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.\" 4. 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.\" $FreeBSD$ 30.\" 31.Dd March 28, 2007 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 allowing an error to be returned to 59transmission attempts. 60Further demand for this mapping causes ARP request retransmissions, that 61are ratelimited to one packet per second. 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). 79.Pp 80ARP entries may be added, deleted or changed with the 81.Xr arp 8 82utility. 83Manually-added entries may be temporary or permanent, 84and may be 85.Dq published , 86in which case the system will respond to ARP requests for that host 87as if it were the target of the request. 88.Pp 89In the past, 90ARP was used to negotiate the use of a trailer encapsulation. 91This is no longer supported. 92.Pp 93ARP watches passively for hosts impersonating the local host (i.e., a host 94which responds to an ARP mapping request for the local host's address). 95.Pp 96Proxy ARP is a feature whereby the local host will respond to requests 97for addresses other than itself, with its own address. 98Normally, proxy ARP in 99.Fx 100is set up on a host-by-host basis using the 101.Xr arp 8 102utility, by adding an entry for each host inside a given subnet for 103which proxying of ARP requests is desired. 104However, the 105.Dq "proxy all" 106feature causes the local host to act as a proxy for 107.Em all 108hosts reachable through some other network interface, 109different from the one the request came in from. 110It may be enabled by setting the 111.Xr sysctl 8 112MIB variable 113.Va net.link.ether.inet.proxyall 114to 1. 115.Sh MIB Variables 116The ARP protocol implements a number of configrable variables in 117.Va net.link.ether.inet 118branch 119of the 120.Xr sysctl 3 121MIB. 122.Bl -tag 123.It Va max_age 124How long an ARP entry is held in the cache until it needs to be refreshed. 125.It Va maxtries 126Number of retransmits before host is considered down and error is returned. 127.It Va useloopback 128If an ARP entry is added for local address, force the traffic to go through 129the loopback interface. 130.It Va proxyall 131Enables ARP proxying for all hosts on net. 132.El 133.Sh DIAGNOSTICS 134.Bl -diag 135.It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" 136ARP has discovered another host on the local network which responds to 137mapping requests for its own Internet address with a different Ethernet 138address, generally indicating that two hosts are attempting to use the 139same Internet address. 140.It "arp: link address is broadcast for IP address %d.%d.%d.%d!" 141ARP requested information for a host, and received an answer indicating 142that the host's ethernet address is the ethernet broadcast address. 143This indicates a misconfigured or broken device. 144.It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s" 145ARP had a cached value for the ethernet address of the referenced host, 146but received a reply indicating that the host is at a new address. 147This can happen normally when host hardware addresses change, 148or when a mobile node arrives or leaves the local subnet. 149It can also indicate a problem with proxy ARP. 150This message can only be issued if the sysctl 151.Va net.link.ether.inet.log_arp_movements 152is set to 1, which is the system's default behaviour. 153.It "arpresolve: can't allocate llinfo for %d.%d.%d.%d" 154The route for the referenced host points to a device upon which ARP is 155required, but ARP was unable to allocate a routing table entry in which 156to store the host's MAC address. 157This usually points to a misconfigured routing table. 158It can also occur if the kernel cannot allocate memory. 159.It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" 160Physical connections exist to the same logical IP network on both if0 and if1. 161It can also occur if an entry already exists in the ARP cache for the IP 162address above, and the cable has been disconnected from if0, then reconnected 163to if1. 164This message can only be issued if the sysctl 165.Va net.link.ether.inet.log_arp_wrong_iface 166is set to 1, which is the system's default behaviour. 167.It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s" 168ARP has received an ARP reply that attempts to overwrite a permanent 169entry in the local ARP table. 170This error will only be logged if the sysctl 171.Va net.link.ether.inet.log_arp_permanent_modify 172is set to 1, which is the system's default behaviour. 173.El 174.Sh SEE ALSO 175.Xr inet 4 , 176.Xr route 4 , 177.Xr arp 8 , 178.Xr ifconfig 8 , 179.Xr route 8 , 180.Xr sysctl 8 181.Rs 182.%A Plummer, D. 183.%B "An Ethernet Address Resolution Protocol" 184.%T RFC826 185.Re 186.Rs 187.%A Leffler, S.J. 188.%A Karels, M.J. 189.%B "Trailer Encapsulations 190.%T RFC893 191.Re 192