Gratuitous ARP

Gratuitous ARP could mean both gratuitous ARP request or gratuitous ARP reply. Gratuitous in this case means a request/reply that is not normally needed according to the ARP specification (RFC 826) but could be used in some cases. A gratuitous ARP request is an AddressResolutionProtocol request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Ordinarily, no reply packet will occur. A gratuitous ARP reply is a reply to which no request has been made.

Gratuitous ARPs are useful for four reasons:

Examples

Example Traffic

Ethernet II, Src: 02:02:02:02:02:02, Dst: ff:ff:ff:ff:ff:ff
    Destination: ff:ff:ff:ff:ff:ff (Broadcast)
    Source: 02:02:02:02:02:02 (02:02:02:02:02:02)
    Type: ARP (0x0806)
    Trailer: 000000000000000000000000000000000000
Address Resolution Protocol (request/gratuitous ARP)
    Hardware type: Ethernet (0x0001)
    Protocol type: IP (0x0800)
    Hardware size: 6
    Protocol size: 4
    Opcode: request (0x0001)
    Sender MAC address: 02:02:02:02:02:02 (02:02:02:02:02:02)
    Sender IP address: 192.168.1.1 (192.168.1.1)
    Target MAC address: ff:ff:ff:ff:ff:ff (Broadcast)
    Target IP address: 192.168.1.1 (192.168.1.1)
0000  ff ff ff ff ff ff 02 02 02 02 02 02 08 06 00 01   ................
0010  08 00 06 04 00 01 02 02 02 02 02 02 c0 a8 01 01   ................
0020  ff ff ff ff ff ff c0 a8 01 01 00 00 00 00 00 00   ................
0030  00 00 00 00 00 00 00 00 00 00 00 00               ............

Discussion

What's a good choice for example MACs? I picked 02:02:02:02:02:02. Is there a better one? -- RandyMcEoin

-The '02' byte at the start of the MAC indicates that this is a 'locally administered address' which has been set by the local user or system. Most normal ethernet devices are allocated a MAC with 00 as the most significant byte.

I updated the article to differentiate between gratuitous ARP request and reply.

Note that some devices will respond to the gratuitous request and some will respond to the gratuitous reply. If one is trying to write software for moving IP addresses around that works with all routers, switches and IP stacks, it is best to send both the request and the reply. These are documented by RFC 2002 and RFC 826. Software implementing the gratuitious ARP function can be found in the Linux-HA source tree. A request may be preceded by a probe to avoid polluting the address space. For an ARP Probe the Sender IP address field is 0.0.0.0. ARP probes were not considered by the original ARP RFC.

-Does the target MAC address ever matter in requests? I gather Solaris uses ff:ff:ff:ff:ff:ff in its standard ARP requests and most other OSes use 00:00:00:00:00:00 instead. Is the use of the ff:ff:ff:ff:ff:ff MAC in the target address above significant in any way? Obviously having a destination address of ff:ff:ff:ff:ff:ff is critical.yes

RFC 3927, which is based on Gratuitous ARP, specifies 00:00:00:00:00:00 for the target MAC. However many simple TCP/IP stacks have an API which permits the specification of only one MAC value, and when the Ethernet Destination field is set to 'broadcast', the ARP target is also set 'broadcast'. Note: Normal ARP requests have the same value in the ARP Packet Target MAC address as in the Ethernet Destination field.

- How can we explain if the source Ethernet MAC address is different from sender's MAC address in a GARP packet? The ARP packet value is for the ARP machine, the Ethernet value is for the Ethernet machine. Originally, they were intended to be redundant information, targeted at different layers. It is possible to consider a hypothetical network appliance that routes ARP packets, where the source Ethernet MAC address changes as the packet is routed, but normally ARP packets are not routed.

Gratuitous_ARP (last edited 2009-01-09 23:01:39 by JeffMorriss)