Understanding Linux Network Internals

Protocol Handler Registration

At system startup and other times when a protocol is registered, the kernel calls dev_add_pack, passing it a data structure of type packet_type, which is defined in include/linux/netdevice.h as follows:

struct packet_type
{
    unsigned short        type;
    struct net_device       *dev;
    int            (*func) (struct sk_buff *, struct net_device *,
                    struct packet_type *);
    void            *af_packet_priv;
    struct list_head    *list;
};

The fields have the following meanings:

type

The protocol code. It can take any of the values listed in the first column of Table 13-1 through 13-4 (i.e., ETH_P_IP). The difference between the protocols belonging to different tables will become clear in the following sections.

dev

Pointer to the device (i.e., eth0) for which the protocol is to be enabled. A setting of NULL means "all devices." Thanks to this parameter, it would be possible to have different handlers for different devices, or associate a handler with one specific device. This is not normally done, but could be useful for testing. PF_PACKET sockets commonly use it to listen only on a specific device. For instance, a command such as tcpdump -i eth0 creates a packet_type instance via a PF_PACKET socket and initializes dev to the net_device instance associated with eth0.

func

The function handler called by netif_receive_skb (see Chapter 10) when it needs to process one frame with skb->protocol=type (an example is ip_rcv). Note that one of func's input parameters is a pointer to a packet_type structure: it ...