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.\" $DragonFly: src/usr.sbin/arp/arp.4,v 1.3 2007/11/23 23:16:37 swildner Exp $ 31.\" 32.Dd April 18, 1994 33.Dt ARP 4 34.Os 35.Sh NAME 36.Nm arp 37.Nd Address Resolution Protocol 38.Sh SYNOPSIS 39.Cd "device ether" 40.Sh DESCRIPTION 41The Address Resolution Protocol (ARP) is used to dynamically 42map between Protocol Addresses (such as IP addresses) and 43Local Network Addresses (such as Ethernet addresses). 44This implementation maps IP addresses to Ethernet, 45ARCnet, 46or Token Ring addresses. 47It is used by all the Ethernet interface drivers. 48.Pp 49ARP caches Internet-Ethernet address mappings. 50When an interface requests a mapping for an address not in the cache, 51ARP queues the message which requires the mapping and broadcasts 52a message on the associated network requesting the address mapping. 53If a response is provided, the new mapping is cached and any pending 54message is transmitted. 55ARP will queue at most one packet while waiting for a response to a 56mapping request; 57only the most recently ``transmitted'' packet is kept. 58If the target host does not respond after several requests, 59the host is considered to be down for a short period (normally 20 seconds), 60allowing an error to be returned to transmission attempts during this 61interval. 62The error is 63.Er EHOSTDOWN 64for a non-responding destination host, and 65.Er EHOSTUNREACH 66for a non-responding router. 67.Pp 68The ARP cache is stored in the system routing table as 69dynamically-created host routes. 70The route to a directly-attached Ethernet network is installed as a 71.Dq cloning 72route (one with the 73.Li RTF_CLONING 74flag set), 75causing routes to individual hosts on that network to be created on 76demand. 77These routes time out periodically (normally 20 minutes after validated; 78entries are not validated when not in use). 79An entry for a host which is not responding is a 80.Dq reject 81route (one with the 82.Li RTF_REJECT 83flag set). 84.Pp 85ARP entries may be added, deleted or changed with the 86.Xr arp 8 87utility. 88Manually-added entries may be temporary or permanent, 89and may be 90.Dq published , 91in which case the system will respond to ARP requests for that host 92as if it were the target of the request. 93.Pp 94In the past, 95ARP was used to negotiate the use of a trailer encapsulation. 96This is no longer supported. 97.Pp 98ARP watches passively for hosts impersonating the local host (i.e. a host 99which responds to an ARP mapping request for the local host's address). 100.Sh DIAGNOSTICS 101.Em "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" : 102ARP has discovered another host on the local network which responds to 103mapping requests for its own Internet address with a different Ethernet 104address, generally indicating that two hosts are attempting to use the 105same Internet address. 106.Pp 107.Em "arp: ether address is broadcast for IP address %d.%d.%d.%d!" : 108ARP requested information for a host, and received an answer indicating 109that the host's ethernet address is the ethernet broadcast address. 110This indicates a misconfigured or broken device. 111.Pp 112.Em "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x" : 113ARP had a cached value for the ethernet address of the referenced host, 114but received a reply indicating that the host is at a new address. This 115can happen normally when host hardware addresses change, or when a mobile 116node arrives or leaves the local subnet. It can also indicate a problem 117with proxy ARP. 118.Pp 119.Em "arpresolve: can't allocate llinfo for %d.%d.%d.%d" : 120The route for the referenced host points to a device upon which ARP is 121required, but ARP was unable to allocate a routing table entry in which 122to store the host's MAC address. This usually points to a misconfigured 123routing table. It can also occur if the kernel cannot allocate memory. 124.Sh SEE ALSO 125.Xr inet 4 , 126.Xr route 4 , 127.Xr arp 8 , 128.Xr ifconfig 8 , 129.Xr route 8 130.Rs 131.%A Plummer, D. 132.%B "An Ethernet Address Resolution Protocol" 133.%T RFC 826 134.Re 135.Rs 136.%A Leffler, S.J. 137.%A Karels, M.J. 138.%B "Trailer Encapsulations" 139.%T RFC 893 140.Re 141