9 Virtual Networking

Oracle VirtualBox provides up to eight virtual PCI Ethernet cards for each virtual machine. For each such card, you can individually select the following:

  • The hardware that will be virtualized.

  • The virtualization mode that the virtual card operates in, with respect to your physical networking hardware on the host.

Four of the network cards can be configured in the Settings for a virtual machine, listed in Machines in Oracle VirtualBox Manager.

You can configure all eight network cards on the command line using VBoxManage modifyvm. See VBoxManage modifyvm.

Configure Networking

Use the Network tool in Oracle VirtualBox Manager to create and manage the networks used by Oracle VirtualBox VMs to communicate each other, the host machine, and external resources.

Note:

You also need to enable the virtual networks to use with each VM you create. See Network Settings.
The network types available are:
  • Host-only. A network for VMs to communicate internally on this machine, but not with external networks.
  • NAT. The default network used by VirtualBox, suitable for most communication between VMs and external networks.
  • Cloud. A network used to connect to Oracle Cloud Infrastructure.

Create a Network

  1. Open the Network tool, or from the File menu select Tools and then click Network Manager.
  2. Click the tab corresponding to the type of network you need to create.
  3. Click Create. A new network adapter will be created with default properties, and shown in the list.
  4. Click Properties and configure the network as required. For more information, see Edit a Network.

Remove a Network

Ensure the network is not in use by an virtual machines (VMs) before removing it.

  1. Open the Network tool, or from the File menu select Tools and then click Network Manager.
  2. Click the tab corresponding to the type of network you need to delete.
  3. Select the network to remove, and then click Remove. The adapter is deleted and the network interface can't be used by VMs.

Edit a Network

  1. Open the Network tool, or from the File menu select Tools and then click Network Manager.
  2. Click the tab corresponding to the type of network you need to edit.
  3. Select the network you need to edit.
  4. Click Properties and configure the network as required.

    Cloud Networks have the following properties.

    • Name. The name used for the cloud network.

    • Provider. The cloud service provider, such as Oracle Cloud Infrastructure.

    • Profile. The cloud profile used to connect to the cloud network. See Creating a Cloud Profile.

    • ID. The OCID for the cloud tunneling network. Click Networkto view the subnets on Oracle Cloud Infrastructure that are available for tunneling traffic.

    Host-only Networks have the following properties.

    • Adapter. The the network adapter for the host-only network.

    • DHCP Server. Settings for the DHCP server used by the host-only network. The DHCP server is built into Oracle VirtualBox and manages IP addresses for the network automatically.

    NAT Networks have the following properties.

    • General Options. Network settings used by the NAT network including the network address and mask of the NAT service interface.

    • Port Forwarding. Port forwarding rules used by the NAT network.

Virtual Networking Hardware

For each card, you can individually select what kind of hardware will be presented to the virtual machine. Oracle VirtualBox can virtualize the following types of networking hardware:

  • AMD PCNet PCI II (Am79C970A) Not available on Arm guests.

  • AMD PCNet FAST III (Am79C973), the default setting on x86 guests. Not available on Arm guests.

  • Intel PRO/1000 MT Desktop (82540EM)

  • Intel PRO/1000 T Server (82543GC)

  • Intel PRO/1000 MT Server (82545EM)

  • Paravirtualized network adapter (virtio-net)

  • Ethernet over USB network adapter (usbnet)

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.

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.

The Paravirtualized network adapter (virtio-net) is special. If you select this adapter, then Oracle VirtualBox does not virtualize common networking hardware that is supported by common guest operating systems. Instead, Oracle 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. Oracle VirtualBox provides support for the industry-standard virtio networking drivers, which are part of the open source KVM project.

The virtio networking drivers are available for the following guest operating systems:

  • 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.

  • For Windows 2000, XP, and Vista, virtio drivers can be downloaded and installed from the KVM project web page:

    http://www.linux-kvm.org/page/WindowsGuestDrivers.

Oracle VirtualBox also has limited support for jumbo frames. 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.

Introduction to Networking Modes

Each of the networking adapters can be separately configured to operate in one of the following modes:

  • Not attached. In this mode, Oracle 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.

  • Network Address Translation (NAT). 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 NAT Limitations.

  • NAT Network. This mode allows you to use NAT between your VMs, as well as with external connections. You first have to create the NAT Network service that you want to use, and then select NAT Network as the network mode. See Oracle VirtualBox NAT Network Service.

  • Bridged networking. This is for more advanced networking needs, such as network simulations and running servers in a guest. When enabled, Oracle 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.

  • Cloud networking. This can be used to connect a local VM to a subnet on a remote cloud service.

  • Generic networking. Rarely used modes that share the same generic network interface, by allowing the user to select a driver which can be included with Oracle VirtualBox or be distributed in an extension pack.

    The following submodes are available:

    • UDP Tunnel: Used to interconnect virtual machines running on different hosts directly, easily, and transparently, over an existing network infrastructure.

    • VDE (Virtual Distributed Ethernet) networking: Used to connect to a Virtual Distributed Ethernet switch on a Linux or a FreeBSD host. At the moment this option requires compilation of Oracle VirtualBox from sources, as the Oracle packages do not include it.

The following table provides an overview of the most important networking modes.

Table 9-1 Overview of Networking Modes

Mode VM→Host VM←Host VM1↔VM2 VM→Net/LAN VM←Net/LAN

Host-only

+

+

+

–

–

Internal

–

–

+

–

–

Bridged

+

+

+

+

+

NAT

+

Port forward

–

+

Port forward

NAT Network

+

Port forward

+

+

Port forward

The following sections describe the available network modes in more detail.

Network Address Translation (NAT)

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 Oracle VirtualBox.

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 Oracle VirtualBox networking engine, which maps traffic from and to the virtual machine transparently. In Oracle 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.

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 Configuring Port Forwarding with NAT.

The network frames sent out by the guest operating system are received by Oracle 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 Oracle VirtualBox application on the host, using an IP address belonging to the host. Oracle VirtualBox listens for replies to the packages sent, and repacks and resends them to the guest machine on its private network.

The virtual machine receives its network address and configuration on the private network from a DHCP server integrated into Oracle 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 Fine Tuning the Oracle VirtualBox NAT Engine.

Configuring Port Forwarding with NAT

As the virtual machine is connected to a private network internal to Oracle 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, Oracle VirtualBox can make selected services available to the world outside the guest through port forwarding. This means that Oracle 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.

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.

To configure port forwarding, go to Network, select the NAT Networks tab, and click the network. On the Port Forwarding tab, click Add. Here, you can map host ports to guest ports to allow network traffic to be routed to a specific port in the guest.

Alternatively, the command line tool VBoxManage can be used. See VBoxManage modifyvm.

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 ssh server in the guest, use the following command: 

VBoxManage modifyvm "VM name" --nat-pf1 "guestssh,tcp,,2222,,22"

In 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 tcp is a mandatory attribute defining which protocol should be used for forwarding, udp could also be used. The name guestssh is purely descriptive and will be auto-generated if omitted. The number after --nat-pf denotes the network card, as with other VBoxManage commands.

To remove this forwarding rule, use the following command: 

VBoxManage modifyvm "VM name" --natpf1 delete "guestssh"

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, as follows: 

VBoxManage modifyvm "VM name" --natpf1 "guestssh,tcp,,2222,10.0.2.19,22"

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.

To forward all incoming traffic from a specific host interface to the guest, specify the IP of that host interface as follows: 

VBoxManage modifyvm "VM name" --natpf1 "guestssh,tcp,127.0.0.1,2222,,22"

This example forwards all TCP traffic arriving on the localhost interface at 127.0.0.1 through port 2222 to port 22 in the guest.

It is possible to configure incoming NAT connections while the VM is running, see VBoxManage controlvm.

PXE Booting with NAT

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

NAT Limitations

There are some limitations of NAT mode which users should be aware of, as follows:

  • ICMP protocol limitations. Some frequently used network debugging tools, such as ping or traceroute, rely on the ICMP protocol for sending and receiving messages. Oracle VirtualBox ICMP support has some limitations, meaning ping should work but some other tools may not work reliably.

  • Receiving of UDP broadcasts. 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 required server in the \\server\share notation.

  • Some protocols are not supported. 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.

  • Forwarding host ports below 1024. On UNIX-based hosts, such as Linux, Oracle Solaris, and macOS, it is not possible to bind to ports below 1024 from applications that are not run by root. As a result, if you try to configure such a port forwarding, the VM will refuse to start.

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 above 1024.

Oracle VirtualBox NAT Network Service

The Network Address Translation (NAT) Network 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.

A NAT Network 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. The following is an example command to create a NAT network: 

VBoxManage natnetwork add --netname natnet1 --network "192.168.15.0/24" --enable

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 Network 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, modify the example command as follows: 

VBoxManage natnetwork add --netname natnet1 --network "192.168.15.0/24" --enable --dhcp on

To add a DHCP server to an existing network, use the following command: 

VBoxManage natnetwork modify --netname natnet1 --dhcp on

To disable the DHCP server, use the following command: 

VBoxManage natnetwork modify --netname natnet1 --dhcp off

A DHCP server provides a list of registered name servers, but does not map servers from the 127/8 network.

To start the NAT Network service, use the following command: 

VBoxManage natnetwork start --netname natnet1

If the network has a DHCP server attached then it will start together with the NAT Network service.

To stop the NAT Network service, together with any DHCP server: 

VBoxManage natnetwork stop --netname natnet1

To delete the NAT Network service: 

VBoxManage natnetwork remove --netname natnet1

This command does not remove the DHCP server if one is enabled on the internal network.

Port-forwarding is supported, using the --port-forward-4 switch for IPv4 and --port-forward-6 for IPv6. For example: 

VBoxManage natnetwork modify \
  --netname natnet1 --port-forward-4 "ssh:tcp:[]:1022:[192.168.15.5]:22"

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 the following command: 

VBoxManage natnetwork modify --netname natnet1 --port-forward-4 delete ssh

It is possible to bind a NAT Network service to specified interface. For example: 

VBoxManage setextradata global "NAT/win-nat-test-0/SourceIp4" 192.168.1.185

To see the list of registered NAT Network services, use the following command: 

VBoxManage list natnetworks

NAT network services can also be created, deleted, and configured using the Network Manager tool in Oracle VirtualBox Manager. Click File, Tools, Network Manager. See Configure Networking.

Note:

Even though the NAT service separates the VM from the host, the VM has access to the host's loopback interface and the network services running on it. The host's loopback interface is accessible as IP address 10.0.2.2 (assuming the default configuration, in other configurations it's the respective address in the configured IPv4 or IPv6 network range). This access to the host's loopback interface can be extremely useful in some cases, for example when running a web application under development in the VM and the database server on the loopback interface on the host. To enable traffic to pass to the host's loopback interface, see VBoxManage, NAT Networking Settings.

Fine Tuning the Oracle VirtualBox NAT Engine

Configuring the Address of a NAT Network Interface

In NAT mode, the guest network interface is assigned to the IPv4 range 10.0.x.0/24 by default where x corresponds to the instance of the NAT interface +2. So x is 2 when there is only one NAT instance active. In that case the guest is assigned to the address 10.0.2.15, the gateway is set to 10.0.2.2 and the name server can be found at 10.0.2.3.

If the NAT network needs to be changed, use the following command: 

$ VBoxManage modifyvm VM-name \
--natnet1 "192.168/16"

This command would reserve the network addresses from 192.168.0.0 to 192.168.254.254 for the first NAT network instance of VM-name The guest IP would be assigned to 192.168.0.15 and the default gateway could be found at 192.168.0.2.

Configuring the Boot Server (Next Server) of a NAT Network Interface

For network booting in NAT mode, by default Oracle VirtualBox uses a built-in TFTP server at the IP address 10.0.2.4. This default behavior should work fine for typical remote-booting scenarios. However, it is possible to change the boot server IP and the location of the boot image with the following commands: 

$ VBoxManage modifyvm VM-name \
--nattftpserver1 10.0.2.2
$ VBoxManage modifyvm VM-name \
--nattftpfile1 /srv/tftp/boot/MyPXEBoot.pxe

Enabling DNS Proxy in NAT Mode

The NAT engine by default offers the same DNS servers to the guest that are configured on the host. In some scenarios, it can be appropriate to hide the DNS server IPs from the guest, for example when this information can change on the host due to expiring DHCP leases. In this case, you can tell the NAT engine to act as DNS proxy using the following command: 

$ VBoxManage modifyvm VM-name --natdnsproxy1 on

Using the Host's Resolver as a DNS Proxy in NAT Mode

For resolving network names, the DHCP server of the NAT engine offers a list of registered DNS servers of the host. If for some reason you need to hide this DNS server list and use the host's resolver settings, thereby forcing the Oracle VirtualBox NAT engine to intercept DNS requests and forward them to host's resolver, use the following command: 

$ VBoxManage modifyvm VM-name --natdnshostresolver1 on

Note that this setting is similar to the DNS proxy mode, however whereas the proxy mode just forwards DNS requests to the appropriate servers, the resolver mode will interpret the DNS requests and use the host's DNS API to query the information and return it to the guest.

User-Defined Host Name Resolving

In some cases it might be useful to intercept the name resolving mechanism, providing a user-defined IP address on a particular DNS request. The intercepting mechanism enables the user to map not only a single host but domains and even more complex naming conventions if required.

The following command sets a rule for mapping a name to a specified IP: 

VBoxManage setextradata VM-name \
"VBoxInternal/Devices/{pcnet,e1000}/0/LUN#0/AttachedDriver/Config/HostResolverMappings/ \
unique-rule-name-of-interception-rule/HostIP" IPv4

VBoxManage setextradata VM-name \
"VBoxInternal/Devices/{pcnet,e1000}/0/LUN#0/AttachedDriver/Config/HostResolverMappings/ \
unique-rule-name/HostName" hostname

The following command sets a rule for mapping a pattern name to a specified IP: 

VBoxManage setextradata VM-name \
"VBoxInternal/Devices/{pcnet,e1000}/0/LUN#0/AttachedDriver/Config/HostResolverMappings/ \
unique-rule-name/HostIP" IPv4

VBoxManage setextradata VM-name \
"VBoxInternal/Devices/{pcnet,e1000}/0/LUN#0/AttachedDriver/Config/HostResolverMappings/ \
unique-rule-name/HostNamePattern" hostpattern

The host name pattern can include the following wildcard characters: pipe (|), question mark (?), and asterisk (*).

This example demonstrates how to instruct the host-resolver mechanism to resolve all domain and probably some mirrors of www.blocked-site.info site with IP 127.0.0.1: 

$ VBoxManage setextradata VM-name \
"VBoxInternal/Devices/e1000/0/LUN#0/AttachedDriver/Config/HostResolverMappings/all_blocked_site/HostIP" 127.0.0.1
$ VBoxManage setextradata VM-name \
"VBoxInternal/Devices/e1000/0/LUN#0/AttachedDriver/Config/HostResolverMappings/all_blocked_site/HostNamePattern" "*.blocked-site.*|*.fb.org"

The host resolver mechanism should be enabled to use user-defined mapping rules, otherwise they do not have any effect.

Configuring Aliasing of the NAT Engine

By default, the NAT core uses aliasing and uses random ports when generating an alias for a connection. This works well for the most protocols like SSH, FTP and so on. Though some protocols might need a more transparent behavior or may depend on the real port number the packet was sent from. You can change the NAT mode by using the following commands: 

$ VBoxManage modifyvm VM-name \
--nataliasmode1 proxyonly
$ VBoxManage modifyvm "Linux Guest" --nataliasmode1 sameports

The first example disables aliasing and switches NAT into transparent mode, the second example enforces preserving of port values. These modes can be combined if necessary.

Bridged Networking

With bridged networking, Oracle VirtualBox uses a device driver on your host system that filters data from your physical network adapter. This driver is therefore called a net filter driver. This enables Oracle 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.

Note:

Even though TAP interfaces are no longer necessary on Linux for bridged networking, you can still use TAP interfaces for certain advanced setups, since you can connect a VM to any host interface.

To enable bridged networking, open the Settings for a virtual machine, go to the Network page and select Bridged Network 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.

Note:

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 Oracle 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 Oracle 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. Oracle VirtualBox examines ARP and DHCP packets in order to learn the IP addresses of virtual machines.

Depending on your host operating system, the following limitations apply:

macOS hosts

  • Functionality is limited when using AirPort, the Mac's wireless networking system, for bridged networking. Currently, Oracle VirtualBox supports only IPv4 and IPv6 over AirPort. For other protocols, such as IPX, you must choose a wired interface.

Linux hosts

  • Functionality is limited when using wireless interfaces for bridged networking. Currently, Oracle VirtualBox supports only IPv4 and IPv6 over wireless. For other protocols, such as IPX, you must choose a wired interface.
  • 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.

  • Some adapters strip VLAN tags in hardware. This does not allow you to use VLAN trunking between VM and the external network with Linux kernels before 2.6.27, or with host operating systems other than Linux.

Oracle Solaris 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 Oracle Solaris networking subsystem. These issues may be addressed in later releases of Oracle Solaris 11.
  • On Oracle Solaris 11 hosts build 159 and above, it is possible to use Oracle Solaris Crossbow Virtual Network Interfaces (VNICs) directly with Oracle VirtualBox without any additional configuration other than each VNIC must be exclusive for every guest network interface.
  • When using VLAN interfaces with Oracle 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.
  • VirtualBox guests can't use bridged networking over an aggregate link (also known as a single link aggregation) on Oracle Solaris 11 FCS hosts. However, you can use dladm to manually create a VNIC over the aggregate link and use that with a VM. This limitation does not exist in Oracle Solaris 11 update 1 (11.1) and later.

Internal Networking

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.

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.

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 Oracle VirtualBox support driver will automatically wire the cards and act as a network switch. The Oracle VirtualBox support driver implements a complete Ethernet switch and supports both broadcast/multicast frames and promiscuous mode.

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:

  • Use the VM's Settings in Oracle VirtualBox Manager. In the Network section, select Internal Network as the networking mode. Select the name of an existing internal network from the drop-down list below, or enter a new name into the Name field.

  • Use the command line, for example: 

    VBoxManage modifyvm "VM name" --nic<x> intnet

    Optionally, you can specify a network name with the command: 

    VBoxManage modifyvm "VM name" --intnet<x> "network name"

    If you do not specify a network name, the network card will be attached to the network intnet by default.

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 Oracle VirtualBox to manage IP addresses for the internal network. See VBoxManage dhcpserver.

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.

Host-Only Networking

Host-only networking 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.

When host-only networking is used, Oracle 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.

Note:

Hosts running recent macOS versions do not support host-only adapters. These adapters are replaced by host-only networks, which define a network mask and an IP address range, where the host network interface receives the lowest address in the range.

The host network interface gets added and removed dynamically by the operating system, whenever a host-only network is used by virtual machines.

On macOS hosts, choose the Host-Only Network option when configuring a network adapter. The Host-Only Adapter option is provided for legacy support.

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 Oracle 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.

To enable a host-only network interface for a virtual machine, do either of the following:

  • Go to the Network section in the virtual machine's Settings and select an Adapter tab. Ensure that the Enable Network Adapter check box is selected and choose Host-Only Adapter for the Attached To field.

  • On the command line, use

    VBoxManage modifyvm VM-name --nicN=hostonly --host-only-adapterN=interface-name.

    See VBoxManage modifyvm.

For host-only networking, as with internal networking, you may find the DHCP server useful that is built into Oracle VirtualBox. This is enabled by default and manages the IP addresses in the host-only network. Without the DHCP server you would need to configure all IP addresses statically.

  • In Oracle VirtualBox Manager you can configure the DHCP server by choosing File, Tools, Network. The Network Manager window lists all host-only networks which are presently in use. Select the network name and then use the DHCP Server tab to configure DHCP server settings. See Configure Networking.

  • Alternatively, you can use the VBoxManage dhcpserver command. See VBoxManage dhcpserver.

Note:

On Linux and macOS hosts the number of host-only interfaces is limited to 128. There is no such limit for Oracle Solaris and Windows hosts.

On Linux, macOS and Solaris Oracle VirtualBox will only allow IP addresses in 192.168.56.0/21 range to be assigned to host-only adapters. For IPv6 only link-local addresses are allowed. If other ranges are required, they can be enabled by creating /etc/vbox/networks.conf and specifying allowed ranges there. For example, to allow 10.0.0.0/8 and 192.168.0.0/16 IPv4 ranges as well as 2001::/64 range put the following lines into /etc/vbox/networks.conf: 

      * 10.0.0.0/8 192.168.0.0/16
      * 2001::/64

Lines starting with the hash # are ignored. The following example allows any addresses, effectively disabling range control: 

      * 0.0.0.0/0 ::/0

If the file exists, but no ranges are specified in it, no addresses will be assigned to host-only adapters. The following example effectively disables all ranges: 

      # No addresses are allowed for host-only adapters

UDP Tunnel Networking

This networking mode enables you to interconnect virtual machines running on different hosts.

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.

UDP Tunnel mode has the following parameters:

  • Source UDP port: 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.

  • Destination address: IP address of the target host of the transmitted data.

  • Destination UDP port: Port number to which the transmitted data is sent.

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.

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: 

        VBoxManage modifyvm "VM 01 on host 1" --nic<x> generic
        VBoxManage modifyvm "VM 01 on host 1" --nic-generic-drv<x> UDPTunnel
        VBoxManage modifyvm "VM 01 on host 1" --nic-property<x> dest=10.0.0.2
        VBoxManage modifyvm "VM 01 on host 1" --nic-property<x> sport=10001
        VBoxManage modifyvm "VM 01 on host 1" --nic-property<x> dport=10002
        VBoxManage modifyvm "VM 02 on host 2" --nic<y> generic
        VBoxManage modifyvm "VM 02 on host 2" --nic-generic-drv<y> UDPTunnel
        VBoxManage modifyvm "VM 02 on host 2" --nic-property<y> dest=10.0.0.1
        VBoxManage modifyvm "VM 02 on host 2" --nic-property<y> sport=10002
        VBoxManage modifyvm "VM 02 on host 2" --nic-property<y> dport=10001

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.

Note:

On UNIX-based hosts, such as Linux, Oracle Solaris, and Mac OS X, it is not possible to bind to ports below 1024 from applications that are not run by root. As a result, if you try to configure such a source UDP port, the VM will refuse to start.

VDE Networking

Virtual Distributed Ethernet (VDE) is a flexible, virtual network infrastructure system, spanning across multiple hosts in a secure way. It enables L2/L3 switching, including spanning-tree protocol, VLANs, and WAN emulation. It is an optional part of Oracle VirtualBox which is only included in the source code.

VDE is a project developed by Renzo Davoli, Associate Professor at the University of Bologna, Italy.

The basic building blocks of the infrastructure are VDE switches, VDE plugs, and VDE wires which interconnect the switches.

The Oracle 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.

The following basic example shows how to connect a virtual machine to a VDE switch.

  1. Create a VDE switch: 

    vde_switch -s /tmp/switch1

  2. Configure VMs using the command-line: 

    VBoxManage modifyvm "VM name" --nic<x> generic
    VBoxManage modifyvm "VM name" --nic-generic-drv<x> VDE

    To connect to an automatically allocated switch port: 

    VBoxManage modifyvm "VM name" --nic-property<x> network=/tmp/switch1

    To connect to a specific switch port n: 

    VBoxManage modifyvm "VM name" --nic-property<x> network=/tmp/switch1[<n>]

    This command can be useful for VLANs.

  3. (Optional) Map between a VDE switch port and a VLAN.

    Using the switch command line: 

    vde$ vlan/create <VLAN>
    vde$ port/setvlan <port> <VLAN>

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.

Note:

For Linux hosts, the shared library libvdeplug.so must be available in the search path for shared libraries.

For more information on setting up VDE networks, please see the documentation accompanying the software. See also http://wiki.virtualsquare.org.

Cloud Networks

Cloud networks can be used for connections from a local VM to a subnet on a remote Oracle Cloud Infrastructure instance. See Configure Networking for details of how to create and configure a cloud network using the Network tool in Oracle VirtualBox Manager.

To enable a cloud network interface for a virtual machine, do either of the following:

  • Go to the Settings for a virtual machine, and under Network, select an Adapter tab. Ensure that the Enable Network Adapter check box is selected and choose Cloud Network for the Attached To field.

  • On the command line, use VBoxManage modifyvm vmname --nic x cloud. See VBoxManage modifyvm.

Limiting Bandwidth for Network Input/Output

Oracle VirtualBox supports limiting of 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.

Note:

Oracle 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.

Limits are configured through VBoxManage. The following example 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: 

VBoxManage bandwidthctl "VM name" add Limit --type network --limit 20m
VBoxManage modifyvm "VM name" --nicbandwidthgroup1 Limit
VBoxManage modifyvm "VM name" --nicbandwidthgroup2 Limit

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.

The limits for each group can be changed while the VM is running, with changes being picked up immediately. The following example changes the limit for the group created in the previous example to 100 Kbps: 

VBoxManage bandwidthctl "VM name" set Limit --limit 100k

To completely disable shaping for the first adapter of VM use the following command: 

VBoxManage modifyvm "VM name" --nicbandwidthgroup1 none

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: 

VBoxManage bandwidthctl "VM name" set Limit --limit 0

Improving Network Performance

Oracle 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.

Note:

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.

Three attachment types: Internal, Bridged, and Host-Only, have nearly identical performance. The Internal type is slightly 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.

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.

Here is a short summary of things to check in order to improve network performance:

  • Whenever possible use the virtio network adapter. Otherwise, use one of the Intel PRO/1000 adapters.

  • Use a Bridged attachment instead of NAT.

  • Ensure segmentation offloading is enabled in the guest OS. Usually it will be enabled by default. You can check and modify offloading settings using the ethtool command on Linux guests.

  • 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.

    To setup a promiscuous mode policy, either set the Promiscuous Mode in the Network section of the VM's Settings for the network adaptor, or use the command line tool VBoxManage. See VBoxManage modifyvm.

    Promiscuous mode policies are as follows:

    • deny, which hides any traffic not intended for the VM's network adaptor. This is the default setting.

    • allow-vms, which hides all host traffic from the VM's network adaptor, but allows it to see traffic from and to other VMs.

    • allow-all, which removes all restrictions. The VM's network adaptor sees all traffic.