The net_device Structure in Detail

The net_device structure is at the very core of the network driver layer and deserves a complete description. At a first reading, however, you can skip this section, because you don’t need a thorough understanding of the structure to get started. This list describes all the fields, but more to provide a reference than to be memorized. The rest of this chapter briefly describes each field as soon as it is used in the sample code, so you don’t need to keep referring back to this section.

struct net_device can be conceptually divided into two parts: visible and invisible. The visible part of the structure is made up of the fields that can be explicitly assigned in static net_device structures. All structures in drivers/net/Space.c are initialized in this way, without using the tagged syntax for structure initialization. The remaining fields are used internally by the network code and usually are not initialized at compilation time, not even by tagged initialization. Some of the fields are accessed by drivers (for example, the ones that are assigned at initialization time), while some shouldn’t be touched.

The Visible Head

The first part of struct net_device is composed of the following fields, in this order:

char name[IFNAMSIZ];

The name of the device. If the name contains a %d format string, the first available device name with the given base is used; assigned numbers start at zero.

unsigned long rmem_end; , unsigned long rmem_start; , unsigned long mem_end; , unsigned long mem_start;

Device memory information. These fields hold the beginning and ending addresses of the shared memory used by the device. If the device has different receive and transmit memories, the mem fields are used for transmit memory and the rmem fields for receive memory. mem_start and mem_end can be specified on the kernel command line at system boot, and their values are retrieved by ifconfig. The rmem fields are never referenced outside of the driver itself. By convention, the end fields are set so that end - start is the amount of available on-board memory.

unsigned long base_addr;

The I/O base address of the network interface. This field, like the previous ones, is assigned during device probe. The ifconfig command can be used to display or modify the current value. The base_addr can be explicitly assigned on the kernel command line at system boot or at load time. The field is not used by the kernel, like the memory fields shown previously.

unsigned char irq;

The assigned interrupt number. The value of dev->irq is printed by ifconfig when interfaces are listed. This value can usually be set at boot or load time and modified later using ifconfig.

unsigned char if_port;

Which port is in use on multiport devices. This field is used, for example, with devices that support both coaxial (IF_PORT_10BASE2) and twisted-pair (IF_PORT_10BASET) Ethernet connections. The full set of known port types is defined in <linux/netdevice.h>.

unsigned char dma;

The DMA channel allocated by the device. The field makes sense only with some peripheral buses, like ISA. It is not used outside of the device driver itself, but for informational purposes (in ifconfig).

unsigned long state;

Device state. The field includes several flags. Drivers do not normally manipulate these flags directly; instead, a set of utility functions has been provided. These functions will be discussed shortly when we get into driver operations.

struct net_device *next;

Pointer to the next device in the global linked list. This field shouldn’t be touched by the driver.

int (*init)(struct net_device *dev);

The initialization function, described earlier.

The Hidden Fields

The net_device structure includes many additional fields, which are usually assigned at device initialization. Some of these fields convey information about the interface, while some exist only for the benefit of the driver (i.e., they are not used by the kernel); other fields, most notably the device methods, are part of the kernel-driver interface.

We will list the three groups separately, independent of the actual order of the fields, which is not significant.

Interface information

Most of the information about the interface is correctly set up by the function ether_setup. Ethernet cards can rely on this general-purpose function for most of these fields, but the flags and dev_addr fields are device specific and must be explicitly assigned at initialization time.

Some non-Ethernet interfaces can use helper functions similar to ether_setup. drivers/net/net_init.c exports a number of such functions, including the following:

void ltalk_setup(struct net_device *dev);

Sets up the fields for a LocalTalk device.

void fc_setup(struct net_device *dev);

Initializes for fiber channel devices.

void fddi_setup(struct net_device *dev);

Configures an interface for a Fiber Distributed Data Interface (FDDI) network.

void hippi_setup(struct net_device *dev);

Prepares fields for a High-Performance Parallel Interface (HIPPI) high-speed interconnect driver.

void tr_configure(struct net_device *dev);

Handles setup for token ring network interfaces. Note that the 2.4 kernel also exports a function tr_setup, which, interestingly, does nothing at all.

Most devices will be covered by one of these classes. If yours is something radically new and different, however, you will need to assign the following fields by hand.

unsigned short hard_header_len;

The hardware header length, that is, the number of octets that lead the transmitted packet before the IP header, or other protocol information. The value of hard_header_len is 14 (ETH_HLEN) for Ethernet interfaces.

unsigned mtu;

The maximum transfer unit (MTU). This field is used by the network layer to drive packet transmission. Ethernet has an MTU of 1500 octets (ETH_DATA_LEN).

unsigned long tx_queue_len;

The maximum number of frames that can be queued on the device’s transmission queue. This value is set to 100 by ether_setup, but you can change it. For example, plip uses 10 to avoid wasting system memory (plip has a lower throughput than a real Ethernet interface).

unsigned short type;

The hardware type of the interface. The type field is used by ARP to determine what kind of hardware address the interface supports. The proper value for Ethernet interfaces is ARPHRD_ETHER, and that is the value set by ether_setup. The recognized types are defined in <linux/if_arp.h>.

unsigned char addr_len; , unsigned char broadcast[MAX_ADDR_LEN]; , unsigned char dev_addr[MAX_ADDR_LEN];

Hardware (MAC) address length and device hardware addresses. The Ethernet address length is six octets (we are referring to the hardware ID of the interface board), and the broadcast address is made up of six 0xff octets; ether_setup arranges for these values to be correct. The device address, on the other hand, must be read from the interface board in a device-specific way, and the driver should copy it to dev_addr. The hardware address is used to generate correct Ethernet headers before the packet is handed over to the driver for transmission. The snull device doesn’t use a physical interface, and it invents its own hardware address.

unsigned short flags;

Interface flags, detailed next.

The flags field is a bit mask including the following bit values. The IFF_ prefix stands for “interface flags.” Some flags are managed by the kernel, and some are set by the interface at initialization time to assert various capabilities and other features of the interface. The valid flags, which are defined in <linux/if.h>, are as follows:


This flag is read-only for the driver. The kernel turns it on when the interface is active and ready to transfer packets.


This flag states that the interface allows broadcasting. Ethernet boards do.


This marks debug mode. The flag can be used to control the verbosity of your printk calls or for other debugging purposes. Although no official driver currently uses this flag, it can be set and reset by user programs via ioctl, and your driver can use it. The misc-progs/netifdebug program can be used to turn the flag on and off.


This flag should be set only in the loopback interface. The kernel checks for IFF_LOOPBACK instead of hardwiring the lo name as a special interface.


This flag signals that the interface is connected to a point-to-point link. It is set by ifconfig. For example, plip and the PPP driver have it set.


This means that the interface can’t perform ARP. For example, point-to-point interfaces don’t need to run ARP, which would only impose additional traffic without retrieving useful information. snull runs without ARP capabilities, so it sets the flag.


This flag is set to activate promiscuous operation. By default, Ethernet interfaces use a hardware filter to ensure that they receive broadcast packets and packets directed to that interface’s hardware address only. Packet sniffers such as tcpdump set promiscuous mode on the interface in order to retrieve all packets that travel on the interface’s transmission medium.


This flag is set by interfaces that are capable of multicast transmission. ether_setup sets IFF_MULTICAST by default, so if your driver does not support multicast, it must clear the flag at initialization time.


This flag tells the interface to receive all multicast packets. The kernel sets it when the host performs multicast routing, only if IFF_MULTICAST is set. IFF_ALLMULTI is read-only for the interface. We’ll see the multicast flags used in Section 14.13 later in this chapter.


These flags are used by the load equalization code. The interface driver doesn’t need to know about them.


These flags signal that the device is capable of switching between multiple media types, for example, unshielded twisted pair (UTP) versus coaxial Ethernet cables. If IFF_AUTOMEDIA is set, the device selects the proper medium automatically.


This flag indicates that the address of this interface can change; used with dialup devices.


This flag indicates that the interface is up and running. It is mostly present for BSD compatibility; the kernel makes little use of it. Most network drivers need not worry about IFF_RUNNING.


This flag is unused in Linux, but it exists for BSD compatibility.

When a program changes IFF_UP, the open or stop device method is called. When IFF_UP or any other flag is modified, the set_multicast_list method is invoked. If the driver needs to perform some action because of a modification in the flags, it must take that action in set_multicast_list. For example, when IFF_PROMISC is set or reset, set_multicast_list must notify the onboard hardware filter. The responsibilities of this device method are outlined in Section 14.13.

The device methods

As happens with the char and block drivers, each network device declares the functions that act on it. Operations that can be performed on network interfaces are listed in this section. Some of the operations can be left NULL, and some are usually untouched because ether_setup assigns suitable methods to them.

Device methods for a network interface can be divided into two groups: fundamental and optional. Fundamental methods include those that are needed to be able to use the interface; optional methods implement more advanced functionalities that are not strictly required. The following are the fundamental methods:

int (*open)(struct net_device *dev);

Opens the interface. The interface is opened whenever ifconfig activates it. The open method should register any system resource it needs (I/O ports, IRQ, DMA, etc.), turn on the hardware, and increment the module usage count.

int (*stop)(struct net_device *dev);

Stops the interface. The interface is stopped when it is brought down; operations performed at open time should be reversed.

int (*hard_start_xmit) (struct sk_buff *skb, struct net_device *dev);

This method initiates the transmission of a packet. The full packet (protocol headers and all) is contained in a socket buffer (sk_buff) structure. Socket buffers are introduced later in this chapter.

int (*hard_header) (struct sk_buff *skb, struct net_device *dev, unsigned short type, void *daddr, void *saddr, unsigned len);

This function builds the hardware header from the source and destination hardware addresses that were previously retrieved; its job is to organize the information passed to it as arguments into an appropriate, device-specific hardware header. eth_header is the default function for Ethernet-like interfaces, and ether_setup assigns this field accordingly.

int (*rebuild_header)(struct sk_buff *skb);

This function is used to rebuild the hardware header before a packet is transmitted. The default function used by Ethernet devices uses ARP to fill the packet with missing information. The rebuild_header method is used rarely in the 2.4 kernel; hard_header is used instead.

void (*tx_timeout)(struct net_device *dev);

This method is called when a packet transmission fails to complete within a reasonable period, on the assumption that an interrupt has been missed or the interface has locked up. It should handle the problem and resume packet transmission.

struct net_device_stats *(*get_stats)(struct net_device *dev);

Whenever an application needs to get statistics for the interface, this method is called. This happens, for example, when ifconfig or netstat -i is run. A sample implementation for snull is introduced in Section 14.12 later in this chapter.

int (*set_config)(struct net_device *dev, struct ifmap *map);

Changes the interface configuration. This method is the entry point for configuring the driver. The I/O address for the device and its interrupt number can be changed at runtime using set_config. This capability can be used by the system administrator if the interface cannot be probed for. Drivers for modern hardware normally do not need to implement this method.

The remaining device operations may be considered optional.

int (*do_ioctl)(struct net_device *dev, struct ifreq *ifr, int cmd);

Perform interface-specific ioctl commands. Implementation of those commands is described later in Section 14.11. The corresponding field in struct net_device can be left as NULL if the interface doesn’t need any interface-specific commands.

void (*set_multicast_list)(struct net_device *dev);

This method is called when the multicast list for the device changes and when the flags change. See Section 14.13 for further details and a sample implementation.

int (*set_mac_address)(struct net_device *dev, void *addr);

This function can be implemented if the interface supports the ability to change its hardware address. Many interfaces don’t support this ability at all. Others use the default eth_mac_addr implementation (from drivers/net/net_init.c). eth_mac_addr only copies the new address into dev->dev_addr, and it will only do so if the interface is not running. Drivers that use eth_mac_addr should set the hardware MAC address from dev->dev_addr when they are configured.

int (*change_mtu)(struct net_device *dev, int new_mtu);

This function is in charge of taking action if there is a change in the MTU (maximum transfer unit) for the interface. If the driver needs to do anything particular when the MTU is changed, it should declare its own function; otherwise, the default will do the right thing. snull has a template for the function if you are interested.

int (*header_cache) (struct neighbour *neigh, struct hh_cache *hh);

header_cache is called to fill in the hh_cache structure with the results of an ARP query. Almost all drivers can use the default eth_header_cache implementation.

int (*header_cache_update) (struct hh_cache *hh, struct net_device *dev, unsigned char *haddr);

This method updates the destination address in the hh_cache structure in response to a change. Ethernet devices use eth_header_cache_update.

int (*hard_header_parse) (struct sk_buff *skb, unsigned char *haddr);

The hard_header_parse method extracts the source address from the packet contained in skb, copying it into the buffer at haddr. The return value from the function is the length of that address. Ethernet devices normally use eth_header_parse.

Utility fields

The remaining struct net_device data fields are used by the interface to hold useful status information. Some of the fields are used by ifconfig and netstat to provide the user with information about the current configuration. An interface should thus assign values to these fields.

unsigned long trans_start; , unsigned long last_rx;

Both of these fields are meant to hold a jiffies value. The driver is responsible for updating these values when transmission begins and when a packet is received, respectively. The trans_start value is used by the networking subsystem to detect transmitter lockups. last_rx is currently unused, but the driver should maintain this field anyway to be prepared for future use.

int watchdog_timeo;

The minimum time (in jiffies) that should pass before the networking layer decides that a transmission timeout has occurred and calls the driver’s tx_timeout function.

void *priv;

The equivalent of filp->private_data. The driver owns this pointer and can use it at will. Usually the private data structure includes a struct net_device_stats item. The field is used in Section 14.2.2, later in this chapter.

struct dev_mc_list *mc_list; , int mc_count;

These two fields are used in handling multicast transmission. mc_count is the count of items in mc_list. See Section 14.13 for further details.

spinlock_t xmit_lock; , int xmit_lock_owner;

The xmit_lock is used to avoid multiple simultaneous calls to the driver’s hard_start_xmit function. xmit_lock_owner is the number of the CPU that has obtained xmit_lock. The driver should make no changes to these fields.

struct module *owner;

The module that “owns” this device structure; it is used to maintain the use count for the module.

There are other fields in struct net_device, but they are not used by network drivers.

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