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

       ethers(5), 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)