4.7 Gratuitous ARP

Another feature of ARP that we can watch is called gratuitous ARP. It occurs when a host sends an ARP request looking for its own IP address. This is usually done when the interface is configured at bootstrap time.

In our internet, if we bootstrap the host bsdi and run tcpdump on the host sun, we see the packet shown in Figure 4.7.

Figure 4.7. Example of gratuitous ARP.

(We specified the -n flag for tcpdump to print numeric dotted -decimal addresses, instead of hostnames.) In terms of the fields in the ARP request, the sender's protocol address and the target's protocol address are identical: 140.252.13.35 for host bsdi. Also, the source address in the Ethernet header, 0:0:c0:6f:2d:40 as shown by tcpdump, equals the sender's hardware address (from Figure 4.4).

Gratuitous ARP provides two features.

It lets a host determine if another host is already configured with the same IP address. The host bsdi is not expecting a reply to this request. But if a reply is received, the error message "duplicate IP address sent from Ethernet address: a:b:c:d:e:f" is logged on the console. This is a warning to the system administrator that one of the systems is misconfigured.
If the host sending the gratuitous ARP has just changed its hardware address (perhaps the host was shut down, the interface card replaced , and then the host was rebooted), this packet causes any other host on the cable that has an entry in its cache for the old hardware address to update its ARP cache entry accordingly . A little known fact of the ARP protocol [Plummer 1982] is that if a host receives an ARP request from an IP address that is already in the receiver's cache, then that cache entry is updated with the sender's hardware address (e.g., Ethernet address) from the ARP request. This is done for any ARP request received by the host. (Recall that ARP requests are broadcast, so this is done by all hosts on the network each time an ARP request is sent.)

[Bhide, Elnozahy, and Morgan 1991] describe an application that can use this feature of ARP to allow a backup file server to take over from a failed server by issuing a gratuitous ARP request with the backup's hardware address and the failed server's IP address. This causes all packets destined for the failed server to be sent to the backup instead, without the client applications being aware that the original server has failed.

Unfortunately the authors then decided against this approach, since it depends on the correct implementation of ARP on all types of clients . They obviously encountered client implementations that did not implement ARP according to its specification.

Monitoring all the systems on the author's subnet shows that SunOS 4.1.3 and 4.4BSD both issue gratuitous ARPs when bootstrapping, but SVR4 does not.