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<chapter>

<title>Remote virtual machines</title>

<sect1>

<title id="vrdp">Remote display (VRDP support)</title>

<para>VirtualBox, the graphical user interface, has a built-in server for

the VirtualBox Remote Desktop Protocol (VRDP). This allows you to see the

output of a virtual machine's window remotely on any other computer and

control the virtual machine from there, as if the virtual machine was

running locally.</para>

<para>VRDP is a backwards-compatible extension to Microsoft's Remote

Desktop Protocol (RDP). Typically graphics updates and audio are sent from

the remote machine to the client, while keyboard and mouse events are sent

back. As a result, you can use any standard RDP client to control the

remote VM.</para>

<para>With <computeroutput>VirtualBox</computeroutput>, the graphical user

interface, the VRDP server is disabled by default, but can easily be

enabled on a per-VM basis either in the "Display" settings (see <xref

linkend="settings-display" />) or with

<computeroutput>VBoxManage</computeroutput>:<screen>VBoxManage modifyvm "VM name" --vrdp on</screen></para>

<para>If you use <computeroutput>VBoxHeadless</computeroutput> (described

further below), VRDP support will be automatically enabled since

VBoxHeadless has no other means of output.</para>

<sect2 id="rdp-viewers">

<title>Common third-party RDP viewers</title>

<para>You can use any standard RDP viewer to connect to such a remote

virtual machine (examples follow below). For this to work, you must

specify the <emphasis role="bold">IP address</emphasis> of your

<emphasis>host</emphasis> system (not of the virtual machine!) as the

server address to connect to, as well as the <emphasis role="bold">port

number</emphasis> that the RDP server is using.</para>

<para>By default, the VRDP server uses the standard RDP TCP port

<computeroutput>3389</computeroutput>. You will need to change the

default port if you run more than one VRDP server, since the port can

only be used by one server at a time; you might also need to change it

on Windows hosts since the default port might already be used by the RDP

server that is built into Windows itself. Ports 5000 through 5050 are

typically not used and might be a good choice.</para>

<para>The port can be changed either in the "Display" settings of the

graphical user interface or with

<computeroutput>--vrdpport</computeroutput> option of the

<computeroutput>VBoxManage modifyvm</computeroutput> command. You can

specify a comma-separated list of ports or ranges of ports. Use a dash

between two port numbers to specify a range. The VRDP server will bind

to <emphasis role="bold">one</emphasis> of available ports from the

specified list. For example, <computeroutput>VBoxManage modifyvm "VM

name" --vrdpport 5000,5010-5012</computeroutput> will configure the

server to bind to one of the ports 5000, 5010, 5011 or 5012. See <xref

linkend="vboxmanage-modifyvm" /> for details.</para>

<para>The actual port used by a running VM can be either queried with

<computeroutput>VBoxManage showvminfo</computeroutput> command or seen

in the GUI on the <computeroutput>Runtime</computeroutput> tab of the

<computeroutput>Session Information Dialog</computeroutput>, which is

accessible via the <computeroutput>Machine</computeroutput> menu of the

VM window.</para>

<para>Here follow examples for the most common RDP viewers:<itemizedlist>

<listitem>

<para>On Windows, you can use the Microsoft Terminal Services

Connector (<computeroutput>mstsc.exe</computeroutput>) that ships

with Windows. You can start it by bringing up the "Run" dialog

(press the Windows key and "R") and typing "mstsc". You can also

find it under "Start" -&gt; "All Programs" -&gt; "Accessories"

-&gt; "Remote Desktop Connection". If you use the "Run" dialog,

you can type in options directly:<screen>mstsc 1.2.3.4[:3389]</screen></para>

<para>Replace "1.2.3.4" with the host IP address, and 3389 with a

different port if necessary.</para>

<note><para>When connecting to localhost in order to test the connection,

the addresses <computeroutput>localhost</computeroutput> and

<computeroutput>127.0.0.1</computeroutput> might not work using

<computeroutput>mstsc.exe</computeroutput>. Instead, the address

<computeroutput>127.0.0.2[:3389]</computeroutput> has to be used.</para></note>

</listitem>

<listitem>

<para>On other systems, you can use the standard open-source

<computeroutput>rdesktop</computeroutput> program. This ships with

most Linux distributions, but VirtualBox also comes with a

modified variant of rdesktop for remote USB support (see <xref

linkend="usb-over-rdp" /> below).</para>

<para>With rdesktop, use a command line such as the

following:<screen>rdesktop -a 16 -N 1.2.3.4:3389</screen></para>

<para>As said for the Microsoft viewer above, replace "1.2.3.4"

with the host IP address, and 3389 with a different port if

necessary. The <computeroutput>-a 16</computeroutput> option

requests a color depth of 16 bits per pixel, which we recommend.

(For best performance, after installation of the guest operating

system, you should set its display color depth to the same value).

The <computeroutput>-N</computeroutput> option enables use of the

NumPad keys.</para>

</listitem>

<listitem>

<para>If you run the KDE desktop, you might prefer

<computeroutput>krdc</computeroutput>, the KDE RDP viewer. The

command line would look like this:<screen>krdc --window --high-quality rdp:/1.2.3.4[:3389]</screen></para>

<para>Again, replace "1.2.3.4" with the host IP address, and 3389

with a different port if necessary. The "rdp:/" bit is required

with krdc to switch it into RDP mode.</para>

</listitem>

</itemizedlist></para>

</sect2>

<sect2 id="vboxheadless">

<title>VBoxHeadless, the VRDP-only server</title>

<para>While the VRDP server that is built into the

<computeroutput>VirtualBox</computeroutput> GUI is perfectly capable of

running virtual machines remotely, it is not convenient to have to run

<computeroutput>VirtualBox</computeroutput> if you never want to have

VMs displayed locally in the first place. In particular, if you are

running servers whose only purpose is to host VMs, and all your VMs are

supposed to run remotely over VRDP, then it is pointless to have a

graphical user interface on the server at all -- especially since, on a

Linux or Solaris host, <computeroutput>VirtualBox</computeroutput> comes

with dependencies on the Qt and SDL libraries, which is inconvenient if

you would rather not have the X Window system on your server at

all.</para>

<para>VirtualBox therefore comes with yet another front-end called

<computeroutput>VBoxHeadless</computeroutput>, which produces no visible

output on the host at all, but instead only delivers VRDP data.<footnote>

<para>Before VirtualBox 1.6, the headless server was called

<computeroutput>VBoxVRDP</computeroutput>. For the sake of backwards

compatibility, the VirtualBox installation still installs an

executable with that name as well.</para>

</footnote></para>

<para>To start a virtual machine with VBoxHeadless, you have two

options:</para>

<itemizedlist>

<listitem>

<para>You can use <screen>VBoxManage startvm "VM name" --type vrdp</screen>

The extra <computeroutput>--type</computeroutput> option causes the

VirtualBox core to use <computeroutput>VBoxHeadless</computeroutput>

as the front-end to the internal virtualization engine.</para>

</listitem>

<listitem>

<para>The alternative is to use

<computeroutput>VBoxHeadless</computeroutput> directly, as

follows:<screen>VBoxHeadless --startvm &lt;uuid|name&gt;</screen></para>

<para>This way of starting the VM has the advantage that you can see

more detailed error messages, especially for early failures before

the VM execution is started. If you have trouble with

<computeroutput>VBoxManage startvm</computeroutput>, it can help

greatly to start <computeroutput>VBoxHeadless</computeroutput>

directly to diagnose the problem cause.</para>

</listitem>

</itemizedlist>

<para>Note that when you use

<computeroutput>VBoxHeadless</computeroutput> to start a VM, since the

headless server has no other means of output, the built-in VRDP server

will <emphasis>always</emphasis> be enabled, regardless of whether you

have enabled the VRDP server in the VM's settings. If this is

undesirable (for example because you want to access the VM via

<computeroutput>ssh</computeroutput> only), start the VM like

this:<screen>VBoxHeadless --startvm &lt;uuid|name&gt; --vrdp=off</screen>To

have the VRDP server use the setting from the VM configuration, as the

other front-ends would, use this:<screen>VBoxHeadless --startvm &lt;uuid|name&gt; --vrdp=config</screen></para>

</sect2>

<sect2>

<title>Step by step: creating a virtual machine on a headless

server</title>

<para>The following instructions may give you an idea how to create a

virtual machine on a headless server over a network connection. We will

create a virtual machine, establish a VRDP connection and install a

guest operating system -- all without having to touch the headless

server. All you need is the following:</para>

<para><orderedlist>

<listitem>

<para>VirtualBox on a server machine with a supported host

operating system; for the following example, we will assume a

Linux server;</para>

</listitem>

<listitem>

<para>an ISO file on the server, containing the installation data

for the guest operating system to install (we will assume Windows

XP in the following example);</para>

</listitem>

<listitem>

<para>a terminal connection to that host over which you can access

a command line (e.g. via <computeroutput>telnet</computeroutput>

or <computeroutput>ssh</computeroutput>);</para>

</listitem>

<listitem>

<para>an RDP viewer on the remote client; see <xref

linkend="rdp-viewers" /> above for examples.</para>

</listitem>

</orderedlist>Note again that on the server machine, since we will

only use the headless server, neither Qt nor SDL nor the X Window system

will be needed.</para>

<para><orderedlist>

<listitem>

<para>On the headless server, create a new virtual machine:</para>

<screen>VBoxManage createvm --name "Windows XP" --ostype WindowsXP --register</screen>

<para>Note that if you do not specify

<computeroutput>--register</computeroutput>, you will have to

manually use the registervm command later.</para>

<para>Note further that you do not need to specify

<computeroutput>--ostype</computeroutput> but doing so selects

some sane default values for certain VM parameters, for example

the RAM size and the type of the virtual network device. To get a

complete list of supported operating systems you can use</para>

<screen>VBoxManage list ostypes</screen>

</listitem>

<listitem>

<para>Make sure the settings for this VM are appropriate for the

guest operating system that we will install. For example:<screen>VBoxManage modifyvm "Windows XP" --memory 256 --acpi on --boot1 dvd --nic1 nat</screen></para>

</listitem>

<listitem>

<para>Create a virtual hard disk for the VM (in this case, 10GB in

size) and register it with VirtualBox:<screen>VBoxManage createhd --filename "WinXP.vdi" --size 10000 --remember</screen></para>

</listitem>

<listitem>

<para>Add an IDE Controller to the new VM:<screen>VBoxManage storagectl "Windows XP" --name "IDE Controller"

--add ide --controller PIIX4</screen></para>

</listitem>

<listitem>

<para>Set this newly created VDI file as the first virtual hard

disk of the new VM:<screen>VBoxManage storageattach "Windows XP" --storagectl "IDE Controller"

--port 0 --device 0 --type hdd --medium "WinXP.vdi"</screen></para>

</listitem>

<listitem>

<para>Register the ISO file that contains the operating system

installation that you want to install later:<screen>VBoxManage openmedium dvd /full/path/to/iso.iso</screen></para>

</listitem>

<listitem>

<para>Attach this ISO to the virtual machine, so it can boot from

it:<screen>VBoxManage storageattach "Windows XP" --storagectl "IDE Controller"

--port 0 --device 1 --type dvddrive --medium /full/path/to/iso.iso</screen></para>

</listitem>

<listitem>

<para>Start the virtual machine using VBoxHeadless:<screen>VBoxHeadless --startvm "Windows XP"</screen></para>

<para>If everything worked, you should see a copyright notice. If,

instead, you are returned to the command line, then something went

wrong.</para>

</listitem>

<listitem>

<para>On the client machine, fire up the RDP viewer and try to

connect to the server (see <xref linkend="rdp-viewers" /> above

for how to use various common RDP viewers).</para>

<para>You should now be seeing the installation routine of your

guest operating system in the RDP viewer.</para>

</listitem>

</orderedlist></para>

</sect2>

<sect2 id="usb-over-rdp">

<title>Remote USB</title>

<para>As a special feature on top of the VRDP support, VirtualBox

supports remote USB devices over the wire as well. That is, the

VirtualBox guest that runs on one computer can access the USB devices of

the remote computer on which the RDP data is being displayed the same

way as USB devices that are connected to the actual host. This allows

for running virtual machines on a VirtualBox host that acts as a server,

where a client can connect from elsewhere that needs only a network

adapter and a display capable of running an RDP viewer. When USB devices

are plugged into the client, the remote VirtualBox server can access

them.</para>

<para>For these remote USB devices, the same filter rules apply as for

other USB devices, as described with <xref linkend="settings-usb" />.

All you have to do is specify "Remote" (or "Any") when setting up these

rules.</para>

<para>Accessing remote USB devices is only possible if the RDP client

supports this extension. On Linux and Solaris hosts, the VirtualBox

installation provides a suitable RDP client called

<computeroutput>rdesktop-vrdp</computeroutput>. RDP clients for other

platforms will be provided in future VirtualBox versions.</para>

<para>To make a remote USB device available to a VM, rdesktop-vrdp

should be started as follows:<screen>rdesktop-vrdp -r usb -a 16 -N my.host.address</screen>Note

that <computeroutput>rdesktop-vrdp</computeroutput> can access USB

devices only through <computeroutput>/proc/bus/usb</computeroutput>.

Please refer to <xref linkend="usb_linux" /> for further details on how

to properly set up the permissions. Furthermore it is advisable to

disable automatic loading of any host driver on the remote host which

might work on USB devices to ensure that the devices are accessible by

the RDP client. If the setup was properly done on the remote host,

plug/unplug events are visible on the VBox.log file of the VM.</para>

</sect2>

<sect2 id="vbox-auth">

<title>RDP authentication</title>

<para>For each virtual machine that is remotely accessible via RDP, you

can individually determine if and how RDP connections are

authenticated.</para>

<para>For this, use <computeroutput>VBoxManage modifyvm</computeroutput>

command with the <computeroutput>--vrdpauthtype</computeroutput> option;

see <xref linkend="vboxmanage-modifyvm" /> for a general introduction.

Three methods of authentication are available:<itemizedlist>

<listitem>

<para>The "null" method means that there is no authentication at

all; any client can connect to the VRDP server and thus the

virtual machine. This is, of course, very insecure and only to be

recommended for private networks.</para>

</listitem>

<listitem>

<para>The "external" method provides external authentication

through a special authentication library.</para>

<para>VirtualBox comes with three default libraries for external

authentication:<itemizedlist>

<listitem>

<para>On Linux hosts,

<computeroutput>VBoxAuth.so</computeroutput> authenticates

users against the host's PAM system.</para>

</listitem>

<listitem>

<para>On Windows hosts,

<computeroutput>VBoxAuth.dll</computeroutput> authenticates

users against the host's WinLogon system.</para>

</listitem>

<listitem>

<para>On Mac OS X hosts,

<computeroutput>VBoxAuth.dylib</computeroutput>

authenticates users against the host's directory

service.<footnote>

<para>Support for Mac OS X was added in version

3.2.</para>

</footnote></para>

</listitem>

</itemizedlist></para>

<para>In other words, the "external" method per default performs

authentication with the user accounts that exist on the host

system. Any user with valid authentication credentials is

accepted, i.e. the username does not have to correspond to the

user running the VM.</para>

</listitem>

<listitem>

<para>An additional library called

<computeroutput>VBoxAuthSimple</computeroutput> performs authentication

against credentials configured in the VM's extra data section. This is

probably the simplest way to get authentication that does not depend

on a running and supported guest (see below). In order to enable

VBoxAuthSimple, issue

<computeroutput>VBoxManage setproperty vrdpauthlibrary "VBoxAuthSimple"</computeroutput>.

To enable the library for a VM, switch authentication to external using

<computeroutput>VBoxManage modifyvm "VM name" --vrdpauthtype external</computeroutput>.

Last but not least, you have to configure users and passwords. Here is an example

for the user "john" with the password "secret":

<computeroutput>VBoxManage setextradata "VM name" "VBoxAuthSimple/users/john" "secret"</computeroutput>

To specify an empty password, use the special reserved value

<computeroutput>[NULL]</computeroutput>.

</para>

</listitem>

<listitem>

<para>Finally, the "guest" authentication method performs

authentication with a special component that comes with the Guest

Additions; as a result, authentication is not performed with the

host users, but with the guest user accounts. This method is

currently still in testing and not yet supported.</para>

</listitem>

</itemizedlist></para>

<para>In addition to the methods described above, you can replace the

default "external authentication module with any other module. For this,

VirtualBox provides a well-defined interface that allows you to write your

own authentication module; see <xref

linkend="vbox-authenticate-sdk" /> for details.</para>

</sect2>

<sect2 id="vrdp-crypt">

<title>RDP encryption</title>

<para>RDP features data stream encryption, which is based on the RC4

symmetric cipher (with keys up to 128bit). The RC4 keys are being

replaced in regular intervals (every 4096 packets).</para>

<para>RDP provides three different authentication methods:<orderedlist>

<listitem>

<para>Historically, RDP4 authentication was used, with which the

RDP client does not perform any checks in order to verify the

identity of the server it connects to. Since user credentials can

be obtained using a man in the middle (MITM) attack, RDP4

authentication is insecure and should generally not be

used.</para>

</listitem>

<listitem>

<para>RDP5.1 authentication employs a server certificate for which

the client possesses the public key. This way it is guaranteed

that the server possess the corresponding private key. However, as

this hard-coded private key became public some years ago, RDP5.1

authentication is also insecure and cannot be recommended.</para>

</listitem>

<listitem>

<para>RDP5.2 authentication is based on TLS 1.0 with

customer-supplied certificates. The server supplies a certificate

to the client which must be signed by a certificate authority (CA)

that the client trusts (for the Microsoft RDP Client 5.2, the CA

has to be added to the Windows Trusted Root Certificate

Authorities database). VirtualBox allows you to supply your own CA

and server certificate and uses OpenSSL for encryption.</para>

</listitem>

</orderedlist></para>

<para>While VirtualBox supports all of the above, only RDP5.2

authentication should be used in environments where security is a

concern. As the client that connects to the server determines what type

of encryption will be used, with rdesktop, the Linux RDP viewer, use the

<computeroutput>-4</computeroutput> or

<computeroutput>-5</computeroutput> options.</para>

</sect2>

<sect2 id="vrdp-multiconnection">

<title>Multiple VRDP connections</title>

<para>The built-in RDP server of VirtualBox supports simultaneous

connections to the same running VM from different clients. All connected

clients see the same screen output and share a mouse pointer and

keyboard focus. This is similar to several people using the same

computer at the same time, taking turns at the keyboard.</para>

<para>The following command enables multiple connection mode: <screen>VBoxManage modifyvm "VM name" --vrdpmulticon on</screen></para>

</sect2>

<sect2 id="vrdp-multimonitor">

<title>Multiple remote monitors</title>

<para>

To access two or more remote VM displays you have to enable the

VRDP multiconnection mode (see <xref linkend="vrdp-multiconnection" />).</para>

<para>The RDP client can select the virtual monitor number to connect to

using the <computeroutput>domain</computeroutput> logon parameter

(<computeroutput>-d</computeroutput>). If

the parameter ends with <computeroutput>@</computeroutput> followed by a

number, the VirtualBox RDP server interprets this number as the screen index.

The primary guest screen is selected with

<computeroutput>@1</computeroutput>, the first secondary screen is

<computeroutput>@2</computeroutput>, etc.</para>

<para>The MS RDP6 client does not let you specify a separate domain

name. Instead, use <computeroutput>domain\username</computeroutput> in

the <computeroutput>Username:</computeroutput> field -- for example,

<computeroutput>@2\name</computeroutput>.

<computeroutput>name</computeroutput> must be supplied, and must be the

name used to log in if the VRDP server is set up to require credentials.

If it is not, you may use any text as the username.</para>

</sect2>

<sect2 id="vrdp-videochannel">

<title>VRDP video redirection</title>

<para>Starting with VirtualBox 3.2, the VRDP server can redirect video

streams from the guest to the RDP client. Video frames are compressed

using the JPEG algorithm allowing a higher compression ratio than

standard RDP bitmap compression methods. It is possible to increase the

compression ratio by lowering the video quality.</para>

<para>Video streams in a guest are detected by the VRDP server

automatically as frequently updated rectangular areas. Therefore, this

method works with any guest operating system without having to install

additional software in the guest.</para>

<para>On the client side, however, currently only the Windows 7 Remote

Desktop Connection client supports this feature. If a client does not

support video redirection, the VRDP server uses regular bitmap

updates.</para>

<para>The following command enables video redirection: <screen>VBoxManage modifyvm "VM name" --vrdpvideochannel on</screen></para>

<para>The quality of the video is defined as a value from 10 to 100

percent, as is common with JPEG compression. The quality can be changed

using the following command: <screen>VBoxManage modifyvm "VM name" --vrdpvideochannelquality 75</screen></para>

</sect2>

<sect2 id="vrdp-customization">

<title>VRDP customization</title>

<para>Starting with VirtualBox 3.2.10, it is possible to disable display output,

mouse and keyboard input, audio, remote USB or clipboard in the VRDP server.</para>

<para>The following commands change corresponding server settings:</para>

<screen>VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableDisplay" 1

VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableInput" 1

VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableUSB" 1

VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableAudio" 1

VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableClipboard" 1</screen>

<para>To reenable a feature use a similar command without the trailing 1. For example:

<screen>VBoxManage setextradata "VM name" "VRDP/Feature/Client/DisableDisplay"</screen></para>

</sect2>

</sect1>

<sect1 id="teleporting">

<title>Teleporting</title>

<para>Starting with version 3.1, VirtualBox supports "teleporting" -- that

is, moving a virtual machine over a network from one VirtualBox host to

another, while the virtual machine is running. This works regardless of

the host operating system that is running on the hosts: you can teleport

virtual machines between Solaris and Mac hosts, for example.</para>

<para>Teleporting requires that a machine be currently running on one

host, which is then called the <emphasis role="bold">"source"</emphasis>.

The host to which the virtual machine will be teleported will then be

called the <emphasis role="bold">"target"</emphasis>; the machine on the

target is then configured to wait for the source to contact the target.

The machine's running state will then be transferred from the source to

the target with minimal downtime.</para>

<para>Teleporting happens over any TCP/IP network; the source and the

target only need to agree on a TCP/IP port which is specified in the

teleporting settings.</para>

<para>At this time, there are a few prerequisites for this to work,

however:<orderedlist>

<listitem>

<para>On the target host, you must configure a virtual machine in

VirtualBox with exactly the same hardware settings as the machine on

the source that you want to teleport. This does not apply to

settings which are merely descriptive, such as the VM name, but

obviously for teleporting to work, the target machine must have the

same amount of memory and other hardware settings. Otherwise

teleporting will fail with an error message.</para>

</listitem>

<listitem>

<para>The two virtual machines on the source and the target must

share the same storage (hard disks as well as floppy and CD/DVD

images). This means that they either use the same iSCSI targets or

that the storage resides somewhere on the network and both hosts

have access to it via NFS or SMB/CIFS.</para>

<para>This also means that neither the source nor the target machine

can have any snapshots.</para>

</listitem>

</orderedlist></para>

<para>Then perform the following steps:<orderedlist>

<listitem>

<para>On the <emphasis>target</emphasis> host, configure the virtual

machine to wait for a teleport request to arrive when it is started,

instead of actually attempting to start the machine. This is done

with the following VBoxManage command:<screen>VBoxManage modifyvm &lt;targetvmname&gt; --teleporter on --teleporterport &lt;port&gt;</screen></para>

<para>where <computeroutput>&lt;targetvmname&gt;</computeroutput> is

the name of the virtual machine on the target host and

<computeroutput>&lt;port&gt;</computeroutput> is a TCP/IP port

number to be used on both the source and the target hosts. For

example, use 6000. For details, see <xref

linkend="vboxmanage-modifyvm-teleport" />.</para>

</listitem>

<listitem>

<para>Start the VM on the target host. You will see that instead of

actually running, it will show a progress dialog. indicating that it

is waiting for a teleport request to arrive.</para>

</listitem>

<listitem>

<para>Start the machine on the <emphasis>source</emphasis> host as

usual. When it is running and you want it to be teleported, issue

the following command on the source host:<screen>VBoxManage controlvm &lt;sourcevmname&gt; teleport --host &lt;targethost&gt; --port &lt;port&gt;</screen></para>

<para>where <computeroutput>&lt;sourcevmname&gt;</computeroutput> is

the name of the virtual machine on the source host (the machine that

is currently running),

<computeroutput>&lt;targethost&gt;</computeroutput> is the host or

IP name of the target host on which the machine is waiting for the

teleport request, and <computeroutput>&lt;port&gt;</computeroutput>

must be the same number as specified in the command on the target

host. For details, see <xref

linkend="vboxmanage-controlvm" />.</para>

</listitem>

</orderedlist></para>

<para>For testing, you can also teleport machines on the same host; in

that case, use "localhost" as the hostname on both the source and the

target host.<note>

<para>In rare cases, if the CPUs of the source and the target are very

different, teleporting can fail with an error message, or the target

may hang. This may happen especially if the VM is running application

software that is highly optimized to run on a particular CPU without

correctly checking that certain CPU features are actually present.

VirtualBox filters what CPU capabilities are presented to the guest

operating system. Advanced users can attempt to restrict these virtual

CPU capabilities with the <computeroutput>VBoxManage --modifyvm

--cpuid</computeroutput> command; see <xref

linkend="vboxmanage-modifyvm-teleport" />.</para>

</note></para>

</sect1>

</chapter>