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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 con-
       vert 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 protocol addresses.  The cache  has
       a limited size so old and less frequently used entries are garbage-collected.  Entries which are marked as per-
       manent 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 for-
       wards 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 only be performed by a process with the CAP_NET_ADMIN capability or an effec-
       tive 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 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 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.

   /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_reachable_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  sec-
              onds.

       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 num-
              ber 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 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 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 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.

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  forward-
       ing 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-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.

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 applicable.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-25                            ARP(7)