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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
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 10 '17 at 15:13
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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

      <...>

      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 (!!!!>) ARP Sender Protocol Address
      and ARP Target Protocol Address are both set to the IP address
      of the cache entry to be updated (<!!!!), 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].

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