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The Paravirtualized network adapter (virtio-net) is special. If you select this adapter, then Oracle VM VirtualBox does not virtualize common networking hardware that is supported by common guest operating systems. Instead, Oracle VM 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 VM VirtualBox provides support for the industry-standard virtio networking drivers, which are part of the open source KVM project.
Oracle VM 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.
Not attached. In this mode, Oracle VM 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.
Bridged networking. This is for more advanced networking needs, such as network simulations and running servers in a guest. When enabled, Oracle VM 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.
Generic networking. Rarely used modes which share the same generic network interface, by allowing the user to select a driver which can be included with Oracle VM VirtualBox or be distributed in an extension pack.
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 VM VirtualBox from sources, as the Oracle packages do not include it.
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 VM 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 VM VirtualBox networking engine, which maps traffic from and to the virtual machine transparently. In Oracle VM 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 network frames sent out by the guest operating system are received by Oracle VM 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 VM VirtualBox application on the host, using an IP address belonging to the host. Oracle VM VirtualBox listens for replies to the packages sent, and repacks and resends them to the guest machine on its private network.
Even though the NAT engine 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. 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.
As the virtual machine is connected to a private network internal to Oracle VM 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 VM VirtualBox can make selected services available to the world outside the guest through port forwarding. This means that Oracle VM 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 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.
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.
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.
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 VM 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 desired 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.
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.
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.
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. The following is an example command to create a NAT network:
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, modify the example command as follows:
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.
With bridged networking, Oracle VM 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 VM 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.
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