Identify gratuitous ARP

Question

I want to identify a gratuitous ARP.

Can I say: If sender IP address and target IP address equal, it's always a gratuitous ARP?

This question seems to be a duplicate of another we've already answered... I'll let the community vote to close if they agree — Mike Pennington, Aug 27 '14 at 10:19
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score 6 · Answer 1 · edited Oct 07 '21 at 06:47

You can identify Gratuitous ARPs by looking at the ARP Sender Protocol Address and ARP Target Protocol Address, so as you mentioned when they are the same it's a gratuitous ARP. See RFC 2002, Section 4.6 for a reference (the emphasis - !!!!> and <!!!! is mine)...

      4.6. ARP, Proxy ARP, and Gratuitous ARP
  &lt;...&gt;

  A Gratuitous ARP [23] is an ARP packet sent by a node in order to
  spontaneously cause other nodes to update an entry in their ARP
  cache.  A gratuitous ARP MAY use either an ARP Request or an ARP
  Reply packet.  In either case, the (!!!!&gt;) ARP Sender Protocol Address
  and ARP Target Protocol Address are both set to the IP address
  of the cache entry to be updated (&lt;!!!!), and the ARP Sender Hardware
  Address is set to the link-layer address to which this cache
  entry should be updated.  When using an ARP Reply packet, the
  Target Hardware Address is also set to the link-layer address to
  which this cache entry should be updated (this field is not used
  in an ARP Request packet).

  In either case, for a gratuitous ARP, the ARP packet MUST be
  transmitted as a local broadcast packet on the local link.  As
  specified in [16], any node receiving any ARP packet (Request or
  Reply) MUST update its local ARP cache with the Sender Protocol
  and Hardware Addresses in the ARP packet, if the receiving node
  has an entry for that IP address already in its ARP cache.  This
  requirement in the ARP protocol applies even for ARP Request
  packets, and for ARP Reply packets that do not match any ARP
  Request transmitted by the receiving node [16].

Identify gratuitous ARP

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