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>.