Gratuitous ARP

A client network interface configured for DHCP will issue a gratuitous ARP request after it has received an IP offered from the DHCP server. It does this to detect if any other local hosts have the same IP address in order to avoid a continued address conflict.

The host in question will issue a gratuitous ARP request packet configured as follows:

Source Hardware Address (SHA): its own MAC address
Source Protocol Address (SPA): the DHCP assigned IP address
Destination Hardware Address: Broadcast MAC FF:FF:FF:FF:FF:FF
Destination Protocol Address: the DHCP assigned IP address

Since hosts are required to receive and process all ARP traffic, you can expect a gratuitous ARP reply if another host already has the IP address recently assigned. At this point the DHCP client will report the address conflict to the DHCP server.

Although the DHCP server will initially send pings to the IP it intends to assign as a preventative measure, it is possible the ARP table on the DHCP server is not accurate. Therefore a local host could be lurking on the network with the IP in question but a MAC address other than what is, or is not, recorded in the ARP table on the DHCP server.


RFC 826: Ethernet Address Resolution Protocol

RFC 5227: IP Address Conflict Detection

Wireshark: Gratuitous ARP