arp

ARP(8)                Linux System Administrator's Manual               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 con-
       tent.

       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 privi-
       lege 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 perma-
       nent 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 sym-
              bolic 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 de-
              fault 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 in-
              terface.  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 an-
              swered.
              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>.

net-tools                         2008-10-03                            ARP(8)