GratuitousARPMessages

The term is ambiguous in more ways than one - see GratuitousARPAmbiguousTerminology

I’m using a strict definition of the term ‘Gratuitous’ here: ARP packets whose Sender IP and Target IP are the same (see IPEqualityAsADefinition) and not 0.0.0.0 (see WhyZeroIPIsExcluded)

This is a host telling the network about its own address. Requests and replies both. See GratuitousRequestsVsReplies

GratuitousARPWithUnicastTargetMAC

• Known as gratuitous ARP, a term used for both requests and replies, often ambiguously. See ARPAnnouncement for the name RFC5227 gave the request @ref
• The requests have their own topic, GratuitousARPRequest
• Sender MAC and Sender IP create a complete mapping so it will trigger ARPCache updates @ref
• An ARPProbe, by contrast, deliberately carries no complete mapping, so updates no cache @ref
• What it’s used for can’t be read off the packet:
  • ARPAnnouncement, as part of ARPDuplicateAddressDetection
  • BroadcastTraffic on a network when a host changes its IP so other hosts can update their ARPCache
  • Sent by some OperatingSystems on startup and to help with LoadBalancing
  • Failover setups, where an address moves to another card or box
  • ARPSpoofing and ARPCachePoisoning
• Shapes:
  • GratuitousARPReply
  • GratuitousARPReplySenderMACMismatch
  • GratuitousARPRequestBlankTarget
  • GratuitousARPRequestBlankTargetSenderMACMismatch
  • GratuitousARPRequestBroadcastTarget
  • GratuitousARPRequestBroadcastTargetSenderMACMismatch
  • GratuitousARPRequestUnicastTarget
  • GratuitousARPRequestUnicastTargetSenderMACMismatch
  • UnicastGratuitousARPReply
  • UnicastGratuitousARPReplySenderMACMismatch
  • UnicastGratuitousARPRequest
  • UnicastGratuitousARPRequestSenderMACMismatch