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 specifying 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  sup‐
       plied  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  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).

       -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 inter‐
              face 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  file‐
              name.   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 host‐
              name, 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 AT uwalt.org>, Bernd Eckenfels <net-tools AT lina.de>.



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