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arp - manipulate the system ARP cache
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]
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.
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], --display [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.
- 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. For pub entries the specified
interface is the interface on which ARP requests will be
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)
- -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
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
Fred N. van Kempen, <firstname.lastname@example.org> with a lot of improvements
from net-tools Maintainer Bernd Eckenfels <email@example.com>.
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