Understanding Linux Network Internals

ARP Protocol Initialization

The ARP protocol is initialized by arp_init in net/ipv4/arp.c.

The skeleton of a general protocol initialization routine was shown in the section "Protocol Initialization and Cleanup" in Chapter 27. In this chapter we'll examine what is ARP-specific.

The first step in the function is to register a table of virtual functions and other general parameters used by ARP; this is done by neigh_table_init. The contents of the table, arp_tbl, are described in the next section.

We saw in Chapter 13 how dev_add_pack is used to install a protocol handler. If you remember how that routine is used, from the following definition of arp_packet_type it should be clear that ARP packets will be processed by the arp_rcv function (defined in the same net/ipv4/arp.c file).

static struct packet_type arp_packet_type = {
    .type:    _ _constant_htons(ETH_P_ARP),
    .func:    arp_rcv,
};

arp_proc_init creates the /proc/net/arp file, which can be read to see the contents of the ARP cache (including proxy destinations).

Figure 28-10. arp_fwd_proxy function

When the kernel is compiled with support for sysctl, the directory /proc/sys/net/ipv4/neigh is created to export the default tuning parameters of the neigh_parms structure by means of neigh_sysctl_register. Note that the first input parameter to the latter is set to NULL, which, as we will see in the section "Directory creation," in Chapter 29, means ...