bsd/man/man4/arp.4

   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