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>.

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