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+.\" Copyright (c) 1985, 1986, 1988, 1994
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
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+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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+.\" SUCH DAMAGE.
+.\"
+.\"	@(#)arp4.4	6.5 (Berkeley) 4/18/94
+.\"
+.Dd April 18, 1994
+.Dt ARP 4
+.Os BSD 4
+.Sh NAME
+.Nm arp
+.Nd Address Resolution Protocol
+.Sh SYNOPSIS
+.Em "pseudo-device ether"
+.Sh 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 currently supports only that combination.
+.Pp
+ARP caches Internet-Ethernet address mappings. 
+When an interface requests 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 request;
+only the most recently ``transmitted'' packet is kept.
+If the target host does not respond after several requests,
+the host is considered to be down for a short period (normally 20 seconds),
+allowing an error to be returned to transmission attempts during this
+interval.
+The error is
+.Li EHOSTDOWN
+for a non-responding destination host, and
+.Li EHOSTUNREACH
+for a non-responding router.
+.Pp
+The ARP cache is stored in the system routing table as
+dynamically-created host routes.
+The route to a directly-attached Ethernet network is installed as a
+.Dq cloning
+route (one with the
+.Li RTF_CLONING
+flag set),
+causing 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
+.Dq reject
+route (one with the
+.Li RTF_REJECT
+flag set).
+.Pp
+ARP entries may be added, deleted or changed with the
+.Xr arp 8
+utility.
+Manually-added entries may be temporary or permanent,
+and may be
+.Dq published ,
+in which case the system will respond to ARP requests for that host
+as if it were the target of the request.
+.Pp
+In the past,
+ARP was used to negotiate the use of a trailer encapsulation.
+This is no longer supported.
+.Pp
+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).
+.Sh DIAGNOSTICS
+.Em "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.
+.Sh SEE ALSO
+.Xr inet 4 ,
+.Xr route 4 ,
+.Xr arp 8 ,
+.Xr ifconfig 8 ,
+.Xr route 8
+.sp
+.Rs
+.%A Plummer, D.
+.%B "An Ethernet Address Resolution Protocol"
+.%T RFC826
+.Re
+.Rs
+.%A Leffler, S.J.
+.%A Karels, M.J.
+.%B "Trailer Encapsulations
+.%T RFC893
+.Re
+