be set. This can result in pam_vbox not waiting for credentials. Consult the appropriate syslog file for more information and use the debug option.

9.2.2.1. VirtualBox Greeter for Ubuntu / LightDM

Starting with version 4.2.12, VirtualBox comes with an own greeter module named vbox-greeter which can be used with LightDM 1.0.1 or later. LightDM is the default display manager since Ubuntu 10.11 and therefore also can be used for automated guest logons.

vbox-greeter does not need the pam_vbox module described above in order to function -- it comes with its own authentication mechanism provided by LightDM. However, to provide maximum of flexibility both modules can be used together on the same guest.

As for the pam_vbox module, vbox-greeter is shipped as part of the Guest Additions but it is not installed and/or activated on the guest OS by default For installing vbox-greeter automatically upon Guest Additions installation, use the --with-autologon switch when starting the VBoxLinuxAdditions.run file:

# ./VBoxLinuxAdditions.run -- --with-autologon

For manual or postponed installation, the vbox-greeter.desktop file has to be copied from /opt/VBoxGuestAdditions-<version>/other/ to the xgreeters directory, usually /usr/share/xgreeters/. Please refer to your guest OS documentation for the correct LightDM greeter directory.

The vbox-greeter module itself already was installed by the VirtualBox Guest Additions installer and resides in /usr/sbin/. To enable vbox-greeter as the standard greeter module, the file /etc/lightdm/lightdm.conf needs to be edited:

[SeatDefaults]
greeter-session=vbox-greeter

Note

The LightDM server needs to be fully restarted in order to get vbox-greeter used as the default greeter. As root, do a service lightdm --full-restart on Ubuntu, or simply restart the guest.

Note

vbox-greeter is independent of the graphical session chosen by the user (like Gnome, KDE, Unity etc). However, it requires FLTK 1.3 for representing its own user interface.

There are numerous guest properties which can be used to further customize the login experience. For automatically logging in users, the same guest properties apply as for pam_vbox, see Section 9.2.2, “Automated Linux/Unix guest logons”.

In addition to the above mentioned guest properties, vbox-greeter allows further customization of its user interface. These special guest properties all reside in /VirtualBox/GuestAdd/Greeter/:

  1. HideRestart: Set to "1" if vbox-greeter should hide the button to restart the guest. This property must be set read-only for the guest (RDONLYGUEST).

  2. HideShutdown: Set to "1" if vbox-greeter should hide the button to shutdown the guest. This property must be set read-only for the guest (RDONLYGUEST).

  3. BannerPath: Path to a .PNG file for using it as a banner on the top. The image size must be 460 x 90 pixels, any bit depth. This property must be set read-only for the guest (RDONLYGUEST).

  4. UseTheming: Set to "1" for turning on the following theming options. This property must be set read-only for the guest (RDONLYGUEST).

  5. Theme/BackgroundColor: Hexadecimal RRGGBB color for the background. This property must be set read-only for the guest (RDONLYGUEST).

  6. Theme/LogonDialog/HeaderColor: Hexadecimal RRGGBB foreground color for the header text. This property must be set read-only for the guest (RDONLYGUEST).

  7. Theme/LogonDialog/BackgroundColor: Hexadecimal RRGGBB color for the logon dialog background. This property must be set read-only for the guest (RDONLYGUEST).

  8. Theme/LogonDialog/ButtonColor: Hexadecimal RRGGBB background color for the logon dialog button. This property must be set read-only for the guest (RDONLYGUEST).

Note

The same restrictions for the guest properties above apply as for the ones specified in the pam_vbox section.

previous page start next page