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.\"	from: @(#)arp4.4	6.5 (Berkeley) 4/18/94
.\"
.Dd April 18, 1994
.Dt ARP 4
.Os
.Sh NAME
.Nm arp
.Nd Address Resolution Protocol
.Sh SYNOPSIS
.Em "pseudo-device ether"
.Sh DESCRIPTION
The Address Resolution Protocol (ARP) is used to dynamically
map between Internet host addresses and 10Mb/s Ethernet addresses.
It is used by all of 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, static 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. A static entry will not
time out, but may be overwritten by network traffic, while a permanent
entry will not time out and can not be overwritten.
.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.
.Pp
.Em "arp info overwritten for %x!! by %x:%x:%x:%x:%x:%x."
An existing route has been overwritten with a new Ethernet address, for
example when the other host has changed Ethernet cards.  If the route
previously was static/non-expiring, the new route will expire normally.
.Pp
.Em "arp: attempt to overwrite permanent entry for %x!! by %x:%x:%x:%x:%x:%x."
As above, but the existing route had been manually setup as permanent.
The routing information is not modified.
.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