Understanding Linux Network Internals

net_device Structure

The net_device data structure stores all information specifically regarding a network device. There is one such structure for each device, both real ones (such as Ethernet NICs) and virtual ones (such as bonding^[1] or VLAN^[†]). In this section, I will use the words interface and device interchangeably, even though the difference between them is important in other contexts.

The net_device structures for all devices are put into a global list to which the global variable dev_base points. The data structure is defined in include/linux/netdevice.h. The registration of network devices is described in Chapter 8. In that chapter, you can find details on how and when most of the net_device fields are initialized.

Like sk_buff, this structure is quite big and includes many feature-specific parameters, along with parameters from many different layers. For this reason, the overall organization of the structure will probably see some changes soon for optimization reasons.

Network devices can be classified into types such as Ethernet cards and Token Ring cards. While certain fields of the net_device structure are set to the same value for all devices of the same type, some fields must be set differently by each model of device. Thus, for almost every type, Linux provides a general function that initializes the parameters whose values stay the same across all models. Each device driver invokes this function in addition to setting those fields that have unique values for its ...