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.\"	from: @(#)arp4.4	6.5 (Berkeley) 4/18/94
.\"
.Dd April 18, 1994
.Dt ARP 4
.Os BSD 4
.Sh NAME
.Nm arp
.Nd Address Resolution Protocol
.Sh SYNOPSIS
.Em "pseudo-device ether"
.Sh DESCRIPTION
The Address Resolution Protocol (ARP) is a protocol used to dynamically
map between Internet host addresses and 10Mb/s Ethernet addresses.
It is used by all the 10Mb/s Ethernet interface drivers.
It is not specific to Internet protocols or to 10Mb/s Ethernet,
but this implementation currently supports only that combination.
.Pp
ARP caches Internet-Ethernet address mappings. 
When an interface requests a mapping for an address not in the cache,
ARP queues the message which requires the mapping and broadcasts
a message on the associated network requesting the address mapping.
If a response is provided, the new mapping is cached and any pending
message is transmitted.
ARP will queue at most one packet while waiting for a response to a
mapping request;
only the most recently ``transmitted'' packet is kept.
If the target host does not respond after several requests,
the host is considered to be down for a short period (normally 20 seconds),
allowing an error to be returned to transmission attempts during this
interval.
The error is
.Li EHOSTDOWN
for a non-responding destination host, and
.Li EHOSTUNREACH
for a non-responding router.
.Pp
The ARP cache is stored in the system routing table as
dynamically-created host routes.
The route to a directly-attached Ethernet network is installed as a
.Dq cloning
route (one with the
.Li RTF_CLONING
flag set),
causing routes to individual hosts on that network to be created on
demand.
These routes time out periodically (normally 20 minutes after validated;
entries are not validated when not in use).
An entry for a host which is not responding is a
.Dq reject
route (one with the
.Li RTF_REJECT
flag set).
.Pp
ARP entries may be added, deleted or changed with the
.Xr arp 8
utility.
Manually-added entries may be temporary or permanent,
and may be
.Dq published ,
in which case the system will respond to ARP requests for that host
as if it were the target of the request.
.Pp
In the past,
ARP was used to negotiate the use of a trailer encapsulation.
This is no longer supported.
.Pp
ARP watches passively for hosts impersonating the local host (i.e. a host
which responds to an ARP mapping request for the local host's address).
.Sh DIAGNOSTICS
.Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
ARP has discovered another host on the local network which responds to
mapping requests for its own Internet address with a different Ethernet
address, generally indicating that two hosts are attempting to use the
same Internet address.
.Sh SEE ALSO
.Xr inet 4 ,
.Xr route 4 ,
.Xr arp 8 ,
.Xr ifconfig 8 ,
.Xr route 8
.sp
.Rs
.%A Plummer, D.
.%B "An Ethernet Address Resolution Protocol"
.%T RFC826
.Re
.Rs
.%A Leffler, S.J.
.%A Karels, M.J.
.%B "Trailer Encapsulations
.%T RFC893
.Re