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. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94 33.\" $FreeBSD$ 34.\" 35.Dd April 18, 1994 36.Dt ARP 4 37.Os BSD 4 38.Sh NAME 39.Nm arp 40.Nd Address Resolution Protocol 41.Sh SYNOPSIS 42.Cd "pseudo-device ether" 43.Sh DESCRIPTION 44The Address Resolution Protocol (ARP) is a protocol used to dynamically 45map between Internet host addresses and 10Mb/s Ethernet addresses. 46It is used by all the 10Mb/s Ethernet interface drivers. 47It is not specific to Internet protocols or to 10Mb/s Ethernet, 48but this implementation currently supports only that combination. 49.Pp 50ARP caches Internet-Ethernet address mappings. 51When an interface requests a mapping for an address not in the cache, 52ARP queues the message which requires the mapping and broadcasts 53a message on the associated network requesting the address mapping. 54If a response is provided, the new mapping is cached and any pending 55message is transmitted. 56ARP will queue at most one packet while waiting for a response to a 57mapping request; 58only the most recently ``transmitted'' packet is kept. 59If the target host does not respond after several requests, 60the host is considered to be down for a short period (normally 20 seconds), 61allowing an error to be returned to transmission attempts during this 62interval. 63The error is 64.Li EHOSTDOWN 65for a non-responding destination host, and 66.Li EHOSTUNREACH 67for a non-responding router. 68.Pp 69The ARP cache is stored in the system routing table as 70dynamically-created host routes. 71The route to a directly-attached Ethernet network is installed as a 72.Dq cloning 73route (one with the 74.Li RTF_CLONING 75flag set), 76causing routes to individual hosts on that network to be created on 77demand. 78These routes time out periodically (normally 20 minutes after validated; 79entries are not validated when not in use). 80An entry for a host which is not responding is a 81.Dq reject 82route (one with the 83.Li RTF_REJECT 84flag set). 85.Pp 86ARP entries may be added, deleted or changed with the 87.Xr arp 8 88utility. 89Manually-added entries may be temporary or permanent, 90and may be 91.Dq published , 92in which case the system will respond to ARP requests for that host 93as if it were the target of the request. 94.Pp 95In the past, 96ARP was used to negotiate the use of a trailer encapsulation. 97This is no longer supported. 98.Pp 99ARP watches passively for hosts impersonating the local host (i.e. a host 100which responds to an ARP mapping request for the local host's address). 101.Sh DIAGNOSTICS 102.Em "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" : 103ARP has discovered another host on the local network which responds to 104mapping requests for its own Internet address with a different Ethernet 105address, generally indicating that two hosts are attempting to use the 106same Internet address. 107.Pp 108.Em "arp: ether address is broadcast for IP address %d.%d.%d.%d!" : 109ARP requested information for a host, and received an answer indicating 110that the host's ethernet address is the ethernet broadcast address. 111This indicates a misconfigured or broken device. 112.Pp 113.Em "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x" : 114ARP had a cached value for the ethernet address of the referenced host, 115but received a reply indicating that the host is at a new address. This 116can happen normally when host hardware addresses change, or when a mobile 117node arrives or leaves the local subnet. It can also indicate a problem 118with proxy ARP. 119.Pp 120.Em "arpresolve: can't allocate llinfo for %d.%d.%d.%d" : 121The route for the referenced host points to a device upon which ARP is 122required, but ARP was unable to allocate a routing table entry in which 123to store the host's MAC address. This usually points to a misconfigured 124routing table. It can also occur if the kernel cannot allocate memory. 125.Sh SEE ALSO 126.Xr inet 4 , 127.Xr route 4 , 128.Xr arp 8 , 129.Xr ifconfig 8 , 130.Xr route 8 131.sp 132.Rs 133.%A Plummer, D. 134.%B "An Ethernet Address Resolution Protocol" 135.%T RFC826 136.Re 137.Rs 138.%A Leffler, S.J. 139.%A Karels, M.J. 140.%B "Trailer Encapsulations 141.%T RFC893 142.Re 143 144