Internet Core Protocols: The Definitive Guide

Reverse ARP (RARP)

Reverse ARP (RARP) is most often used with diskless workstations that do not have a permanent IP address, or that do not have sufficient resources for storing a permanent IP address. Whenever such a device needs to obtain an IP address for its own use (generally at boot time), it issues an ARP broadcast that contains its own hardware address in both the Sender and Recipient Hardware Address fields. Note that the Sender and Recipient Protocol Address fields would be zeroed out.

A special host (called a RARP server) watches for RARP broadcasts (RARP packets have their own unique ethertype of 8035). When one is seen, the server attempts to find an entry for the requesting device’s hardware address in a local table of IP-to-hardware address mappings. If a match is found, it returns a RARP response to the requesting device, providing it with the IP address needed to continue the boot process. This process is illustrated in Figure 3.4.

Figure 3.4. An overview of Reverse ARP

Although RARP was functional enough to be useful, it did not offer much in the way of comprehensive resource configuration. RARP did not provide for automatically determining the subnet mask in use on the network, the routers available, or much of anything else. For this reason, RARP is used much less frequently today than it has been in the past, with most devices having long ago moved to the BOOTP ...