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<chapter id="networkingdetails">
  <title>Virtual networking</title>

  <para>As briefly mentioned in <xref linkend="settings-network" />,
  VirtualBox provides up to eight virtual PCI Ethernet cards for each virtual
  machine. For each such card, you can individually select<orderedlist>
      <listitem>
        <para>the hardware that will be virtualized as well as</para>
      </listitem>

      <listitem>
        <para>the virtualization mode that the virtual card will be operating
        in with respect to your physical networking hardware on the
        host.</para>
      </listitem>
    </orderedlist></para>

  <para>Four of the network cards can be configured in the "Network" section
  of the settings dialog in the graphical user interface of VirtualBox. You
  can configure all eight network cards on the command line via VBoxManage
  modifyvm; see <xref linkend="vboxmanage-modifyvm" />.</para>

  <para>This chapter explains the various networking settings in more
  detail.</para>

  <sect1 id="nichardware">
    <title>Virtual networking hardware</title>

    <para>For each card, you can individually select what kind of
    <emphasis>hardware</emphasis> will be presented to the virtual machine.
    VirtualBox can virtualize the following six types of networking
    hardware:<itemizedlist>
        <listitem>
          <para>AMD PCNet PCI II (Am79C970A);</para>
        </listitem>

        <listitem>
          <para>AMD PCNet FAST III (Am79C973, the default);</para>
        </listitem>

        <listitem>
          <para>Intel PRO/1000 MT Desktop (82540EM);</para>
        </listitem>

        <listitem>
          <para>Intel PRO/1000 T Server (82543GC);</para>
        </listitem>

        <listitem>
          <para>Intel PRO/1000 MT Server (82545EM);</para>
        </listitem>

        <listitem>
          <para>Paravirtualized network adapter (virtio-net).</para>
        </listitem>
      </itemizedlist></para>

    <para>The PCNet FAST III is the default because it is supported by nearly
    all operating systems out of the box, as well as the GNU GRUB boot
    manager. As an exception, the Intel PRO/1000 family adapters are chosen
    for some guest operating system types that no longer ship with drivers for
    the PCNet card, such as Windows Vista.</para>

    <para>The Intel PRO/1000 MT Desktop type works with Windows Vista and
    later versions. The T Server variant of the Intel PRO/1000 card is
    recognized by Windows XP guests without additional driver installation.
    The MT Server variant facilitates OVF imports from other platforms.</para>

    <para>The <emphasis role="bold">"Paravirtualized network adapter
    (virtio-net)"</emphasis> is special. If you select this, then VirtualBox
    does <emphasis>not</emphasis> virtualize common networking hardware (that
    is supported by common guest operating systems out of the box). Instead,
    VirtualBox then expects a special software interface for virtualized
    environments to be provided by the guest, thus avoiding the complexity of
    emulating networking hardware and improving network performance. Starting
    with version 3.1, VirtualBox provides support for the industry-standard
    "virtio" networking drivers, which are part of the open-source KVM
    project.</para>

    <para>The "virtio" networking drivers are available for the following
    guest operating systems:</para>

    <para><itemizedlist>
        <listitem>
          <para>Linux kernels version 2.6.25 or later can be configured to
          provide virtio support; some distributions also back-ported virtio
          to older kernels.</para>
        </listitem>

        <listitem>
          <para>For Windows 2000, XP and Vista, virtio drivers can be
          downloaded and installed from the KVM project web page.<footnote>
              <para><ulink
              url="http://www.linux-kvm.org/page/WindowsGuestDrivers">http://www.linux-kvm.org/page/WindowsGuestDrivers</ulink>.</para>
            </footnote></para>
        </listitem>
      </itemizedlist></para>

    <para>VirtualBox also has limited support for so-called <emphasis
    role="bold">jumbo frames</emphasis>, i.e. networking packets with more
    than 1500 bytes of data, provided that you use the Intel card
    virtualization and bridged networking. In other words, jumbo frames are
    not supported with the AMD networking devices; in those cases, jumbo
    packets will silently be dropped for both the transmit and the receive
    direction. Guest operating systems trying to use this feature will observe
    this as a packet loss, which may lead to unexpected application behavior
    in the guest. This does not cause problems with guest operating systems in
    their default configuration, as jumbo frames need to be explicitly
    enabled.</para>
  </sect1>

  <sect1 id="networkingmodes">
    <title>Introduction to networking modes</title>

    <para>Each of the eight networking adapters can be separately configured
    to operate in one of the following modes:<glosslist>
        <glossentry>
          <glossterm>Not attached</glossterm>

          <glossdef>
            <para>In this mode, VirtualBox reports to the guest that a network
            card is present, but that there is no connection -- as if no
            Ethernet cable was plugged into the card. This way it is possible
            to "pull" the virtual Ethernet cable and disrupt the connection,
            which can be useful to inform a guest operating system that no
            network connection is available and enforce a
            reconfiguration.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Network Address Translation (NAT)</glossterm>

          <glossdef>
            <para>If all you want is to browse the Web, download files and
            view e-mail inside the guest, then this default mode should be
            sufficient for you, and you can safely skip the rest of this
            section. Please note that there are certain limitations when using
            Windows file sharing (see <xref linkend="nat-limitations" /> for
            details).</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>NAT Network</glossterm>

          <glossdef>
            <para>The NAT network is a new NAT flavour introduced in
              VirtualBox 4.3. See
              <xref linkend="network_nat_service" xrefstyle="template: %n" />
              for details.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Bridged networking</glossterm>

          <glossdef>
            <para>This is for more advanced networking needs such as network
            simulations and running servers in a guest. When enabled,
            VirtualBox connects to one of your installed network cards and
            exchanges network packets directly, circumventing your host
            operating system's network stack.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Internal networking</glossterm>

          <glossdef>
            <para>This can be used to create a different kind of
            software-based network which is visible to selected virtual
            machines, but not to applications running on the host or to the
            outside world.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Host-only networking</glossterm>

          <glossdef>
            <para>This can be used to create a network containing the host and
            a set of virtual machines, without the need for the host's
            physical network interface. Instead, a virtual network interface
            (similar to a loopback interface) is created on the host,
            providing connectivity among virtual machines and the host.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Generic networking</glossterm>

          <glossdef>
            <para>Rarely used modes share the same generic network interface,
            by allowing the user to select a driver which can be included with
            VirtualBox or be distributed in an extension pack.</para>

            <para>At the moment there are potentially two available
            sub-modes:</para>

            <para><glosslist>
                <glossentry>
                  <glossterm>UDP Tunnel</glossterm>

                  <glossdef>
                    <para>This can be used to interconnect virtual machines
                    running on different hosts directly, easily and
                    transparently, over existing network
                    infrastructure.</para>
                  </glossdef>
                </glossentry>

                <glossentry>
                  <glossterm>VDE (Virtual Distributed Ethernet)
                  networking</glossterm>

                  <glossdef>
                    <para>This option can be used to connect to a Virtual
                    Distributed Ethernet switch on a Linux or a FreeBSD host.
                    At the moment this needs compiling VirtualBox from
                    sources, as the Oracle packages do not include it.</para>
                  </glossdef>
                </glossentry>
              </glosslist></para>
          </glossdef>
        </glossentry>
      </glosslist></para>

    <para>The following sections describe the available network modes in more
    detail.</para>
  </sect1>

  <sect1 id="network_nat">
    <title>Network Address Translation (NAT)</title>

    <para>Network Address Translation (NAT) is the simplest way of accessing
    an external network from a virtual machine. Usually, it does not require
    any configuration on the host network and guest system. For this reason,
    it is the default networking mode in VirtualBox.</para>

    <para>A virtual machine with NAT enabled acts much like a real computer
    that connects to the Internet through a router. The "router", in this
    case, is the VirtualBox networking engine, which maps traffic from and to
    the virtual machine transparently. In VirtualBox this router is placed
    between each virtual machine and the host. This separation maximizes
    security since by default virtual machines cannot talk to each
    other.</para>

    <para>The disadvantage of NAT mode is that, much like a private network
    behind a router, the virtual machine is invisible and unreachable from the
    outside internet; you cannot run a server this way unless you set up port
    forwarding (described below).</para>

    <para>The network frames sent out by the guest operating system are
    received by VirtualBox's NAT engine, which extracts the TCP/IP data and
    resends it using the host operating system. To an application on the host,
    or to another computer on the same network as the host, it looks like the
    data was sent by the VirtualBox application on the host, using an IP
    address belonging to the host. VirtualBox listens for replies to the
    packages sent, and repacks and resends them to the guest machine on its
    private network.</para>

    <para>The virtual machine receives its network address and configuration
    on the private network from a DHCP server integrated into VirtualBox. The
    IP address thus assigned to the virtual machine is usually on a completely
    different network than the host. As more than one card of a virtual
    machine can be set up to use NAT, the first card is connected to the
    private network 10.0.2.0, the second card to the network 10.0.3.0 and so
    on. If you need to change the guest-assigned IP range for some reason,
    please refer to <xref linkend="changenat" />.</para>

    <sect2 id="natforward">
      <title>Configuring port forwarding with NAT</title>

      <para>As the virtual machine is connected to a private network internal
      to VirtualBox and invisible to the host, network services on the guest
      are not accessible to the host machine or to other computers on the same
      network. However, like a physical router, VirtualBox can make selected
      services available to the world outside the guest through <emphasis
      role="bold">port forwarding.</emphasis> This means that VirtualBox
      listens to certain ports on the host and resends all packets which
      arrive there to the guest, on the same or a different port.</para>

      <para>To an application on the host or other physical (or virtual)
      machines on the network, it looks as though the service being proxied is
      actually running on the host. This also means that you cannot run the
      same service on the same ports on the host. However, you still gain the
      advantages of running the service in a virtual machine -- for example,
      services on the host machine or on other virtual machines cannot be
      compromised or crashed by a vulnerability or a bug in the service, and
      the service can run in a different operating system than the host
      system.</para>

      <para>To configure Port Forwarding you can use the graphical Port
      Forwarding editor which can be found in the Network Settings dialog
      for Network Adaptors configured to use NAT. Here you can map host
      ports to guest ports to allow network traffic to be routed to a
      specific port in the guest.</para>

      <para>Alternatively command line tool <computeroutput>VBoxManage</computeroutput> could be used;
      for details, please refer to <xref linkend="vboxmanage-modifyvm" />.</para>

      <para>You will need to know which ports on the guest the service uses
      and to decide which ports to use on the host (often but not always you
      will want to use the same ports on the guest and on the host). You can
      use any ports on the host which are not already in use by a service. For
      example, to set up incoming NAT connections to an
      <computeroutput>ssh</computeroutput> server in the guest, use the
      following command: <screen>VBoxManage modifyvm "VM name" --natpf1 "guestssh,tcp,,2222,,22"</screen>With
      the above example, all TCP traffic arriving on port 2222 on any host
      interface will be forwarded to port 22 in the guest. The protocol name
      <computeroutput>tcp</computeroutput> is a mandatory attribute defining
      which protocol should be used for forwarding
      (<computeroutput>udp</computeroutput> could also be used). The name
      <computeroutput>guestssh</computeroutput> is purely descriptive and will
      be auto-generated if omitted. The number after
      <computeroutput>--natpf</computeroutput> denotes the network card, like
      in other parts of VBoxManage.</para>

      <para>To remove this forwarding rule again, use the following command:
      <screen>VBoxManage modifyvm "VM name" --natpf1 delete "guestssh"</screen></para>

      <para>If for some reason the guest uses a static assigned IP address not
      leased from the built-in DHCP server, it is required to specify the
      guest IP when registering the forwarding rule: <screen>VBoxManage modifyvm "VM name" --natpf1 "guestssh,tcp,,2222,10.0.2.19,22"</screen>This
      example is identical to the previous one, except that the NAT engine is
      being told that the guest can be found at the 10.0.2.19 address.</para>

      <para>To forward <emphasis>all</emphasis> incoming traffic from a
      specific host interface to the guest, specify the IP of that host
      interface like this:<screen>VBoxManage modifyvm "VM name" --natpf1 "guestssh,tcp,127.0.0.1,2222,,22"</screen>This
      forwards all TCP traffic arriving on the localhost interface (127.0.0.1)
      via port 2222 to port 22 in the guest.</para>

      <para>It is possible to configure incoming NAT connections while the
      VM is running, see <xref linkend="vboxmanage-controlvm"/>.</para>
    </sect2>

    <sect2 id="nat-tftp">
      <title>PXE booting with NAT</title>

      <para>PXE booting is now supported in NAT mode. The NAT DHCP server
      provides a boot file name of the form
      <computeroutput>vmname.pxe</computeroutput> if the directory
      <computeroutput>TFTP</computeroutput> exists in the directory where the
      user's <computeroutput>VirtualBox.xml</computeroutput> file is kept. It
      is the responsibility of the user to provide
      <computeroutput>vmname.pxe</computeroutput>.</para>
    </sect2>

    <sect2 id="nat-limitations">
      <title>NAT limitations</title>

      <para>There are four <emphasis role="bold">limitations</emphasis> of NAT
      mode which users should be aware of:</para>

      <glosslist>
        <glossentry>
          <glossterm>ICMP protocol limitations:</glossterm>

          <glossdef>
            <para>Some frequently used network debugging tools (e.g.
            <computeroutput>ping</computeroutput> or tracerouting) rely on the
            ICMP protocol for sending/receiving messages. While ICMP support
            has been improved with VirtualBox 2.1
            (<computeroutput>ping</computeroutput> should now work), some
            other tools may not work reliably.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Receiving of UDP broadcasts is not reliable:</glossterm>

          <glossdef>
            <para>The guest does not reliably receive broadcasts, since, in
            order to save resources, it only listens for a certain amount of
            time after the guest has sent UDP data on a particular port. As a
            consequence, NetBios name resolution based on broadcasts does not
            always work (but WINS always works). As a workaround, you can use
            the numeric IP of the desired server in the
            <computeroutput>\\server\share</computeroutput> notation.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Protocols such as GRE are unsupported:</glossterm>

          <glossdef>
            <para>Protocols other than TCP and UDP are not supported. This
            means some VPN products (e.g. PPTP from Microsoft) cannot be used.
            There are other VPN products which use simply TCP and UDP.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Forwarding host ports &lt; 1024 impossible:</glossterm>

          <glossdef>
            <para>On Unix-based hosts (e.g. Linux, Solaris, Mac OS X) it is
            not possible to bind to ports below 1024 from applications that
            are not run by <computeroutput>root</computeroutput>. As a result,
            if you try to configure such a port forwarding, the VM will refuse
            to start.</para>
          </glossdef>
        </glossentry>
      </glosslist>

      <para>These limitations normally don't affect standard network use. But
      the presence of NAT has also subtle effects that may interfere with
      protocols that are normally working. One example is NFS, where the
      server is often configured to refuse connections from non-privileged
      ports (i.e. ports not below 1024).</para>
    </sect2>
  </sect1>

  <sect1 id="network_nat_service">
    <title>Network Address Translation Service</title>
    
    <para>The Network Address Translation (NAT) service works in a similar way
    to a home router, grouping the systems using it into a network and
    preventing systems outside of this network from directly accessing systems
    inside it, but letting systems inside communicate with each other and with
    systems outside using TCP and UDP over IPv4 and IPv6.</para>

    <para>A NAT service is attached to an internal network. Virtual machines
    which are to make use of it should be attached to that internal network.
    The name of internal network is chosen when the NAT service is created and
    the internal network will be created if it does not already exist.  An
    example command to create a NAT network is:
    </para>
    <para><screen>VBoxManage natnetwork add --netname natnet1 --network "192.168.15.0/24" --enable</screen></para>
    <para>
    Here, "natnet1" is the name of the internal network to be used and
    "192.168.15.0/24" is the network address and mask of the NAT service
    interface.  By default in this static configuration the gateway will be
    assigned the address 192.168.15.1 (the address following the interface
    address), though this is subject to change.  To attach a DHCP server to the
    internal network, we modify the example as follows:</para>
    <para><screen>VBoxManage natnetwork add --netname natnet1 --network "192.168.15.0/24" --enable --dhcp on</screen></para>
    <para> or to add a DHCP server to the network after creation:</para>
    <para><screen>VBoxManage natnetwork modify --netname natnet1 --dhcp on</screen></para>
    <para>To disable it again, use:</para>
    <para><screen>VBoxManage natnetwork modify --netname natnet1 --dhcp off</screen></para>
    <para>DHCP server provides list of registered nameservers, but doesn't map 
    servers from 127/8 network.</para>
    <para>To start the NAT service, use the following command:</para>
    <para><screen>VBoxManage natnetwork start --netname natnet1</screen></para>
    <para>If the network has a DHCP server attached then it will start together
    with the NAT network service.</para>
    <para><screen>VBoxManage natnetwork stop --netname natnet1</screen> stops
    the NAT network service, together with DHCP server if any.</para>
    <para>To delete the NAT network service use:</para>
    <para><screen>VBoxManage natnetwork remove --netname natnet1</screen></para>
    <para>This command does not remove the DHCP server if one is enabled on the
    internal network.</para>
    <para>Port-forwarding is supported (using the
    <computeroutput>--port-forward-4</computeroutput> switch for IPv4 and
    <computeroutput>--port-forward-6</computeroutput>
    for IPv6):</para>
    <para><screen>VBoxManage natnetwork modify --netname natnet1 --port-forward-4 "ssh:tcp:[]:1022:[192.168.15.5]:22"</screen></para>
    <para>This adds a port-forwarding rule from the host's TCP 1022 port to
    the port 22 on the guest with IP address 192.168.15.5. Host port, guest port and guest IP
    are mandatory. To delete the rule, use:</para>
    <para><screen>VBoxManage natnetwork modify --netname natnet1 --port-forward-4 delete ssh</screen></para>
    <para>It's possible to bind NAT service to specified interface:</para>
    <screen>VBoxManage setextradata global "NAT/win-nat-test-0/SourceIp4" 192.168.1.185</screen>
    <para>To see the list of registered NAT networks, use:</para>
    <para><screen>VBoxManage list natnetworks</screen></para>
  </sect1>

  <sect1 id="network_bridged">
    <title>Bridged networking</title>

    <para>With bridged networking, VirtualBox uses a device driver on your
    <emphasis>host</emphasis> system that filters data from your physical
    network adapter. This driver is therefore called a "net filter" driver.
    This allows VirtualBox to intercept data from the physical network and
    inject data into it, effectively creating a new network interface in
    software. When a guest is using such a new software interface, it looks to
    the host system as though the guest were physically connected to the
    interface using a network cable: the host can send data to the guest
    through that interface and receive data from it. This means that you can
    set up routing or bridging between the guest and the rest of your
    network.</para>

    <para>For this to work, VirtualBox needs a device driver on your host
    system. The way bridged networking works has been completely rewritten
    with VirtualBox 2.0 and 2.1, depending on the host operating system. From
    the user perspective, the main difference is that complex configuration is
    no longer necessary on any of the supported host operating
    systems.<footnote>
        <para>For Mac OS X and Solaris hosts, net filter drivers were already
        added in VirtualBox 2.0 (as initial support for Host Interface
        Networking on these platforms). With VirtualBox 2.1, net filter
        drivers were also added for the Windows and Linux hosts, replacing the
        mechanisms previously present in VirtualBox for those platforms;
        especially on Linux, the earlier method required creating TAP
        interfaces and bridges, which was complex and varied from one
        distribution to the next. None of this is necessary anymore. Bridged
        network was formerly called "Host Interface Networking" and has been
        renamed with version 2.2 without any change in functionality.</para>
      </footnote></para>

    <para><note>
        <para>Even though TAP is no longer necessary on Linux with bridged
        networking, you <emphasis>can</emphasis> still use TAP interfaces for
        certain advanced setups, since you can connect a VM to any host
        interface -- which could also be a TAP interface.</para>
      </note>To enable bridged networking, all you need to do is to open the
    Settings dialog of a virtual machine, go to the "Network" page and select
    "Bridged network" in the drop down list for the "Attached to" field.
    Finally, select desired host interface from the list at the bottom of the
    page, which contains the physical network interfaces of your systems. On a
    typical MacBook, for example, this will allow you to select between "en1:
    AirPort" (which is the wireless interface) and "en0: Ethernet", which
    represents the interface with a network cable.</para>

    <note><para>Bridging to a wireless interface is done differently from
        bridging to a wired interface, because most wireless adapters do not
        support promiscuous mode. All traffic has to use the MAC address of the
        host's wireless adapter, and therefore VirtualBox needs to replace the
        source MAC address in the Ethernet header of an outgoing packet to make
        sure the reply will be sent to the host interface. When VirtualBox sees
        an incoming packet with a destination IP address that belongs to one of
        the virtual machine adapters it replaces the destination MAC address in
        the Ethernet header with the VM adapter's MAC address and passes it on.
        VirtualBox examines ARP and DHCP packets in order to learn the IP
        addresses of virtual machines.</para></note>

    <para>Depending on your host operating system, the following limitations
    should be kept in mind:<itemizedlist>
        <listitem>
          <para>On <emphasis role="bold">Macintosh</emphasis> hosts,
          functionality is limited when using AirPort (the Mac's wireless
          networking) for bridged networking. Currently, VirtualBox supports
          only IPv4 and IPv6 over AirPort. For other protocols (such as IPX),
          you must choose a wired interface.</para>
        </listitem>

        <listitem>
          <para>On <emphasis role="bold">Linux</emphasis> hosts, functionality
          is limited when using wireless interfaces for bridged networking.
          Currently, VirtualBox supports only IPv4 and IPv6 over wireless.
          For other protocols (such as IPX), you must choose a wired
          interface.</para>

          <para>Also, setting the MTU to less than 1500 bytes on wired
          interfaces provided by the sky2 driver on the Marvell Yukon II EC
          Ultra Ethernet NIC is known to cause packet losses under certain
          conditions.</para>

          <para>Some adapters strip VLAN tags in hardware. This does not allow
          to use VLAN trunking between VM and the external network with
          pre-2.6.27 Linux kernels nor with host operating systems other than
          Linux.</para>
        </listitem>

        <listitem>
          <para>On <emphasis role="bold">Solaris</emphasis> hosts, there is no
          support for using wireless interfaces. Filtering guest traffic using
          IPFilter is also not completely supported due to technical
          restrictions of the Solaris networking subsystem. These issues would
          be addressed in a future release of Solaris 11.</para>

          <para>Starting with VirtualBox 4.1, on Solaris 11 hosts (build 159
          and above), it is possible to use Solaris' Crossbow Virtual Network
          Interfaces (VNICs) directly with VirtualBox without any additional
          configuration other than each VNIC must be exclusive for every guest
          network interface.</para>

          <para>Starting with VirtualBox 2.0.4 and up to VirtualBox 4.0, VNICs
          can be used but with the following caveats:</para>

          <itemizedlist>
            <listitem>
              <para>A VNIC cannot be shared between multiple guest network
              interfaces, i.e. each guest network interface must have its own,
              exclusive VNIC.</para>
            </listitem>

            <listitem>
              <para>The VNIC and the guest network interface that uses the
              VNIC must be assigned identical MAC addresses.</para>
            </listitem>
          </itemizedlist>

          <para>When using VLAN interfaces with VirtualBox, they must be named
          according to the PPA-hack naming scheme (e.g. "e1000g513001"), as
          otherwise the guest may receive packets in an unexpected
          format.</para>
        </listitem>
      </itemizedlist></para>
  </sect1>

  <sect1 id="network_internal">
    <title>Internal networking</title>

    <para>Internal Networking is similar to bridged networking in that the VM
    can directly communicate with the outside world. However, the "outside
    world" is limited to other VMs on the same host which connect to the same
    internal network.</para>

    <para>Even though technically, everything that can be done using internal
    networking can also be done using bridged networking, there are security
    advantages with internal networking. In bridged networking mode, all
    traffic goes through a physical interface of the host system. It is
    therefore possible to attach a packet sniffer (such as Wireshark) to the
    host interface and log all traffic that goes over it. If, for any reason,
    you prefer two or more VMs on the same machine to communicate privately,
    hiding their data from both the host system and the user, bridged
    networking therefore is not an option.</para>

    <para>Internal networks are created automatically as needed, i.e. there is
    no central configuration. Every internal network is identified simply by
    its name. Once there is more than one active virtual network card with the
    same internal network ID, the VirtualBox support driver will automatically
    "wire" the cards and act as a network switch. The VirtualBox support
    driver implements a complete Ethernet switch and supports both
    broadcast/multicast frames and promiscuous mode.</para>

    <para>In order to attach a VM's network card to an internal network, set
    its networking mode to "internal networking". There are two ways to
    accomplish this:</para>

    <para><itemizedlist>
        <listitem>
          <para>You can use a VM's "Settings" dialog in the VirtualBox
          graphical user interface. In the "Networking" category of the
          settings dialog, select "Internal Networking" from the drop-down
          list of networking modes. Now select the name of an existing
          internal network from the drop-down below or enter a new name into
          the entry field.</para>
        </listitem>

        <listitem>
          <para>You can use <screen>VBoxManage modifyvm "VM name" --nic&lt;x&gt; intnet</screen>
          Optionally, you can specify a network name with the command <screen>VBoxManage modifyvm "VM name" --intnet&lt;x&gt; "network name"</screen>
          If you do not specify a network name, the network card will be
          attached to the network <computeroutput>intnet</computeroutput> by
          default.</para>
        </listitem>
      </itemizedlist></para>

    <para>Unless you configure the (virtual) network cards in the guest
    operating systems that are participating in the internal network to use
    static IP addresses, you may want to use the DHCP server that is built
    into VirtualBox to manage IP addresses for the internal network. Please
    see <xref linkend="vboxmanage-dhcpserver" /> for details.</para>

    <para>As a security measure, the Linux implementation of internal
    networking only allows VMs running under the same user ID to establish an
    internal network.</para>
  </sect1>

  <sect1 id="network_hostonly">
    <title>Host-only networking</title>

    <para>Host-only networking is another networking mode that was added with
    version 2.2 of VirtualBox. It can be thought of as a hybrid between the
    bridged and internal networking modes: as with bridged networking, the
    virtual machines can talk to each other and the host as if they were
    connected through a physical Ethernet switch. Similarly, as with internal
    networking however, a physical networking interface need not be present,
    and the virtual machines cannot talk to the world outside the host since
    they are not connected to a physical networking interface.</para>

    <para>Instead, when host-only networking is used, VirtualBox creates a new
    software interface on the host which then appears next to your existing
    network interfaces. In other words, whereas with bridged networking an
    existing physical interface is used to attach virtual machines to, with
    host-only networking a new "loopback" interface is created on the host.
    And whereas with internal networking, the traffic between the virtual
    machines cannot be seen, the traffic on the "loopback" interface on the
    host can be intercepted.</para>

    <para>Host-only networking is particularly useful for preconfigured
    virtual appliances, where multiple virtual machines are shipped together
    and designed to cooperate. For example, one virtual machine may contain a
    web server and a second one a database, and since they are intended to
    talk to each other, the appliance can instruct VirtualBox to set up a
    host-only network for the two. A second (bridged) network would then
    connect the web server to the outside world to serve data to, but the
    outside world cannot connect to the database.</para>

    <para>To change a virtual machine's virtual network interface to "host
    only" mode:<itemizedlist>
        <listitem>
          <para>either go to the "Network" page in the virtual machine's
          settings notebook in the graphical user interface and select
          "Host-only networking", or</para>
        </listitem>

        <listitem>
          <para>on the command line, type <computeroutput>VBoxManage modifyvm
          "VM name" --nic&lt;x&gt; hostonly</computeroutput>; see <xref
          linkend="vboxmanage-modifyvm" /> for details.</para>
        </listitem>
    </itemizedlist></para>

    <para>Before you can attach a VM to a host-only network you have to
      create at least one host-only interface, either from the GUI:
      "File" &rarr; "Preferences" &rarr; "Network" &rarr; "Host-only network"
      &rarr; "(+)Add host-only network", or via command line with</para>
      <screen>VBoxManage hostonlyif create</screen> 
      <para>see <xref linkend="vboxmanage-hostonlyif" /> for details.</para>

    <para>For host-only networking, like with internal networking, you may
    find the DHCP server useful that is built into VirtualBox. This can be
    enabled to then manage the IP addresses in the host-only network since
    otherwise you would need to configure all IP addresses
    statically.<itemizedlist>
        <listitem>
          <para>In the VirtualBox graphical user interface, you can configure
          all these items in the global settings via "File" &rarr; "Preferences"
          &rarr; "Network", which lists all host-only networks which are
          presently in use. Click on the network name and then on the "Edit"
          button to the right, and you can modify the adapter and DHCP
          settings.</para>
        </listitem>

        <listitem>
          <para>Alternatively, you can use <computeroutput>VBoxManage
          dhcpserver</computeroutput> on the command line; please see <xref
          linkend="vboxmanage-dhcpserver" /> for details.</para>
        </listitem>
      </itemizedlist>
    </para>

    <para><note><para>On Linux and Mac OS X hosts the number of host-only
    interfaces is limited to 128. There is no such limit for Solaris and
    Windows hosts.</para></note></para>
  </sect1>

  <sect1 id="network_udp_tunnel">
    <title>UDP Tunnel networking</title>

    <para>This networking mode allows to interconnect virtual machines running
    on different hosts.</para>

    <para>Technically this is done by encapsulating Ethernet frames sent or
    received by the guest network card into UDP/IP datagrams, and sending them
    over any network available to the host.</para>

    <para>UDP Tunnel mode has three parameters:<glosslist>
        <glossentry>
          <glossterm>Source UDP port</glossterm>

          <glossdef>
            <para>The port on which the host listens. Datagrams arriving on
            this port from any source address will be forwarded to the
            receiving part of the guest network card.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Destination address</glossterm>

          <glossdef>
            <para>IP address of the target host of the transmitted
            data.</para>
          </glossdef>
        </glossentry>

        <glossentry>
          <glossterm>Destination UDP port</glossterm>

          <glossdef>
            <para>Port number to which the transmitted data is sent.</para>
          </glossdef>
        </glossentry>
      </glosslist></para>

    <para>When interconnecting two virtual machines on two different hosts,
    their IP addresses must be swapped. On single host, source and destination
    UDP ports must be swapped.</para>

    <para>In the following example host 1 uses the IP address 10.0.0.1 and
    host 2 uses IP address 10.0.0.2. Configuration via command-line:<screen>        VBoxManage modifyvm "VM 01 on host 1" --nic&lt;x&gt; generic
        VBoxManage modifyvm "VM 01 on host 1" --nicgenericdrv&lt;x&gt; UDPTunnel
        VBoxManage modifyvm "VM 01 on host 1" --nicproperty&lt;x&gt; dest=10.0.0.2
        VBoxManage modifyvm "VM 01 on host 1" --nicproperty&lt;x&gt; sport=10001
        VBoxManage modifyvm "VM 01 on host 1" --nicproperty&lt;x&gt; dport=10002</screen>
    and <screen>        VBoxManage modifyvm "VM 02 on host 2" --nic&lt;y&gt; generic
        VBoxManage modifyvm "VM 02 on host 2" --nicgenericdrv&lt;y&gt; UDPTunnel
        VBoxManage modifyvm "VM 02 on host 2" --nicproperty&lt;y&gt; dest=10.0.0.1
        VBoxManage modifyvm "VM 02 on host 2" --nicproperty&lt;y&gt; sport=10002
        VBoxManage modifyvm "VM 02 on host 2" --nicproperty&lt;y&gt; dport=10001</screen></para>

    <para>Of course, you can always interconnect two virtual machines on the
    same host, by setting the destination address parameter to 127.0.0.1 on
    both. It will act similarly to "Internal network" in this case, however
    the host can see the network traffic which it could not in the normal
    Internal network case.</para>

    <para><note><para>On Unix-based hosts (e.g. Linux, Solaris, Mac OS X) it is
    not possible to bind to ports below 1024 from applications that are not run
    by <computeroutput>root</computeroutput>. As a result, if you try to
    configure such a source UDP port, the VM will refuse to
    start.</para></note></para>
  </sect1>

  <sect1 id="network_vde">
    <title>VDE networking</title>

    <para>Virtual Distributed Ethernet (VDE<footnote>
        <para>VDE is a project developed by Renzo Davoli, Associate Professor
        at the University of Bologna, Italy.</para>
      </footnote>) is a flexible, virtual network infrastructure system,
    spanning across multiple hosts in a secure way. It allows for L2/L3
    switching, including spanning-tree protocol, VLANs, and WAN emulation. It
    is an optional part of VirtualBox which is only included in the source
    code.</para>

    <para>The basic building blocks of the infrastructure are VDE switches,
    VDE plugs and VDE wires which inter-connect the switches.</para>

    <para>The VirtualBox VDE driver has one parameter:<glosslist>
        <glossentry>
          <glossterm>VDE network</glossterm>

          <glossdef>
            <para>The name of the VDE network switch socket to which the VM
            will be connected.</para>
          </glossdef>
        </glossentry>
      </glosslist></para>

    <para>The following basic example shows how to connect a virtual machine
    to a VDE switch:</para>

    <para><orderedlist>
        <listitem>
          <para>Create a VDE switch: <screen>vde_switch -s /tmp/switch1</screen></para>
        </listitem>

        <listitem>
          <para>Configuration via command-line: <screen>VBoxManage modifyvm "VM name" --nic&lt;x&gt; generic</screen>
          <screen>VBoxManage modifyvm "VM name" --nicgenericdrv&lt;x&gt; VDE</screen>
          To connect to automatically allocated switch port, use: <screen>VBoxManage modifyvm "VM name" --nicproperty&lt;x&gt; network=/tmp/switch1</screen>
          To connect to specific switch port &lt;n&gt;, use: <screen>VBoxManage modifyvm "VM name" --nicproperty&lt;x&gt; network=/tmp/switch1[&lt;n&gt;]</screen>
          The latter option can be useful for VLANs.</para>
        </listitem>

        <listitem>
          <para>Optionally map between VDE switch port and VLAN: (from switch
          CLI) <screen>vde$ vlan/create &lt;VLAN&gt;</screen> <screen>vde$ port/setvlan &lt;port&gt; &lt;VLAN&gt;</screen></para>
        </listitem>
      </orderedlist></para>

    <para>VDE is available on Linux and FreeBSD hosts only. It is only
    available if the VDE software and the VDE plugin library from the
    VirtualSquare project are installed on the host system<footnote>
        <para>For Linux hosts, the shared library libvdeplug.so must be
        available in the search path for shared libraries</para>
      </footnote>. For more information on setting up VDE networks, please see
    the documentation accompanying the software.<footnote>
        <para><ulink
        url="http://wiki.virtualsquare.org/wiki/index.php/VDE_Basic_Networking">http://wiki.virtualsquare.org/wiki/index.php/VDE_Basic_Networking</ulink>.</para>
      </footnote></para>
  </sect1>

  <sect1 id="network_bandwidth_limit">
    <title>Limiting bandwidth for network I/O</title>

    <para>Starting with version 4.2, VirtualBox allows for limiting the
    maximum bandwidth used for network transmission. Several network adapters
    of one VM may share limits through bandwidth groups. It is possible
    to have more than one such limit.</para>
    <note><para>VirtualBox shapes VM traffic only in the transmit direction,
        delaying the packets being sent by virtual machines. It does not limit
        the traffic being received by virtual machines.</para>
    </note>

    <para>Limits are configured through
    <computeroutput>VBoxManage</computeroutput>. The example below creates a
    bandwidth group named "Limit", sets the limit to 20 Mbit/s and assigns the
    group to the first and second adapters of the VM:<screen>VBoxManage bandwidthctl "VM name" add Limit --type network --limit 20m
VBoxManage modifyvm "VM name" --nicbandwidthgroup1 Limit
VBoxManage modifyvm "VM name" --nicbandwidthgroup2 Limit</screen></para>

    <para>All adapters in a group share the bandwidth limit, meaning that in the
    example above the bandwidth of both adapters combined can never exceed 20
    Mbit/s. However, if one adapter doesn't require bandwidth the other can use the
    remaining bandwidth of its group.</para>

    <para>The limits for each group can be changed while the VM is running,
    with changes being picked up immediately. The example below changes the
    limit for the group created in the example above to 100 Kbit/s:<screen>VBoxManage bandwidthctl "VM name" set Limit --limit 100k</screen></para>

    <para>To completely disable shaping for the first adapter of VM use the
      following command:
      <screen>VBoxManage modifyvm "VM name" --nicbandwidthgroup1 none</screen></para>

    <para>It is also possible to disable shaping for all adapters assigned to a
      bandwidth group while VM is running, by specifying the zero limit for the
      group. For example, for the bandwidth group named "Limit" use:
      <screen>VBoxManage bandwidthctl "VM name" set Limit --limit 0</screen></para>
  </sect1>
  <sect1 id="network_performance">
    <title>Improving network performance</title>

    <para>VirtualBox provides a variety of virtual network adapters that can be
      "attached" to the host's network in a number of ways. Depending on which
      types of adapters and attachments are used the network performance will
      be different. Performance-wise the <emphasis>virtio</emphasis> network
      adapter is preferable over <emphasis>Intel PRO/1000</emphasis> emulated
      adapters, which are preferred over <emphasis>PCNet</emphasis> family of
      adapters. Both <emphasis>virtio</emphasis> and <emphasis>Intel PRO/1000
      </emphasis> adapters enjoy the benefit of segmentation and checksum
      offloading. Segmentation offloading is essential for high performance as
      it allows for less context switches, dramatically increasing the sizes
      of packets that cross VM/host boundary.</para>
    <note><para>Neither <emphasis>virtio</emphasis> nor <emphasis>Intel PRO/1000
        </emphasis> drivers for Windows XP support segmentation
        offloading. Therefore Windows XP guests never reach the same
        transmission rates as other guest types. Refer to MS Knowledge base
        article 842264 for additional information.</para>
    </note>
    <para>Three attachment types: <emphasis>internal</emphasis>,
      <emphasis>bridged</emphasis> and <emphasis>host-only</emphasis>, have
      nearly identical performance, the <emphasis>internal</emphasis> type
      being a little bit faster and using less CPU cycles as the packets never
      reach the host's network stack. The <emphasis>NAT</emphasis> attachment
      is the slowest (and safest) of all attachment types as it provides
      network address translation. The generic driver attachment is special and
      cannot be considered as an alternative to other attachment types.</para>
    <para>The number of CPUs assigned to VM does not improve network
      performance and in some cases may hurt it due to increased concurrency in
      the guest.</para>
    <para>Here is the short summary of things to check in order to improve
      network performance:</para>
    <para><orderedlist>
        <listitem>
          <para>Whenever possible use <emphasis>virtio</emphasis> network
            adapter, otherwise use one of <emphasis>Intel PRO/1000</emphasis>
            adapters;</para>
        </listitem>
        <listitem>
          <para>Use <emphasis>bridged</emphasis> attachment instead of
            <emphasis>NAT</emphasis>;</para>
        </listitem>
        <listitem>
          <para>Make sure segmentation offloading is enabled in the guest OS.
            Usually it will be enabled by default. You can check and modify
            offloading settings using <computeroutput>ethtool</computeroutput>
            command in Linux guests.</para>
        </listitem>
        </orderedlist></para>
  </sect1>
</chapter>
