Understanding Linux Network Internals

Gratuitous ARP

Normally an ARPOP_REQUEST is sent because the sender wants to talk to a given IP address and needs to find out the associated L2 address. But sometimes the sender generates an ARPOP_REQUEST to inform the receivers about some information, instead of asking for information. This is called gratuitous ARP and is commonly used in the following situations:

• Change of L2 address

• Duplicate address detection

• Virtual IP

Each is described in the subsections that follow.

Change of L2 Address

We already saw in the section "Reasons That Neighboring Protocols Are Needed" in Chapter 26, that a change of L2 address (which invalidates neighbour entries for other nodes on the network) cannot be detected without the help of a protocol. Instead of waiting for the old association to expire and forcing each node to start a new protocol transaction (and therefore suffer a temporary black hole), it makes sense to trigger the update of the association in advance. The node that changed the address accomplishes the update through gratuitous ARP. See net/irda/irlan/irlan_eth.c for an example.

Figure 28-2. Example of ARP usage

Duplicate Address Detection

No two hosts on a local network should have the same L3 address, but this problem can happen, especially in big networks with a mix of static and dynamic (that is, DHCP-based) configurations. The most common reasons for duplicate addresses are the presence ...