arp - manipulate the system ARP cache at Linux.org

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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]

NOTE
This program is obsolete. For replacement check ip neigh.

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 net‐
work 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 inter‐
faces, you can use the arp -Ds address ifname form. In that case the hardware address is taken from the inter‐
face 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
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 spec‐
ified 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