ARP

ARP(8)               Linux System Administrator’s Manual              ARP(8)

NAME

arp – manipulate the system ARP cache

SYNOPSIS

arp [-vn] [-H type] [-i if] [-ae] [hostname]

arp [-v] [-i if] -d hostname [pub]

arp [-v] [-H type] [-i if] -s hostname hw_addr [temp]

arp [-v] [-H type] [-i if] -s hostname hw_addr [netmask nm] pub

arp [-v] [-H type] [-i if] -Ds hostname ifname [netmask nm] pub

arp [-vnD] [-H type] [-i if] -f [filename]

DESCRIPTION

Arp manipulates or displays the kernel’s IPv4 network neighbour
cache. It can add entries to the table, delete one or display the
current content.

ARP stands for Address Resolution Protocol, which is used to find the
media access control address of a network neighbour for a given IPv4
Address.

MODES

arp with no mode specifier will print the current content of the
table. It is possible to limit the number of entries printed, by
specifying an hardware address type, interface name or host address.

arp -d address will delete a ARP table entry. Root or netadmin
privilege is required to do this. The entry is found by IP address.
If a hostname is given, it will be resolved before looking up the
entry in the ARP table.

arp -s address hw_addr is used to set up a new table entry. The
format of the hw_addr parameter is dependent on the hardware class,
but for most classes one can assume that the usual presentation can
be used.  For the Ethernet class, this is 6 bytes in hexadecimal,
separated by colons. When adding proxy arp entries (that is those
with the publish flag set) a netmask may be specified to proxy arp
for entire subnets. This is not good practice, but is supported by
older kernels because it can be useful. If the temp flag is not
supplied entries will be permanent stored into the ARP cache. To
simplify setting up entries for one of your own network interfaces,
you can use the arp -Ds address ifname form. In that case the
hardware address is taken from the interface with the specified name.

OPTIONS

-v, –verbose
Tell the user what is going on by being verbose.

-n, –numeric
shows numerical addresses instead of trying to determine
symbolic host, port or user names.

-H type, –hw-type type, -t type
When setting or reading the ARP cache, this optional parameter
tells arp which class of entries it should check for.  The
default value of this parameter is ether (i.e. hardware code
0x01 for IEEE 802.3 10Mbps Ethernet).  Other values might
include network technologies such as ARCnet (arcnet) , PROnet
(pronet) , AX.25 (ax25) and NET/ROM (netrom).

-a     Use alternate BSD style output format (with no fixed columns).

-e     Use default Linux style output format (with fixed columns).

-D, –use-device
Instead of a hw_addr, the given argument is the name of an
interface.  arp will use the MAC address of that interface for
the table entry. This is usually the best option to set up a
proxy ARP entry to yourself.

-i If, –device If
Select an interface. When dumping the ARP cache only entries
matching the specified interface will be printed. When setting
a permanent or temp ARP entry this interface will be
associated with the entry; if this option is not used, the
kernel will guess based on the routing table. For pub entries
the specified interface is the interface on which ARP requests
will be answered.
NOTE: This has to be different from the interface to which the
IP datagrams will be routed.  NOTE: As of kernel 2.2.0 it is
no longer possible to set an ARP entry for an entire subnet.
Linux instead does automagic proxy arp when a route exists and
it is forwarding. See arp(7) for details. Also the dontpub
option which is available for delete and set operations cannot
be used with 2.4 and newer kernels.

-f filename, –file filename
Similar to the -s option, only this time the address info is
taken from file filename.  This can be used if ARP entries for
a lot of hosts have to be set up.  The name of the data file
is very often /etc/ethers, but this is not official. If no
filename is specified /etc/ethers is used as default.

The format of the file is simple; it only contains ASCII text
lines with a hostname, and a hardware address separated by
whitespace. Additionally the pub, temp and netmask flags can
be used.

In all places where a hostname is expected, one can also enter an IP
address in dotted-decimal notation.

As a special case for compatibility the order of the hostname and the
hardware address can be exchanged.

Each complete entry in the ARP cache will be marked with the C flag.
Permanent entries are marked with M and published entries have the P
flag.

EXAMPLES

/usr/sbin/arp -i eth0 -Ds 10.0.0.2 eth1 pub

This will answer ARP requests for 10.0.0.2 on eth0 with the MAC
address for eth1.

/usr/sbin/arp -i eth1 -d 10.0.0.1

Delete the ARP table entry for 10.0.0.1 on interface eth1. This will
match published proxy ARP entries and permanent entries.

FILES

/proc/net/arp
/etc/networks
/etc/hosts
/etc/ethers

SEE ALSO

rarp(8), route(8), ifconfig(8), netstat(8)

AUTHORS

Fred N. van Kempen <[email protected]>, Bernd Eckenfels
<[email protected]>.

COLOPHON

This page is part of the net-tools (networking utilities) project.
Information about the project can be found at
⟨http://net-tools.sourceforge.net/⟩.  If you have a bug report for
this manual page, see ⟨http://net-tools.sourceforge.net/⟩.  This page
was obtained from the project’s upstream Git repository
(git://git.code.sf.net/p/net-tools/code) on 2014-12-30.  If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to [email protected]

net-tools                        2008-10-03                           ARP(8)

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