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



NAME
       arp - Linux ARP kernel module.

DESCRIPTION
       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 mecha-
       nism  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  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
       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 for-
       ward 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 uni-
       cast probe is sent ucast_solicit times.  If that fails too, it will broadcast  a  new  ARP
       request to the network.  Requests are sent only when there is data queued for sending.

       Linux will automatically add a nonpermanent 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 added.

   Ioctls
       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 be performed only by a process
       with the CAP_NET_ADMIN capability or an effective UID of 0.

       arp_pa must be an AF_INET address 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 device.

              +-------------------------------------+
              |             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 sup-
       port 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.

   /proc interfaces
       ARP supports a range of /proc interfaces to configure parameters on a global or per-inter-
       face  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 solicita-
              tion 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_reachable_time/2.  An entry's
              validity will be extended if it receives positive feedback from higher level proto-
              cols.   Defaults  to 30 seconds.  This file is now obsolete in favor of base_reach-
              able_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 collector 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 prevents ARP
              cache thrashing if there is more than one potential mapping (generally due to  net-
              work 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 flooding 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 sec-
              ond.  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.

VERSIONS
       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 2.2.

       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.

BUGS
       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 connection-ori-
       ented protocols implemented 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.

SEE ALSO
       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  applica-
       ble.

COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2008-11-25                                     ARP(7)

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