arp

ARP(8)                 Linux System Administrator's Manual                ARP(8)



NAME
       arp - manipulate the system ARP cache

SYNOPSIS
       arp [-vn] [-H type] [-i if] [-ae] [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 specifying an
       hardware address type, interface name or host address.

       arp -d address will delete a ARP table entry. Root or netadmin privilege
       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 simplify 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, -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     Use alternate BSD style output format (with no fixed columns).

       -e     Use default Linux style output format (with 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
              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.  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.  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.

EXAMPLES
       /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                          2008-10-03                             ARP(8)