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       This kernel protocol module implements the Address Resolu­
       tion Protocol defined in RFC 826.  It is used  to  convert
       between   Layer2  hardware  addresses  and  IPv4  protocol
       addresses on directly connected networks.  The  user  nor­
       mally doesn't interact directly with this module except to
       configure it; instead it provides a service for other pro­
       tocols in the kernel.

       A  user process can receive ARP packets by using packet(7)
       sockets.  There is also a mechanism for managing  the  ARP
       cache  in  user-space by using netlink(7) sockets. The ARP
       table can also be controlled via ioctl (2) on any  PF_INET

       The ARP module maintains a cache of mappings between hard­
       ware addresses and protocol addresses.  The  cache  has  a
       limited  size  so old and less frequently used entries are
       garbage-collected.  Entries which are marked as  permanent
       are never deleted by the garbage-collector.  The cache can
       be directly manipulated by  the  use  of  ioctls  and  its
       behaviour can be tuned by the sysctls defined below.

       When there is no positive feedback for an existing mapping
       after some time (see the sysctls below) a neighbour  cache
       entry  is considered stale.  Positive feedback can be got­
       ten from a higher layer; for example from a successful TCP
       ACK. Other protocols can signal forward progress using the
       MSG_CONFIRM flag to sendmsg(2).  When there is no  forward
       progress  ARP  tries  to reprobe.  It first tries to ask a
       local arp daemon app_solicit  times  for  an  updated  MAC
       address.  If that fails and an old MAC address is known an
       unicast probe is send ucast_solicit times. If  that  fails
       too  it  will  broadcast a new ARP request to the network.
       Requests are only send when there is data queued for send­

       Linux  will  automatically  add  a non-permanent proxy arp
       entry when it receives a request for an  address  it  for­
       wards  to and proxy arp is enabled on the receiving inter­
       face. When there is a reject route for the target no proxy
       arp entry is added.


       Three  ioctls  are available on all PF_INET sockets.  They
       take a pointer to a struct arpreq as their parameter.

       struct arpreq
           struct sockaddr arp_pa;      /* protocol address */
           struct sockaddr arp_ha;      /* hardware address */

       |             arp_flags               |
       |flag            | meaning            |
       |ATF_COM         | Lookup complete    |
       |ATF_PERM        | Permanent entry    |
       |ATF_PUBL        | Publish entry      |
       |ATF_USETRAILERS | Trailers requested |
       |ATF_NETMASK     | Use a netmask      |
       |ATF_DONTPUB     | Don't answer       |

       If the ATF_NETMASK flag is set, then arp_netmask should be
       valid.  Linux 2.2  does  not  support  proxy  network  ARP
       entries,  so  this  should  be  set to 0xffffffff, or 0 to
       remove an existing proxy arp  entry.   ATF_USETRAILERS  is
       obsolete and should not be used.


       ARP supports a sysctl interface to configure parameters on
       a global or  per-interface  basis.   The  sysctls  can  be
       accessed      by      reading      or      writing     the
       /proc/sys/net/ipv4/neigh/*/* files or with  the  sysctl(2)
       interface.   Each  interface  in  the  system  has its own
       directory in /proc/sys/net/ipv4/neigh/.   The  setting  in
       the  `default'  directory  is  used  for all newly created
       devices.  Unless otherwise specified time related  sysctls
       are specified in seconds.

              The  maximum  number  of  jiffies  to  delay before
              replying to a IPv6 neighbour solicitation  message.
              Anycast  support  is not yet implemented.  Defaults
              to 1 second.

              The maximum number of probes to send  to  the  user
              space  ARP  daemon via netlink before dropping back
              to multicast probes (see mcast_solicit).   Defaults
              to 0.

              Once  a neighbour has been found, the entry is con­

              Determines how often to check for  stale  neighbour
              entries.   When  a  neighbour  entry  is considered
              stale it is resolved again before sending  data  to
              it.  Defaults to 60 seconds.

              The  minimum  number  of entries to keep in the ARP
              cache.  The garbage collector will not run if there
              are fewer than this number of entries in the cache.
              Defaults to 128.

              The soft maximum number of entries to keep  in  the
              ARP  cache.   The  garbage collector will allow the
              number of entries to  exceed  this  for  5  seconds
              before  collection  will be performed.  Defaults to

              The hard maximum number of entries to keep  in  the
              ARP  cache.   The garbage collector will always run
              if there are more than this number  of  entries  in
              the cache.  Defaults to 1024.

              The  minimum number of jiffies to keep an ARP entry
              in the cache.  This prevents ARP cache thrashing if
              there is more than one potential mapping (generally
              due to network misconfiguration).   Defaults  to  1

              The  maximum  number  of  attempts  to  resolve  an
              address by multicast/broadcast before  marking  the
              entry as unreachable.  Defaults to 3.

              When  an  ARP request for a known proxy-ARP address
              is received, delay up to proxy_delay jiffies before
              replying.  This is used to prevent network flooding
              in some cases.  Defaults to 0.8 seconds.

              The maximum number of packets which may  be  queued
              to proxy-ARP addresses.  Defaults to 64.

              The  number  of jiffies to delay before retransmit­
              ting a request.  Defaults to 1 second.

       second, on most other architectures it is 1/100s.

       There is no way to  signal  positive  feedback  from  user
       space.  This  means  connection  oriented protocols imple­
       mented in user space will generate excessive ARP  traffic,
       because ndisc will regularly reprobe the MAC address.  The
       same problem applies for some kernel protocols  (e.g.  NFS
       over UDP).

       This man page mashes IPv4 specific and shared between IPv4
       and IPv6 functionality together.


       The struct arpreq changed in  Linux  2.0  to  include  the
       arp_dev  member  and the ioctl numbers changed at the same
       time.  Support for the old ioctls  was  dropped  in  Linux

       Support  for  proxy  arp entries for networks (netmask not
       equal 0xffffffff) was dropped in Linux 2.2. It is replaced
       by  automatic proxy arp setup by the kernel for all reach­
       able hosts on other interfaces (when forwarding and  proxy
       arp is enabled for the interface).

       The neigh/* sysctls did not exist before Linux 2.2.



       RFC826 for a description of ARP.
       RFC2461  for a description of IPv6 neighbour discovery and
       the base algorithms used.

       Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applica­

Linux Man Page              1999-06-03                     ARP(7)



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