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.\"     @(#)arp.8	8.1 (Berkeley) 6/6/93
.\" $FreeBSD$
.\"
.Dd January 31, 2013
.Dt ARP 8
.Os
.Sh NAME
.Nm arp
.Nd address resolution display and control
.Sh SYNOPSIS
.Nm
.Op Fl n
.Op Fl i Ar interface
.Ar hostname
.Nm
.Op Fl n
.Op Fl i Ar interface
.Fl a
.Nm
.Fl d Ar hostname
.Op Cm pub
.Nm
.Fl d
.Op Fl i Ar interface
.Fl a
.Nm
.Fl s Ar hostname ether_addr
.Op Cm temp
.Op Cm blackhole No \&| Cm reject
.Op Cm pub
.Nm
.Fl S Ar hostname ether_addr
.Op Cm temp
.Op Cm blackhole No \&| Cm reject
.Op Cm pub
.Nm
.Fl f Ar filename
.Sh DESCRIPTION
The
.Nm
utility displays and modifies the Internet-to-Ethernet address translation
tables used by the address resolution protocol
.Pq Xr arp 4 .
With no flags, the program displays the current
.Tn ARP
entry for
.Ar hostname .
The host may be specified by name or by number,
using Internet dot notation.
.Pp
Available options:
.Bl -tag -width indent
.It Fl a
The program displays or deletes all of the current
.Tn ARP
entries.
.It Fl d
A super-user may delete an entry for the host called
.Ar hostname
with the
.Fl d
flag.
If the
.Cm pub
keyword is specified, only the
.Dq published
.Tn ARP
entry
for this host will be deleted.
.Pp
Alternatively, the
.Fl d
flag may be combined with the
.Fl a
flag to delete all entries.
.It Fl i Ar interface
Limit the operation scope to the
.Tn ARP
entries on
.Ar interface .
Applicable only to the following operations:
display one, display all, delete all.
.It Fl n
Show network addresses as numbers (normally
.Nm
attempts to display addresses symbolically).
.It Fl s Ar hostname ether_addr
Create an
.Tn ARP
entry for the host called
.Ar hostname
with the Ethernet address
.Ar ether_addr .
The Ethernet address is given as six hex bytes separated by colons.
The entry will be permanent unless the word
.Cm temp
is given in the command.
If the word
.Cm pub
is given, the entry will be
.Dq published ;
i.e., this system will
act as an
.Tn ARP
server,
responding to requests for
.Ar hostname
even though the host address is not its own.
In this case the
.Ar ether_addr
can be given as
.Cm auto
in which case the interfaces on this host will be examined,
and if one of them is found to occupy the same subnet, its
Ethernet address will be used.
.Pp
If the
.Cm reject
keyword is specified the entry will be marked so that traffic to
the host will be discarded and the sender will be notified the
host is unreachable.
The
.Cm blackhole
keyword is similar in that traffic is discarded but the sender is
not notified.
These can be used to block external traffic to a host without
using a firewall.
.It Fl S Ar hostname ether_addr
Is just like
.Fl s
except any existing
.Tn ARP
entry for this host will be deleted first.
.It Fl f Ar filename
Cause the file
.Ar filename
to be read and multiple entries to be set in the
.Tn ARP
tables.
Entries
in the file should be of the form
.Pp
.Bd -ragged -offset indent -compact
.Ar hostname ether_addr
.Op Cm temp
.Op Cm blackhole No \&| Cm reject
.Op Cm pub
.Ed
.Pp
with argument meanings as given above.
Leading whitespace and empty lines are ignored.
A
.Ql #
character will mark the rest of the line as a comment.
.El
.Sh SEE ALSO
.Xr inet 3 ,
.Xr arp 4 ,
.Xr ifconfig 8 ,
.Xr ndp 8
.Sh HISTORY
The
.Nm
utility appeared in
.Bx 4.3 .