1.\" Copyright (c) 1985, 1986, 1988, 1994 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" %sccs.include.redist.roff% 5.\" 6.\" @(#)arp4.4 6.5 (Berkeley) 04/18/94 7.\" 8.Dd 9.Dt ARP 4 10.Os BSD 4 11.Sh NAME 12.Nm arp 13.Nd Address Resolution Protocol 14.Sh SYNOPSIS 15.Em "pseudo-device ether" 16.Sh DESCRIPTION 17The Address Resolution Protocol (ARP) is a protocol used to dynamically 18map between Internet host addresses and 10Mb/s Ethernet addresses. 19It is used by all the 10Mb/s Ethernet interface drivers. 20It is not specific to Internet protocols or to 10Mb/s Ethernet, 21but this implementation currently supports only that combination. 22.Pp 23ARP caches Internet-Ethernet address mappings. 24When an interface requests a mapping for an address not in the cache, 25ARP queues the message which requires the mapping and broadcasts 26a message on the associated network requesting the address mapping. 27If a response is provided, the new mapping is cached and any pending 28message is transmitted. 29ARP will queue at most one packet while waiting for a response to a 30mapping request; 31only the most recently ``transmitted'' packet is kept. 32If the target host does not respond after several requests, 33the host is considered to be down for a short period (normally 20 seconds), 34allowing an error to be returned to transmission attempts during this 35interval. 36The error is 37.Li EHOSTDOWN 38for a non-responding destination host, and 39.Li EHOSTUNREACH 40for a non-responding router. 41.Pp 42The ARP cache is stored in the system routing table as 43dynamically-created host routes. 44The route to a directly-attached Ethernet network is installed as a 45.Dq cloning 46route (one with the 47.Li RTF_CLONING 48flag set), 49causing routes to individual hosts on that network to be created on 50demand. 51These routes time out periodically (normally 20 minutes after validated; 52entries are not validated when not in use). 53An entry for a host which is not responding is a 54.Dq reject 55route (one with the 56.Li RTF_REJECT 57flag set). 58.Pp 59ARP entries may be added, deleted or changed with the 60.Xr arp 8 61utility. 62Manually-added entries may be temporary or permanent, 63and may be 64.Dq published , 65in which case the system will respond to ARP requests for that host 66as if it were the target of the request. 67.Pp 68In the past, 69ARP was used to negotiate the use of a trailer encapsulation. 70This is no longer supported. 71.Pp 72ARP watches passively for hosts impersonating the local host (i.e. a host 73which responds to an ARP mapping request for the local host's address). 74.Sh DIAGNOSTICS 75.Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x." 76ARP has discovered another host on the local network which responds to 77mapping requests for its own Internet address with a different Ethernet 78address, generally indicating that two hosts are attempting to use the 79same Internet address. 80.Sh SEE ALSO 81.Xr inet 4 , 82.Xr route 4 , 83.Xr arp 8 , 84.Xr ifconfig 8 , 85.Xr route 8 86.sp 87.Rs 88.%A Plummer, D. 89.%B "An Ethernet Address Resolution Protocol" 90.%T RFC826 91.Re 92.Rs 93.%A Leffler, S.J. 94.%A Karels, M.J. 95.%B "Trailer Encapsulations 96.%T RFC893 97.Re 98 99