6 Virtual networking • Linux kernels version 2.6.25 or later can be configured to provide virtio support some distributions also back-ported virtio to older kernels. • For Windows 2000, XP and Vista, virtio drivers can be downloaded and installed from the KVM project web page.2 VirtualBox also has limited support for so-called jumbo frames, 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. 6.2 Introduction to networking modes Each of the eight networking adapters can be separately configured to operate in one of the following five modes: Not attached 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. Network Address Translation (NAT) 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 chapter 6.3.3, NAT limitations, page 86 for details). Bridged networking 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. Internal networking 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. Host-only networking 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. VDE (Virtual Distributed Ethernet) networking This option can be used to connect to a Virtual Distributed Ethernet switch on a Linux or a FreeBSD host. It is only available if the VDE software and the VDE plugin library from the VirtualSquare project are installed on the host system. For more information on setting up VDE networks, please see the documentation accompanying the software. The following sections describe the available network modes in more detail. 2http://www.linux-kvm.org/page/WindowsGuestDrivers. 84