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ARP(7)                     Linux Programmer's Manual                    ARP(7)

       arp - Linux ARP kernel module.

       This  kernel  protocol module implements the Address Resolution Protocol defined in
       RFC 826.  It is used to convert between Layer2 hardware addresses and IPv4 protocol
       addresses  on  directly  connected  networks.   The  user normally doesn't interact
       directly with this module except to configure it; instead it provides a service for
       other protocols 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 AF_INET socket.

       The  ARP module maintains a cache of mappings between hardware addresses and proto-
       col 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 behavior can be tuned by the /proc interfaces described below.

       When there is no positive feedback for an existing mapping after some time (see the
       /proc interfaces below), a neighbor cache  entry  is  considered  stale.   Positive
       feedback  can be gotten 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, a unicast probe is sent ucast_solicit
       times.  If that fails too, it will broadcast a new  ARP  request  to  the  network.
       Requests are only sent when there is data queued for sending.

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

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

           struct arpreq {
               struct sockaddr arp_pa;      /* protocol address */
               struct sockaddr arp_ha;      /* hardware address */
               int             arp_flags;   /* flags */
               struct sockaddr arp_netmask; /* netmask of protocol address */
               char            arp_dev[16];

       SIOCSARP,  SIOCDARP  and  SIOCGARP respectively set, delete and get an ARP mapping.
       Setting and deleting ARP maps are privileged operations and may only  be  performed
       by a process with the CAP_NET_ADMIN capability or an effective UID of 0.

       arp_pa  must  be an AF_INET socket and arp_ha must have the same type as the device
       which is specified in arp_dev.  arp_dev is a zero-terminated string which  names  a

              |             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

   /proc interfaces
       ARP supports a range of /proc interfaces to configure parameters  on  a  global  or
       per-interface  basis.   The  interfaces  can  be accessed by reading or writing the
       /proc/sys/net/ipv4/neigh/*/* files.  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
       interfaces are specified in seconds.

       anycast_delay (since Linux 2.2)
              The  maximum  number  of jiffies to delay before replying to a IPv6 neighbor
              solicitation message.  Anycast support is not yet implemented.  Defaults  to
              1 second.

       app_solicit (since Linux 2.2)
              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.

       base_reachable_time (since Linux 2.2)
              Once  a  neighbor has been found, the entry is considered to be valid for at
              least  a  random  value  between  base_reachable_time/2  and   3*base_reach-
              able_time/2.   An  entry's validity will be extended if it receives positive
              feedback from higher level protocols.  Defaults to 30 seconds.  This file is
              now obsolete in favor of base_reachable_time_ms.

       base_reachable_time_ms (since Linux 2.6.12)
              As  for base_reachable_time, but measures time in milliseconds.  Defaults to
              30000 milliseconds.

       delay_first_probe_time (since Linux 2.2)
              Delay before first probe after it has been decided that a neighbor is stale.
              Defaults to 5 seconds.

       gc_interval (since Linux 2.2)
              How  frequently the garbage collector for neighbor entries should attempt to
              run.  Defaults to 30 seconds.

       gc_stale_time (since Linux 2.2)
              Determines how often to check for stale neighbor entries.  When  a  neighbor
              entry  is  considered stale, it is resolved again before sending data to it.
              Defaults to 60 seconds.

       gc_thresh1 (since Linux 2.2)
              The minimum number of entries to keep in the ARP cache.  The garbage collec-
              tor  will  not  run  if  there  are fewer than this number of entries in the
              cache.  Defaults to 128.

       gc_thresh2 (since Linux 2.2)
              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 512.

       gc_thresh3 (since Linux 2.2)
              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.

       locktime (since Linux 2.2)
              The minimum number of jiffies to keep an ARP entry in the cache.  This  pre-
              vents  ARP cache thrashing if there is more than one potential mapping (gen-
              erally due to network misconfiguration).  Defaults to 1 second.

       mcast_solicit (since Linux 2.2)
              The maximum number of attempts to resolve an address by  multicast/broadcast
              before marking the entry as unreachable.  Defaults to 3.

       proxy_delay (since Linux 2.2)
              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 flood-
              ing in some cases.  Defaults to 0.8 seconds.

       proxy_qlen (since Linux 2.2)
              The  maximum  number  of packets which may be queued to proxy-ARP addresses.
              Defaults to 64.

       retrans_time (since Linux 2.2)
              The number of jiffies to delay before retransmitting a request.  Defaults to
              1 second.  This file is now obsolete in favor of retrans_time_ms.

       retrans_time_ms (since Linux 2.6.12)
              The  number  of  milliseconds  to  delay  before  retransmitting  a request.
              Defaults to 1000 milliseconds.

       ucast_solicit (since Linux 2.2)
              The maximum number of attempts to send unicast probes before asking the  ARP
              daemon (see app_solicit).  Defaults to 3.

       unres_qlen (since Linux 2.2)
              The  maximum  number  of  packets  which  may  be queued for each unresolved
              address by other network layers.  Defaults to 3.

       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 reachable hosts on other interfaces  (when  forwarding  and  proxy  arp  is
       enabled for the interface).

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

       Some  timer  settings  are  specified in jiffies, which is architecture- and kernel
       version-dependent; see time(7).

       There is no way to signal positive feedback from user space.   This  means  connec-
       tion-oriented protocols implemented in user space will generate excessive ARP traf-
       fic, 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

       capabilities(7), ip(7)

       RFC 826 for a description of ARP.
       RFC 2461 for a description of IPv6 neighbor discovery and the base algorithms used.

       Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2008-11-25                            ARP(7)

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