arp Command with Examples in Linux

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This tutorial explains Linux “arp” command, options and its usage with examples.

arp – manipulate the system Address Resolution Protocol (ARP) cache

Description

“arp” manipulates the kernel’s ARP cache in various ways. The primary options are clearing an address mapping entry and manually setting up one. For debugging purposes, the arp program also allows a complete dump of the ARP cache.


Synopsis

arp [-evn] [-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 ifa [netmask nm] pub
arp [-vnD] [-H type] [-i if] -f [filename]

Note : This program is obsolete. For replacement check ip neighbor.

Options

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-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 [hostname], –all [hostname]
Shows the entries of the specified hosts. If the hostname parameter is not used, all entries will be displayed. The entries will be displayed in alternate (BSD) style.

-d hostname, –delete hostname

Remove any entry for the specified host. This can be used if the indicated host is brought down, for example.

-D, –use-device

Use the interface ifa’s hardware address.

-e
Shows the entries in default (Linux) style.

-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.
NOTE: This has to be different from the interface to which the IP datagrams will be routed.

-s hostname hw_addr, –set hostname
Manually create an ARP address mapping entry(static entry) for host hostname with hardware address set to hw_addr 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.

-f filename, –file filename
Similar to the -s option, only this time the address info is taken from file filename 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.

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.

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Example Usage :

1. This command lists all the peers connected at various interfaces alongwith their MAC Addresses and IP addresses.

$ arp -a
? (192.168.211.2) at 00:50:56:ef:9f:a2 [ether] on eth0
? (192.168.211.254) at 00:50:56:fb:74:a6 [ether] on eth0

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

$arp -i eth0 -Ds 10.0.0.2 eth1 pub

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

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$arp -i eth1 -d 10.0.0.1

4. Add a static entry in arp table

$arp -s 157.55.85.212   00-aa-00-62-c6-09

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If you wish to look at all Linux commands and their usage examples, go to Linux Commands Tutorial.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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