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.\"	from: @(#)arp4.4	6.5 (Berkeley) 4/18/94
.\"
.Dd October 12, 2016
.Dt ARP 4
.Os
.Sh NAME
.Nm arp
.Nd Address Resolution Protocol
.Sh SYNOPSIS
.In netinet/if_ether.h
.Sh DESCRIPTION
The Address Resolution Protocol (ARP) is a protocol used to dynamically
map between Internet host addresses and Ethernet addresses.
It is used by all the Ethernet interface drivers.
It is not specific to Internet protocols or to 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
.Er EHOSTDOWN
for a non-responding destination host, and
.Er EHOSTUNREACH
for a non-responding router.
.Pp
Each ARP cache entry is stored in a network interface which a response
of ARP comes in.
ARP cache entires time out periodically (normally 20 minutes after validated;
entries are not validated when not in use).
.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.
.Sh DIAGNOSTICS
.Bl -diag
ARP implements Address Conflict Detection.
When an address is first added to the host, it is marked tentative and
ARP probes the network to discover if another host has the address.
If another host replies with the same address, then the local address is
marked duplicate and the host will not use it.
Otherwise the tentative
mark is removed and the host can start using the address.
.Pp
ARP will defend the host's active address when a conflicting message is
received.
However, if another conflicting message for the address is found within
a 10 second period, then the address is marked duplicate and the host will
stop using it.
.Pp
For some systems such as a router or a server,
it is desirable never to give up an assigned address.
This can be achieved by setting the
.Xr sysctl 7
variable
.Dv net.inet.ip_dad_count
to 0.
.Pp
In all of the above cases, ARP will log diagnostic messages which include
the hardware address of the conflicting host.
.El
.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 RFC 826
.Re
.Rs
.%A Leffler, S.J.
.%A Karels, M.J.
.%B "Trailer Encapsulations"
.%T RFC 893
.Re
.Rs
.%A Cheshire, S.
.%B "IPv4 Address Conflict Detection"
.%T RFC 5227
.Re
.Sh HISTORY
Since
.Nx 8.0 ,
the ARP cache was not stored in the routing table.
.Pp
Address Conflict Detection was added in
.Nx 8.0 .