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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 pa-
rameter 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 prac-
tice, 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 en-
tries 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 in-
clude 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 op-
tion 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 inter-
face 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 file-
name. 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 host-
name, 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 <waltje AT uwalt.org>, Bernd Eckenfels <net-tools AT lina.de>.
net-tools 2008-10-03 ARP(8)
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