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ARP(4)		       FreeBSD Kernel Interfaces Manual			ARP(4)

     arp -- Address Resolution Protocol

     pseudo-device ether

     The Address Resolution Protocol (ARP) is used to dynamically map between
     Internet host addresses and Ethernet addresses.  It is used by all	of the
     Ethernet interface	drivers.  It is	not specific to	Internet protocols or
     to	Ethernet, but this implementation currently supports only that combi-

     ARP caches	Internet-Ethernet address mappings.  When an interface re-
     quests a mapping for an address not in the	cache, ARP queues the message
     which requires the	mapping	and broadcasts a message on the	associated
     network requesting	the address mapping.  If a response is provided, the
     new mapping is cached and any pending message is transmitted.  ARP	will
     queue at most one packet while waiting for	a response to a	mapping	re-
     quest; only the most recently transmitted packet is kept.	If the target
     host does not respond after several requests, the host is considered to
     be	down for a short period	(normally 20 seconds), allowing	an error to be
     returned to transmission attempts during this interval.  The error	is
     EHOSTDOWN for a non-responding destination	host, and EHOSTUNREACH for a
     non-responding router.

     The ARP cache is stored in	the system routing table as dynamically	cre-
     ated host routes.	The route to a directly	attached Ethernet network is
     installed as a "cloning" route (one with the RTF_CLONING flag set), caus-
     ing routes	to individual hosts on that network to be created on demand.
     These routes time out periodically	(normally 20 minutes after validated;
     entries are not validated when not	in use).  An entry for a host which is
     not responding is a "reject" route	(one with the RTF_REJECT flag set).

     ARP entries may be	added, deleted or changed with the arp(8) utility.
     Manually added entries may	be temporary, static or	permanent, and may be
     "published", in which case	the system will	respond	to ARP requests	for
     that host as if it	were the target	of the request.	 A static entry	will
     not time out, but may be overwritten by network traffic, while a perma-
     nent entry	will not time out and cannot be	overwritten.

     ARP watches passively for hosts impersonating the local host (i.e., a
     host which	responds to an ARP mapping request for the local host's	ad-

     duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x
     ARP has discovered	another	host on	the local network which	responds to
     mapping requests for its own Internet address with	a different Ethernet
     address, generally	indicating that	two hosts are attempting to use	the
     same Internet address.

     arp info overwritten for %x!! by %x:%x:%x:%x:%x:%x	on %x  An existing
     route has been overwritten	with a new Ethernet address, for example when
     the other host has	changed	Ethernet cards.	 If the	route previously was
     static/non-expiring, the new route	will expire normally.

     arp: attempt to overwrite permanent entry for %x!!	by %x:%x:%x:%x:%x:%x
     on	%x  As above, but the existing route had been manually set up as per-
     manent.  The routing information is not modified.

     arp: attempt to overwrite entry for %x!! on %x by %x:%x:%x:%x:%x:%x on
     %x	 ARP has noticed an attempt to overwrite a host's routing entry	on one
     interface with a routing entry for	a different interface.	The routing
     information is not	modified.

     arp: received reply to broadcast or multicast address  ARP	received a re-
     sponse which is a broadcast or multicast address.	This might indicate an
     ARP spoofing attempt.

     arp: ether	address	is broadcast for IP address %s!	 ARP requested infor-
     mation for	a host,	and received an	answer indicating that the host's Eth-
     ernet address is the Ethernet broadcast address.  This indicates a	mis-
     configured	or broken device.

     arp: ether	address	is multicast for IP address %s!	 ARP requested infor-
     mation for	a host,	and received an	answer indicating that the host's Eth-
     ernet address is the Ethernet multicast address.  This indicates a	mis-
     configured	or broken device.

     arp: attempt to add entry for %s on %s by %s on %s	 This usually indi-
     cates there is more than one interface connected to the same hub, or that
     the networks have somehow been short-circuited (e.g. IPs that should have
     been present on interface one are present on interface two).

     arplookup:	unable to enter	address	for %s	An IP received on the inter-
     face does not match the network/netmask of	the interface.	This indicates
     a netmask problem.

     inet(4), route(4),	arp(8),	ifconfig(8), route(8)

     David C. Plummer, An Ethernet Address Resolution Protocol,	RFC 826,
     November 1982.

FreeBSD	13.0			January	2, 2021			  FreeBSD 13.0


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