A Look at How Logical Network Interfaces Attach to the Linux Kernel

Alessandro Rubini

Saturday, April 15th, 2000

Share This:

Figure One: The data flow through insane (INterface SAmple for Network Errors), which simulates random packet loss, or intermittent network failure.

In the Linux (or Unix) world, most network interfaces, such as eth0 and ppp0, are associated with a physical device that is in charge of transmitting and receiving data packets. However, some logical network interfaces don’t feature any physical packet transmission. The most well-known examples of these “virtual” interfaces are the shaper and eql interfaces. This month, we’ll look at how this kind of interface attaches to the kernel and to the packet-transmission mechanism.

From the kernel’s point of view, a network interface is a software object that can process outgoing packets, with the actual transmission mechanism hidden inside the interface driver. Even though most interfaces are associated with physical devices (or, for the loopback interface, to a software-only data loop), it is possible to design network-interface drivers that rely on other interfaces to perform actual packet transmission. The idea of a “virtual” interface can be useful to implement special-purpose processing on data packets while avoiding hacking the network subsystem of the kernel. Although some of what can be accomplished by a virtual interface is more easily implemented by writing a netfilter module, not everything can be implemented by netfilters, and the virtual interface…

1. Load Balancing for Application Server Administrators
2. T6 Broadband uses Linux Routers to Maximize Availability
3. Linux Helps Telephone Co-op Expand
4. Aardman Animations' Observer Adventure
5. Virtualizing Service Provider Networks with Vyatta