1.\" $NetBSD: arp.4,v 1.7 2002/02/08 01:38:46 ross 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 39.Sh NAME 40.Nm arp 41.Nd Address Resolution Protocol 42.Sh SYNOPSIS 43.Fd #include \*[Lt]netinet/if_ether.h\*[Gt] 44.Sh DESCRIPTION 45The Address Resolution Protocol (ARP) is a protocol used to dynamically 46map between Internet host addresses and Ethernet addresses. 47It is used by all the Ethernet interface drivers. 48It is not specific to Internet protocols or to Ethernet, 49but this implementation currently supports only that combination. 50.Pp 51ARP caches Internet-Ethernet address mappings. 52When an interface requests a mapping for an address not in the cache, 53ARP queues the message which requires the mapping and broadcasts 54a message on the associated network requesting the address mapping. 55If a response is provided, the new mapping is cached and any pending 56message is transmitted. 57ARP will queue at most one packet while waiting for a response to a 58mapping request; 59only the most recently ``transmitted'' packet is kept. 60If the target host does not respond after several requests, 61the host is considered to be down for a short period (normally 20 seconds), 62allowing an error to be returned to transmission attempts during this 63interval. 64The error is 65.Er EHOSTDOWN 66for a non-responding destination host, and 67.Er EHOSTUNREACH 68for a non-responding router. 69.Pp 70The ARP cache is stored in the system routing table as 71dynamically-created host routes. 72The route to a directly-attached Ethernet network is installed as a 73.Dq cloning 74route (one with the 75.Dv RTF_CLONING 76flag set), 77causing routes to individual hosts on that network to be created on 78demand. 79These routes time out periodically (normally 20 minutes after validated; 80entries are not validated when not in use). 81An entry for a host which is not responding is a 82.Dq reject 83route (one with the 84.Dv RTF_REJECT 85flag set). 86.Pp 87ARP entries may be added, deleted or changed with the 88.Xr arp 8 89utility. 90Manually-added entries may be temporary or permanent, 91and may be 92.Dq published , 93in which case the system will respond to ARP requests for that host 94as if it were the target of the request. 95.Pp 96In the past, 97ARP was used to negotiate the use of a trailer encapsulation. 98This is no longer supported. 99.Pp 100ARP watches passively for hosts impersonating the local host (i.e. a host 101which responds to an ARP mapping request for the local host's address). 102.Sh DIAGNOSTICS 103.Bl -diag 104.It "duplicate IP address %x sent from ethernet address %x:%x:%x:%x:%x:%x." 105ARP has discovered another host on the local network which responds to 106mapping requests for its own Internet address with a different Ethernet 107address, generally indicating that two hosts are attempting to use the 108same Internet address. 109.El 110.Sh SEE ALSO 111.Xr inet 4 , 112.Xr route 4 , 113.Xr arp 8 , 114.Xr ifconfig 8 , 115.Xr route 8 116.sp 117.Rs 118.%A Plummer, D. 119.%B "An Ethernet Address Resolution Protocol" 120.%T RFC826 121.Re 122.Rs 123.%A Leffler, S.J. 124.%A Karels, M.J. 125.%B "Trailer Encapsulations" 126.%T RFC893 127.Re 128