ARP(8)							     Linux Programmer's Manual							    ARP(8)

NAME

       arp - manipulate the system ARP cache

SYNOPSIS

       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 ifname [netmask nm] pub

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

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 network 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 specify-
       ing an hardware address type, interface name or host address.

       arp -d address will delete a ARP table entry. Root or netadmin priveledge 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 simplyfy setting up entries for one of your own network interfaces, you can use the arp -Ds address
       ifname form. In that case the hardware address is taken from the interface 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
	      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 tech-
	      nologies such as ARCnet (arcnet) , PROnet (pronet) , AX.25 (ax25) and NET/ROM (netrom).

       -a     Use alternate BSD style output format (with no fixed columns).

       -D, --use-device
	      Instead of a hw_addr, the given argument is the name of an interface.  arp will use the MAC address of that interface for the  table
	      entry. This is usually the best option to set up a proxy ARP entry to yourself.

       -i If, --device If
	      Select an interface. When dumping the ARP cache only entries matching the specified interface will be printed. When setting a perma-
	      nent 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.  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  for-
	      warding.	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.

EXSAMPLES

       /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

SEE ALSO

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

AUTHORS

       Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>, Bernd Eckenfels <net-tools@lina.inka.de>.

net-tools							    2007-12-01								    ARP(8)