NAME
arp - manipulate the system ARP cache
SYNOPSIS
arp [-vn] [-H type] [-i if] [-a] [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
priveledge 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 simplyfy 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
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).
-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.
EXSAMPLES
/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 <waltje@uwalt.nl.mugnet.org>, Bernd Eckenfels
<net-tools@lina.inka.de>.