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	1	.\" $NetBSD: arp.4,v 1.2 1995/03/01 11:50:56 chopps Exp $
	2	.\"
	3	.\" Copyright (c) 1985, 1986, 1988, 1994
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" from: @(#)arp4.4 6.5 (Berkeley) 4/18/94
	35	.\"
	36	.Dd April 18, 1994
	37	.Dt ARP 4
	38	.Os BSD 4
	39	.Sh NAME
	40	.Nm arp
	41	.Nd Address Resolution Protocol
	42	.Sh SYNOPSIS
	43	.Em "pseudo-device ether"
	44	.Sh DESCRIPTION
	45	The Address Resolution Protocol (ARP) is a protocol used to dynamically
	46	map between Internet host addresses and 10Mb/s Ethernet addresses.
	47	It is used by all the 10Mb/s Ethernet interface drivers.
	48	It is not specific to Internet protocols or to 10Mb/s Ethernet,
	49	but this implementation currently supports only that combination.
	50	.Pp
	51	ARP caches Internet-Ethernet address mappings.
	52	When an interface requests a mapping for an address not in the cache,
	53	ARP queues the message which requires the mapping and broadcasts
	54	a message on the associated network requesting the address mapping.
	55	If a response is provided, the new mapping is cached and any pending
	56	message is transmitted.
	57	ARP will queue at most one packet while waiting for a response to a
	58	mapping request;
	59	only the most recently ``transmitted'' packet is kept.
	60	If the target host does not respond after several requests,
	61	the host is considered to be down for a short period (normally 20 seconds),
	62	allowing an error to be returned to transmission attempts during this
	63	interval.
	64	The error is
	65	.Li EHOSTDOWN
	66	for a non-responding destination host, and
	67	.Li EHOSTUNREACH
	68	for a non-responding router.
	69	.Pp
	70	The ARP cache is stored in the system routing table as
	71	dynamically-created host routes.
	72	The route to a directly-attached Ethernet network is installed as a
	73	.Dq cloning
	74	route (one with the
	75	.Li RTF_CLONING
	76	flag set),
	77	causing routes to individual hosts on that network to be created on
	78	demand.
	79	These routes time out periodically (normally 20 minutes after validated;
	80	entries are not validated when not in use).
	81	An entry for a host which is not responding is a
	82	.Dq reject
	83	route (one with the
	84	.Li RTF_REJECT
	85	flag set).
	86	.Pp
	87	ARP entries may be added, deleted or changed with the
	88	.Xr arp 8
	89	utility.
	90	Manually-added entries may be temporary or permanent,
	91	and may be
	92	.Dq published ,
	93	in which case the system will respond to ARP requests for that host
	94	as if it were the target of the request.
	95	.Pp
	96	In the past,
	97	ARP was used to negotiate the use of a trailer encapsulation.
	98	This is no longer supported.
	99	.Pp
	100	ARP watches passively for hosts impersonating the local host (i.e. a host
	101	which responds to an ARP mapping request for the local host's address).
	102	.Sh DIAGNOSTICS
	103	.Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
	104	ARP has discovered another host on the local network which responds to
	105	mapping requests for its own Internet address with a different Ethernet
	106	address, generally indicating that two hosts are attempting to use the
	107	same Internet address.
	108	.Sh SEE ALSO
	109	.Xr inet 4 ,
	110	.Xr route 4 ,
	111	.Xr arp 8 ,
	112	.Xr ifconfig 8 ,
	113	.Xr route 8
	114	.sp
	115	.Rs
	116	.%A Plummer, D.
	117	.%B "An Ethernet Address Resolution Protocol"
	118	.%T RFC826
	119	.Re
	120	.Rs
	121	.%A Leffler, S.J.
	122	.%A Karels, M.J.
	123	.%B "Trailer Encapsulations
	124	.%T RFC893
	125	.Re