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ARP(4)			 BSD Kernel Interfaces Manual			ARP(4)

NAME
     arp -- Address Resolution Protocol

SYNOPSIS
     #include <netinet/if_ether.h>

DESCRIPTION
     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.

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

DIAGNOSTICS
     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.

SEE ALSO
     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.

BSD				April 18, 1994				   BSD

NAME | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO

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