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.
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
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.
-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.
