140 lines
4.9 KiB
Groff
140 lines
4.9 KiB
Groff
.\" $NetBSD: arp.4,v 1.14 2016/10/16 05:32:30 wiz Exp $
|
|
.\"
|
|
.\" Copyright (c) 1985, 1986, 1988, 1994
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" 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 .
|