source: vbox/trunk/doc/manual/en_US/user_Networking.xml@ 75206

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<chapter id="networkingdetails">

  <title>Virtual Networking</title>

  <para>
    As 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 the
    following:
  </para>

  <itemizedlist>

    <listitem>
      <para>
        The hardware that will be virtualized.
      </para>
    </listitem>

    <listitem>
      <para>
        The virtualization mode that the virtual card operates in, with
        respect to your physical networking hardware on the host.
      </para>
    </listitem>

  </itemizedlist>

  <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 using 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 types of
      networking hardware:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          AMD PCNet PCI II (Am79C970A)
        </para>
      </listitem>

      <listitem>
        <para>
          AMD PCNet FAST III (Am79C973), the default setting
        </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>
      The PCNet FAST III is the default because it is supported by
      nearly all operating systems, as well as by 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 Paravirtualized network adapter (virtio-net) is special. If
      you select this adapter, then VirtualBox does
      <emphasis>not</emphasis> virtualize common networking hardware
      that is supported by common guest operating systems. Instead,
      VirtualBox 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 <emphasis>virtio</emphasis>
      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>

    <itemizedlist>

      <listitem>
        <para>
          Linux kernels version 2.6.25 or later can be configured to
          provide virtio support. Some distributions have 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:
        </para>

        <para>
          <ulink
              url="http://www.linux-kvm.org/page/WindowsGuestDrivers"/>.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      VirtualBox also has limited support for <emphasis>jumbo
      frames</emphasis>. These are networking packets with more than
      1500 bytes of data, provided that you use the Intel card
      virtualization and bridged networking. 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 networking adapters can be separately configured to
      operate in one of the following modes:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Not attached.</emphasis> In this mode,
          VirtualBox reports to the guest that a network card is
          present, but that there is no connection. This is as if no
          Ethernet cable was plugged into the card. Using this mode, 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>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Network Address Translation
          (NAT)</emphasis>. If all you want is to browse the Web,
          download files, and view email inside the guest, then this
          default mode should be sufficient for you, and you can skip
          the rest of this section. Please note that there are certain
          limitations when using Windows file sharing. See
          <xref linkend="nat-limitations" />.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">NAT Network.</emphasis> The NAT network
          is a new NAT flavour introduced in VirtualBox 4.3. See
          <xref linkend="network_nat_service"/>.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Bridged networking.</emphasis> 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>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Internal networking.</emphasis> 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>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Host-only networking.</emphasis> 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>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold"> Generic networking.</emphasis> Rarely
          used modes which 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>
          The following sub-modes are available:
        </para>

        <itemizedlist>

          <listitem>
            <para>
              <emphasis role="bold">UDP Tunnel:</emphasis> Used to
              interconnect virtual machines running on different hosts
              directly, easily, and transparently, over an existing
              network infrastructure.
            </para>
          </listitem>

          <listitem>
            <para>
              <emphasis role="bold">VDE (Virtual Distributed Ethernet)
              networking:</emphasis> Used to connect to a Virtual
              Distributed Ethernet switch on a Linux or a FreeBSD host.
              At the moment this option requires compilation of
              VirtualBox from sources, as the Oracle packages do not
              include it.
            </para>
          </listitem>

        </itemizedlist>
      </listitem>

    </itemizedlist>

    <para>
      <xref linkend="table-networking-modes"/> provides a quick overview
      of the most important networking modes.
    </para>

    <table id="table-networking-modes">
      <title>Overview of Networking Modes</title>
      <tgroup cols="6">
        <colspec align="left" />
        <colspec align="center" />
        <colspec align="center" />
        <colspec align="center" />
        <colspec align="center" />
        <colspec align="center" />
        <thead valign="middle">
          <row>
            <entry></entry>
            <entry><emphasis role="bold">VM&rarr;Host</emphasis></entry>
            <entry><emphasis role="bold">VM&larr;Host</emphasis></entry>
            <entry><emphasis role="bold">VM1&harr;VM2</emphasis></entry>
            <entry><emphasis role="bold">VM&rarr;Net/LAN</emphasis></entry>
            <entry><emphasis role="bold">VM&larr;Net/LAN</emphasis></entry>
          </row>
        </thead>
        <tbody valign="middle">
          <row>
            <entry>Host-only</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry align="center"><emphasis role="bold">+</emphasis></entry>
            <entry>&ndash;</entry>
            <entry>&ndash;</entry>
          </row>
          <row>
            <entry>Internal</entry>
            <entry>&ndash;</entry>
            <entry>&ndash;</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry>&ndash;</entry>
            <entry>&ndash;</entry>
          </row>
          <row>
            <entry>Bridged</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
          </row>
          <row>
            <entry>NAT</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><link linkend="natforward">Port forward</link></entry>
            <entry>&ndash;</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><link linkend="natforward">Port forward</link></entry>
          </row>
          <row>
            <entry>NATservice</entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><link linkend="network_nat_service">Port forward</link></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><emphasis role="bold">+</emphasis></entry>
            <entry><link linkend="network_nat_service">Port forward</link></entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <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. See <xref linkend="natforward"/>.
    </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, see
      <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>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, the command line tool
        <computeroutput>VBoxManage</computeroutput> can be used. See
        <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. You may 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, as with other VBoxManage command.
      </para>

      <para>
        To remove this forwarding rule, 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 some limitations of NAT mode which users should be
        aware of, as follows:
      </para>

      <itemizedlist>

        <listitem>
          <para>
            <emphasis role="bold">ICMP protocol limitations.</emphasis>
            Some frequently used network debugging tools, such as
            <computeroutput>ping</computeroutput> or tracerouting, rely
            on the ICMP protocol for sending and receiving messages.
            While ICMP support has been improved with VirtualBox 2.1,
            meaning <computeroutput>ping</computeroutput> should now
            work, some other tools may not work reliably.
          </para>
        </listitem>

        <listitem>
          <para>
            <emphasis role="bold">Receiving of UDP
            broadcasts.</emphasis> The guest does not reliably receive
            UDP broadcasts. 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>
        </listitem>

        <listitem>
          <para>
            <emphasis role="bold">Some protocols are not
            supported.</emphasis> Protocols other than TCP and UDP are
            not supported. GRE is not supported. This means some VPN
            products, such as PPTP from Microsoft, cannot be used. There
            are other VPN products which use only TCP and UDP.
          </para>
        </listitem>

        <listitem>
          <para>
            <emphasis role="bold">Forwarding host ports below
            1024.</emphasis> On Unix-based hosts, such as Linux,
            Solaris, and 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>
        </listitem>

      </itemizedlist>

      <para>
        These limitations normally do not 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, which are those 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>
      To add a DHCP server to an existing network:
    </para>

    <para>
<screen>VBoxManage natnetwork modify --netname natnet1 --dhcp on</screen>
    </para>

    <para>
      To disable the DHCP server:
    </para>

    <para>
<screen>VBoxManage natnetwork modify --netname natnet1 --dhcp off</screen>
    </para>

    <para>
      A DHCP server provides a list of registered nameservers, but does
      not map servers from the 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>
      To stops the NAT network service, together with any DHCP server:
    </para>

    <para>
<screen>VBoxManage natnetwork stop --netname natnet1</screen>
    </para>

    <para>
      To delete the NAT network service:
    </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.
      For example:
    </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 is possible to bind a NAT service to specified interface. For
      example:
    </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
      <emphasis>net filter</emphasis> 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>

    <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.
      </para>
    </note>

    <para>
      To enable bridged networking, open the Settings dialog of a
      virtual machine, go to the Network page and select
      <emphasis role="bold">Bridged Network</emphasis> in the drop-down
      list for the Attached To field. Select a 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
      apply:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Mac OS X hosts.</emphasis> Functionality
          is limited when using AirPort, the Mac's wireless networking
          system, 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>
          <emphasis role="bold">Linux hosts.</emphasis> 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>
          <emphasis role="bold">Solaris hosts.</emphasis> 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. For example, 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, such as e1000g513001.
          Otherwise, the guest may receive packets in an unexpected
          format.
        </para>
      </listitem>

    </itemizedlist>

  </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. 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 <emphasis>wire</emphasis> 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>

    <itemizedlist>

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

      <listitem>
        <para>
          Use the command line, for example:
        </para>

<screen>VBoxManage modifyvm "VM name" --nic&lt;x&gt; intnet</screen>

        <para>
          Optionally, you can specify a network name with the command:
        </para>

<screen>VBoxManage modifyvm "VM name" --intnet&lt;x&gt; "network name"</screen>

        <para>
          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>
      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. See <xref linkend="vboxmanage-dhcpserver" />.
    </para>

    <para>
      As a security measure, by default, the Linux implementation of
      internal networking only allows VMs running under the same user ID
      to establish an internal network. However, it is possible to
      create a shared internal networking interface, accessible by users
      with different user IDs.
    </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.
      As with internal networking, 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>
      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
      <emphasis>loopback</emphasis> 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, do either of the following:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          Go to the Network page in the virtual machine's Settings
          dialog and select <emphasis role="bold">Host-Only
          Networking</emphasis>.
        </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" />.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      Before you can attach a VM to a host-only network you have to
      create at least one host-only interface. You can use the GUI for
      this. Choose <emphasis role="bold">File</emphasis>,
      <emphasis role="bold">Preferences</emphasis>,
      <emphasis role="bold">Network</emphasis>,
      <emphasis role="bold">Host-Only Network</emphasis>,
      <emphasis role="bold">(+)Add Host-Only Network</emphasis>.
    </para>

    <para>
      Alternatively, you can use the command line:
    </para>

<screen>VBoxManage hostonlyif create</screen>

    <para>
      See <xref linkend="vboxmanage-hostonlyif" />.
    </para>

    <para>
      For host-only networking, as 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.
    </para>

    <itemizedlist>

      <listitem>
        <para>
          In the VirtualBox graphical user interface, you can configure
          all these items in the global settings by choosing
          <emphasis role="bold">File</emphasis>,
          <emphasis role="bold">Preferences</emphasis>,
          <emphasis role="bold">Network</emphasis>. This lists all
          host-only networks which are presently in use. Click on the
          network name and then on
          <emphasis role="bold">Edit</emphasis>. You can then modify the
          adapter and DHCP settings.
        </para>
      </listitem>

      <listitem>
        <para>
          Alternatively, you can use <computeroutput>VBoxManage
          dhcpserver</computeroutput> on the command line. See
          <xref
          linkend="vboxmanage-dhcpserver" />.
        </para>
      </listitem>

    </itemizedlist>

    <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>

  </sect1>

  <sect1 id="network_udp_tunnel">

    <title>UDP Tunnel Networking</title>

    <para>
      This networking mode allows you 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 the following parameters:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Source UDP port:</emphasis> 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>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Destination address:</emphasis> IP
          address of the target host of the transmitted data.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Destination UDP port:</emphasis> Port
          number to which the transmitted data is sent.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      When interconnecting two virtual machines on two different hosts,
      their IP addresses must be swapped. On a 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. To configure using the
      command-line:
    </para>

<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>

<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>
      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 an internal network in
      this case. However, the host can see the network traffic which it
      could not in the normal internal network case.
    </para>

    <note>
      <para>
        On Unix-based hosts, such as Linux, Solaris, and 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>

  </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 a single parameter: VDE network.
      This is the name of the VDE network switch socket to which the VM
      will be connected.
    </para>

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

    <orderedlist>

      <listitem>
        <para>
          Create a VDE switch:
        </para>

<screen>vde_switch -s /tmp/switch1</screen>
      </listitem>

      <listitem>
        <para>
          Configure VMs using the command-line:
        </para>

<screen>VBoxManage modifyvm "VM name" --nic&lt;x&gt; generic</screen>

<screen>VBoxManage modifyvm "VM name" --nicgenericdrv&lt;x&gt; VDE</screen>

        <para>
          To connect to an automatically allocated switch port:
        </para>

<screen>VBoxManage modifyvm "VM name" --nicproperty&lt;x&gt; network=/tmp/switch1</screen>

        <para>
          To connect to a specific switch port
          <replaceable>n</replaceable>:
        </para>

<screen>VBoxManage modifyvm "VM name" --nicproperty&lt;x&gt; network=/tmp/switch1[&lt;n&gt;]</screen>

        <para>
          This command can be useful for VLANs.
        </para>
      </listitem>

      <listitem>
        <para>
          (Optional) Map between a VDE switch port and a VLAN.
        </para>

        <para>
          Using the switch command line:
        </para>

<screen>vde$ vlan/create &lt;VLAN&gt;</screen>

<screen>vde$ port/setvlan &lt;port&gt; &lt;VLAN&gt;</screen>
      </listitem>

    </orderedlist>

    <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 Mbps
      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 Mbps. However, if one adapter does not 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 Kbps:

<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 virtio network
      adapter is preferable over Intel PRO/1000 emulated adapters, which
      are preferred over the PCNet family of adapters. Both virtio and
      Intel PRO/1000 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 the VM/host boundary.
    </para>

    <note>
      <para>
        Neither virtio nor Intel PRO/1000 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: Internal, Bridged, and Host-Only, have
      nearly identical performance. The Internal type is a little bit
      faster and uses less CPU cycles as the packets never reach the
      host's network stack. The NAT attachment type is the slowest and
      most secure 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 a short summary of things to check in order to improve
      network performance:
    </para>

    <orderedlist>

      <listitem>
        <para>
          Whenever possible use the virtio network adapter. Otherwise,
          use one of the Intel PRO/1000 adapters.
        </para>
      </listitem>

      <listitem>
        <para>
          Use a Bridged attachment instead of NAT.
        </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 the
          <computeroutput>ethtool</computeroutput> command on Linux
          guests.
        </para>
      </listitem>

      <listitem>
        <para>
          Perform a full, detailed analysis of network traffic on the
          VM's network adaptor using a third party tool such as
          Wireshark. To do this, a promiscuous mode policy needs to be
          used on the VM's network adaptor. Use of this mode is only
          possible on the following network types: NAT Network, Bridged
          Adapter, Internal Network, and Host-Only Adapter.
        </para>

        <para>
          To setup a promiscuous mode policy, either select from the
          drop down list located in the Network Settings dialog for the
          network adaptor or use the command line tool
          <computeroutput>VBoxManage</computeroutput>. See
          <xref linkend="vboxmanage-modifyvm" />.
        </para>

        <para>
          Promiscuous mode policies are as follows:
        </para>

        <itemizedlist>

          <listitem>
            <para>
              <computeroutput>deny</computeroutput>, which hides any
              traffic not intended for the VM's network adaptor. This is
              the default setting.
            </para>
          </listitem>

          <listitem>
            <para>
              <computeroutput>allow-vms</computeroutput>, which hides
              all host traffic from the VM's network adaptor, but allows
              it to see traffic from and to other VMs.
            </para>
          </listitem>

          <listitem>
            <para>
              <computeroutput>allow-all</computeroutput>, which removes
              all restrictions. The VM's network adaptor sees all
              traffic.
            </para>
          </listitem>

        </itemizedlist>
      </listitem>

    </orderedlist>

  </sect1>

</chapter>