Understanding Linux Network Internals

Transmitting and Receiving ARP Packets

The functions used to send and receive ARP packets are:

arp_send

The neighboring subsystem calls neigh_ops->solicit to transmit a solicitation request. In the case of ARP, the solicit function (arp_solicit) is a simple wrapper around arp_send. arp_send fills in the ARP header and payload and uses the dev_queue_xmit function to transmit the request.

arp_rcv

Because ARP is a protocol in its own right (unlike ND for IPv6), it registers a handler in arp_init. The next section describes arp_rcv in detail, along with how the two main ARP packet types are processed.

As shown in Figure 28-13, both transmission and reception of ARP packets can be controlled by Netfilter.

The dotted lines between arp_rcv and arp_send indicate that in some cases, the reception of an ARP packet triggers the transmission of at least one other ARP packet. This occurs when:

• Bridging is configured. A bridge receiving an ARP packet may just forward it to other bridge interfaces without processing it.

• The ingress packet is an ARPOP_REQUEST and the neighboring subsystem decides it can reply according to its configuration. The subsystem generates an ARPOP_REPLY.

As Figure 28-13 shows, arp_send is also triggered by external events and a few kernel features such as bonding; details are provided in later sections.

Transmitting ARP Packets: Introduction to arp_send

arp_send is the routine provided by ARP to transmit both solicitation requests and replies, as shown in Figure 28-14. Chapter ...