arp - manipulate the system ARP cache


SYNOPSIS

       arp [-evn] [-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 ifa  [netmask  nm]
       pub

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


DESCRIPTION

       Arp  manipulates  the  kernel's ARP cache in various ways.
       The primary options are clearing an address mapping  entry
       and  manually setting up one.  For debugging purposes, the
       arp program also allows a complete dump of the ARP  cache.


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 (arc­
              net) , PROnet (pronet) , AX.25 (ax25)  and  NET/ROM
              (netrom).

       -a [hostname], --display [hostname]
              Shows  the  entries of the specified hosts.  If the
              hostname parameter is not used, all entries will be
              displayed.  The entries will be displayed in alter­
              nate (BSD) style.

       -d hostname, --delete hostname
              Remove any entry for the specified host.  This  can
              be  used if the indicated host is brought down, for
              example.

              Use the interface ifa's hardware address.

       -e     Shows the entries in default (Linux) style.

       -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.

       -s hostname hw_addr, --set hostname
              Manually create an ARP address  mapping  entry  for
              host  hostname with hardware address set to hw_addr
              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  ker­
              nels  because it can be useful. If the temp flag is
              not supplied entries will be permanent stored  into
              the ARP cache.
              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.

       -f filename, --file filename
              Similar  to  the  -s  option,  only  this  time the
              address info is taken from file  filename  set  up.
              The   name   of   the   data  file  is  very  often
              /etc/ethers, but this is not official. If no  file­
              name 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 host­
       name and the hardware address can be exchanged.

       the  C  flag. Permanent entries are marked with M and pub­
       lished entries have the P flag.


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>  with  a
       lot of improvements from net-tools Maintainer Bernd Ecken­
       fels <net-tools@lina.inka.de>.


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