Understanding Linux Network Internals

Processing Ingress ARP Packets

As explained in the section "ARP Protocol Initialization," ARP registers the arp_rcv routine as its protocol handler. Let's see how this handler processes incoming ARP packets.

The ARP packet can be accessed from the skb buffer that is the function's input argument; in particular, the ARP header is at skb->nh.arph. The function's first task is to make sure the ARP packet is not fragmented; that is, that it can be accessed linearly in memory. This task is necessary because sometimes the skb buffer is fragmented in memory.^[*] If it is, arp_rcv calls the generic routine pskb_may_pull to make sure there is enough room in the main buffer for the ARP header and payload.

int arp_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt)
{
    struct arphdr *arp;

    /* ARP header, plus 2 device addresses, plus 2 IP addresses.  */
    if (!pskb_may_pull(skb, (sizeof(struct arphdr) +
                 (2 * dev->addr_len) +
                 (2 * sizeof(u32)))))
        goto freeskb;

An input ARP packet is dropped by arp_rcv if one of the following conditions is met:

• It was received on a device that does not use ARP (i.e., one tagged with the IFF_NOARP flag).

  The loopback interface is a special case within this category. Packets sent to and from the loopback interface are classified with the PACKET_LOOPBACK type. Since such an interface is virtual and does not have a hardware address, there is no need to use ARP.

• It was not addressed to the receiving interface (i.e., the destination address was not the receiving ...