126 lines
4.8 KiB
Groff
126 lines
4.8 KiB
Groff
.\" Copyright (c) 1985, 1986, 1988, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" from: @(#)arp4.4 6.5 (Berkeley) 4/18/94
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.\" $Id: arp4.4,v 1.2 1994/05/13 08:09:23 mycroft Exp $
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.\"
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.Dd April 18, 1994
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.Dt ARP 4
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.Os BSD 4
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.Sh NAME
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.Nm arp
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.Nd Address Resolution Protocol
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.Sh SYNOPSIS
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.Em "pseudo-device ether"
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.Sh DESCRIPTION
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The Address Resolution Protocol (ARP) is a protocol used to dynamically
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map between Internet host addresses and 10Mb/s Ethernet addresses.
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It is used by all the 10Mb/s Ethernet interface drivers.
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It is not specific to Internet protocols or to 10Mb/s Ethernet,
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but this implementation currently supports only that combination.
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.Pp
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ARP caches Internet-Ethernet address mappings.
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When an interface requests a mapping for an address not in the cache,
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ARP queues the message which requires the mapping and broadcasts
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a message on the associated network requesting the address mapping.
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If a response is provided, the new mapping is cached and any pending
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message is transmitted.
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ARP will queue at most one packet while waiting for a response to a
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mapping request;
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only the most recently ``transmitted'' packet is kept.
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If the target host does not respond after several requests,
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the host is considered to be down for a short period (normally 20 seconds),
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allowing an error to be returned to transmission attempts during this
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interval.
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The error is
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.Li EHOSTDOWN
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for a non-responding destination host, and
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.Li EHOSTUNREACH
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for a non-responding router.
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.Pp
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The ARP cache is stored in the system routing table as
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dynamically-created host routes.
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The route to a directly-attached Ethernet network is installed as a
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.Dq cloning
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route (one with the
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.Li RTF_CLONING
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flag set),
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causing routes to individual hosts on that network to be created on
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demand.
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These routes time out periodically (normally 20 minutes after validated;
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entries are not validated when not in use).
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An entry for a host which is not responding is a
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.Dq reject
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route (one with the
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.Li RTF_REJECT
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flag set).
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.Pp
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ARP entries may be added, deleted or changed with the
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.Xr arp 8
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utility.
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Manually-added entries may be temporary or permanent,
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and may be
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.Dq published ,
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in which case the system will respond to ARP requests for that host
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as if it were the target of the request.
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.Pp
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In the past,
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ARP was used to negotiate the use of a trailer encapsulation.
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This is no longer supported.
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.Pp
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ARP watches passively for hosts impersonating the local host (i.e. a host
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which responds to an ARP mapping request for the local host's address).
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.Sh DIAGNOSTICS
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.Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
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ARP has discovered another host on the local network which responds to
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mapping requests for its own Internet address with a different Ethernet
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address, generally indicating that two hosts are attempting to use the
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same Internet address.
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.Sh SEE ALSO
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.Xr inet 4 ,
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.Xr route 4 ,
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.Xr arp 8 ,
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.Xr ifconfig 8 ,
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.Xr route 8
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.sp
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.Rs
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.%A Plummer, D.
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.%B "An Ethernet Address Resolution Protocol"
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.%T RFC826
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.Re
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.Rs
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.%A Leffler, S.J.
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.%A Karels, M.J.
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.%B "Trailer Encapsulations
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.%T RFC893
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.Re
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