1.\" Copyright (c) 1985, 1986, 1988, 1994 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94 29.\" $FreeBSD: src/usr.sbin/arp/arp.4,v 1.4.2.5 2003/03/12 22:08:13 trhodes Exp $ 30.\" 31.Dd April 18, 1994 32.Dt ARP 4 33.Os 34.Sh NAME 35.Nm arp 36.Nd Address Resolution Protocol 37.Sh SYNOPSIS 38.Cd "device ether" 39.Sh DESCRIPTION 40The Address Resolution Protocol (ARP) is used to dynamically 41map between Protocol Addresses (such as IP addresses) and 42Local Network Addresses (such as Ethernet addresses). 43This implementation maps IP addresses to Ethernet, 44ARCnet, 45or Token Ring addresses. 46It is used by all the Ethernet interface drivers. 47.Pp 48ARP caches Internet-Ethernet address mappings. 49When an interface requests a mapping for an address not in the cache, 50ARP queues the message which requires the mapping and broadcasts 51a message on the associated network requesting the address mapping. 52If a response is provided, the new mapping is cached and any pending 53message is transmitted. 54ARP will queue at most one packet while waiting for a response to a 55mapping request; 56only the most recently ``transmitted'' packet is kept. 57If the target host does not respond after several requests, 58the host is considered to be down for a short period (normally 20 seconds), 59allowing an error to be returned to transmission attempts during this 60interval. 61The error is 62.Er EHOSTDOWN 63for a non-responding destination host, and 64.Er EHOSTUNREACH 65for a non-responding router. 66.Pp 67The ARP cache is stored in the system routing table as 68dynamically-created host routes. 69The route to a directly-attached Ethernet network is installed as a 70.Dq cloning 71route (one with the 72.Li RTF_CLONING 73flag set), 74causing routes to individual hosts on that network to be created on 75demand. 76These routes time out periodically (normally 20 minutes after validated; 77entries are not validated when not in use). 78An entry for a host which is not responding is a 79.Dq reject 80route (one with the 81.Li RTF_REJECT 82flag set). 83.Pp 84ARP entries may be added, deleted or changed with the 85.Xr arp 8 86utility. 87Manually-added entries may be temporary or permanent, 88and may be 89.Dq published , 90in which case the system will respond to ARP requests for that host 91as if it was the target of the request. 92.Pp 93In the past, 94ARP was used to negotiate the use of a trailer encapsulation. 95This is no longer supported. 96.Pp 97ARP watches passively for hosts impersonating the local host (i.e. a host 98which responds to an ARP mapping request for the local host's address). 99.Sh DIAGNOSTICS 100.Em "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" : 101ARP has discovered another host on the local network which responds to 102mapping requests for its own Internet address with a different Ethernet 103address, generally indicating that two hosts are attempting to use the 104same Internet address. 105.Pp 106.Em "arp: ether address is broadcast for IP address %d.%d.%d.%d!" : 107ARP requested information for a host, and received an answer indicating 108that the host's ethernet address is the ethernet broadcast address. 109This indicates a misconfigured or broken device. 110.Pp 111.Em "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x" : 112ARP had a cached value for the ethernet address of the referenced host, 113but received a reply indicating that the host is at a new address. This 114can happen normally when host hardware addresses change, or when a mobile 115node arrives or leaves the local subnet. It can also indicate a problem 116with proxy ARP. 117.Pp 118.Em "arpresolve: can't allocate llinfo for %d.%d.%d.%d" : 119The route for the referenced host points to a device upon which ARP is 120required, but ARP was unable to allocate a routing table entry in which 121to store the host's MAC address. This usually points to a misconfigured 122routing table. It can also occur if the kernel cannot allocate memory. 123.Sh SEE ALSO 124.Xr inet 4 , 125.Xr route 4 , 126.Xr arp 8 , 127.Xr ifconfig 8 , 128.Xr route 8 129.Rs 130.%A Plummer, D. 131.%B "An Ethernet Address Resolution Protocol" 132.%T RFC 826 133.Re 134.Rs 135.%A Leffler, S.J. 136.%A Karels, M.J. 137.%B "Trailer Encapsulations" 138.%T RFC 893 139.Re 140