arp - manipulate the system ARP cache

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 ifa [netmask nm] pub

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

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.

-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 [hostname], --display [hostname]

Shows the entries of the specified hosts. If the hostname parameter is not used, all entries will be displayed.

-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.

-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.

-s hostname hw_addr, --set hostname

Manually create an ARP address mapping 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. 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.

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 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.

/proc/net/arp,

/etc/networks

/etc/hosts

/etc/ethers

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

Fred N. van Kempen, <waltje@uwalt.nl.mugnet.org> with a lot of improvements from net-tools Maintainer Bernd Eckenfels <net-tools@lina.inka.de>.