Understanding Linux Network Internals

Virtual Devices

A virtual device is an abstraction built on top of one or more real devices. The association between virtual devices and real devices can be many-to-many, as shown by the three models in Figure 5-4. It is also possible to build virtual devices on top of other virtual devices. However, not all combinations are meaningful or are supported by the kernel.

Figure 5-4. Possible relationship between virtual and real devices

Examples of Virtual Devices

Linux allows you to define different kinds of virtual devices. Here are a few examples:

Bonding

With this feature, a virtual device bundles a group of physical devices and makes them behave as one.

802.1Q

This is an IEEE standard that extends the 802.3/Ethernet header with the so-called VLAN header, allowing for the creation of Virtual LANs.

Bridging

A bridge interface is a virtual representation of a bridge. Details are in Part IV.

Aliasing interfaces

Originally, the main purpose for this feature was to allow a single real Ethernet interface to span several virtual interfaces (eth0:0, eth0:1, etc.), each with its own IP configuration. Now, thanks to improvements to the networking code, there is no need to define a new virtual interface to configure multiple IP addresses on the same NIC. However, there may be cases (notably routing) where having different virtual NICs on the same NIC would make life easier, perhaps allowing simpler ...