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

NAME
     arp -- Address Resolution Protocol

SYNOPSIS
     pseudo-device ether

DESCRIPTION
     The Address Resolution Protocol (ARP) is a	protocol used to dynamically
     map between Internet host addresses and 10Mb/s Ethernet addresses.	 It is
     used by all the 10Mb/s Ethernet interface drivers.	 It is not specific to
     Internet protocols	or to 10Mb/s Ethernet, but this	implementation cur-
     rently 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
     arp: %x:%x:%x:%x:%x:%x is using my	IP address %d.%d.%d.%d!: ARP has dis-
     covered another host on the local network which responds to mapping re-
     quests for	its own	Internet address with a	different Ethernet address,
     generally indicating that two hosts are attempting	to use the same	Inter-
     net address.

     arp: ether	address	is broadcast for IP address %d.%d.%d.%d!: ARP re-
     quested information for a host, and received an answer indicating that
     the host's	ethernet address is the	ethernet broadcast address.  This in-
     dicates a misconfigured or	broken device.

     arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x: ARP
     had a cached value	for the	ethernet address of the	referenced host, but
     received a	reply indicating that the host is at a new address.  This can
     happen normally when host hardware	addresses change, or when a mobile
     node arrives or leaves the	local subnet.  It can also indicate a problem
     with proxy	ARP.

     arpresolve: can't allocate	llinfo for %d.%d.%d.%d:	The route for the ref-
     erenced host points to a device upon which	ARP is required, but ARP was
     unable to allocate	a routing table	entry in which to store	the host's MAC
     address.  This usually points to a	misconfigured routing table.  It can
     also occur	if the kernel cannot allocate memory.

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

     Plummer, D., "RFC826", An Ethernet	Address	Resolution Protocol.

     Leffler, S.J.  and	Karels,	M.J., "RFC893",	Trailer	Encapsulations.

BSD				April 18, 1994				   BSD

NAME | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO

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