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

     arp -- Address Resolution Protocol

     #include <netinet/if_ether.h>

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

     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 consid-
     ered to be	down for a short period	(normally 20 seconds), allowing	an er-
     ror 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.

     Each ARP cache entry is stored in a network interface which a response of
     ARP comes in.  ARP	cache entires time out periodically (normally 20 min-
     utes after	validated; entries are not validated when not in use).

     ARP entries may be	added, deleted or changed with the arp(8) utility.
     Manually-added entries may	be temporary 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.

     In	the past, ARP was used to negotiate the	use of a trailer encapsula-
     tion.  This is no longer supported.

     ARP implements Address Conflict Detection.	 When an address is first
     added to the host,	it is marked tentative and ARP probes the network to
     discover if another host has the address.	If another host	replies	with
     the same address, then the	local address is marked	duplicate and the host
     will not use it.  Otherwise the tentative mark is removed and the host
     can start using the address.

     ARP will defend the host's	active address when a conflicting message is
     received.	However, if another conflicting	message	for the	address	is
     found within a 10 second period, then the address is marked duplicate and
     the host will stop	using it.

     For some systems such as a	router or a server, it is desirable never to
     give up an	assigned address.  This	can be achieved	by setting the
     sysctl(7) variable	net.inet.ip_dad_count to 0.

     In	all of the above cases,	ARP will log diagnostic	messages which include
     the hardware address of the conflicting host.

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

     Plummer, D., "RFC 826", An	Ethernet Address Resolution Protocol.

     Leffler, S.J.  and	Karels,	M.J., "RFC 893", Trailer Encapsulations.

     Cheshire, S., "RFC	5227", IPv4 Address Conflict Detection.

     Since NetBSD 8.0, the ARP cache was not stored in the routing table.

     Address Conflict Detection	was added in NetBSD 8.0.

FreeBSD	13.0		       October 12, 2016			  FreeBSD 13.0


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