.\"	$OpenBSD: arp.8,v 1.24 2009/04/29 07:10:38 jmc Exp $
.\"	$NetBSD: arp.8,v 1.7 1995/03/01 11:50:59 chopps Exp $
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.\"	from: @(#)arp.8	8.1 (Berkeley) 6/6/93
.\"
.Dd $Mdocdate: April 29 2009 $
.Dt ARP 8
.Os
.Sh NAME
.Nm arp
.Nd address resolution display and control
.Sh SYNOPSIS
.Nm arp
.Op Fl adn
.Ar hostname
.Nm arp
.Op Fl F
.Op Fl f Ar file
.Fl s Ar hostname ether_addr
.Op Cm temp | permanent
.Op Cm pub
.Sh DESCRIPTION
The
.Nm
program displays and modifies the Internet-to-Ethernet address translation
tables used by the address resolution protocol (ARP).
.Pp
.Nm
displays the current ARP entry for
.Ar hostname
when no optional parameters are supplied.
.Ar hostname
may be specified by name or by number,
using Internet dot notation.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a
Display all of the current ARP entries.
See also the
.Fl d
option below.
.It Fl d
Delete an entry for the host called
.Ar hostname .
Alternatively, the
.Fl d
flag may be combined with the
.Fl a
flag to delete all entries, with hostname lookups automatically
disabled.
Only the superuser may delete entries.
.It Fl F
Force existing entries for the given host to be overwritten
(only relevant to the
.Fl f
and
.Fl s
options).
.It Fl f Ar file
Process entries from
.Ar file
to be set in the ARP tables.
Any entries in the file that already exist for a given host
will not be overwritten unless
.Fl F
is given.
Entries in the file should be of the form:
.Bd -filled -offset indent
.Ar hostname ether_addr
.Op Cm temp | permanent
.Op Cm pub
.Ed
.Pp
The entry will be static (will not time out) unless the word
.Cm temp
is given in the command.
A static ARP entry can be overwritten by network traffic, unless the word
.Cm permanent
is given.
If the word
.Cm pub
is given, the entry will be
.Dq published ;
that is, this system will act as an ARP server,
responding to requests for
.Ar hostname
even though the host address is not its own.
This behavior has traditionally been called
.Em proxy ARP .
.It Fl n
Show network addresses as numbers (normally
.Nm
attempts to display addresses symbolically).
.It Xo
.Fl s Ar hostname ether_addr
.Op Cm temp | permanent
.Op Cm pub
.Xc
Create an ARP entry for the host called
.Ar hostname
with the Ethernet address
.Ar ether_addr .
The Ethernet address is given as six hexadecimal bytes separated by
colons.
.Pp
The
.Cm permanent , pub ,
or
.Cm temp
modifiers may be specified with meanings as given above.
.Pp
If the entry already exists for the given host, it will not
be replaced unless
.Fl F
is given.
.El
.Sh EXAMPLES
View the current
.Xr arp 4
table,
showing network addresses symbolically:
.Pp
.Dl $ arp -a
.Pp
Create a permanent
entry (one that cannot be overwritten by other network traffic):
.Pp
.Dl # arp -s 10.0.0.2 00:90:27:bb:cc:dd permanent
.Pp
Create proxy ARP
entries on interface fxp0
(MAC address 00:90:27:bb:cc:dd),
for IP addresses 204.1.2.3 and 204.1.2.4:
.Bd -literal -offset indent
# arp -s 204.1.2.3 00:90:27:bb:cc:dd pub
# arp -s 204.1.2.4 00:90:27:bb:cc:dd pub
.Ed
.Sh SEE ALSO
.Xr inet 3 ,
.Xr arp 4 ,
.Xr ifconfig 8 ,
.Xr ndp 8
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .