ARP(7)                    (2020-08-13)                     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 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 hard-
          ware addresses and protocol addresses.  The cache has a lim-
          ited 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 behav-
          ior 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
          unicast 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 send-
          ing.

          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.

     Page 1                        Linux             (printed 5/24/22)

     ARP(7)                    (2020-08-13)                     ARP(7)

              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 priv-
          ileged 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.
               tab(:) allbox; c s l l.  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 param-
          eters 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.

               The maximum number of jiffies to delay before replying
               to a IPv6 neighbor solicitation message.  Anycast sup-
               port 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 multi-
               cast probes (see mcast_solicit). Defaults to 0.

               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

     Page 2                        Linux             (printed 5/24/22)

     ARP(7)                    (2020-08-13)                     ARP(7)

               entry's validity will be extended if it receives posi-
               tive feedback from higher level protocols.  Defaults to
               30 seconds.  This file is now obsolete in favor of
               base_reachable_time_ms.

               As for base_reachable_time, but measures time in mil-
               liseconds.  Defaults to 30000 milliseconds.

               Delay before first probe after it has been decided that
               a neighbor is stale.  Defaults to 5 seconds.

               How frequently the garbage collector for neighbor
               entries should attempt to run.  Defaults to 30 seconds.

               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.

               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 512.

               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 second.

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

     Page 3                        Linux             (printed 5/24/22)

     ARP(7)                    (2020-08-13)                     ARP(7)

               When an ARP request for a known proxy-ARP address is
               received, delay up to proxy_delay jiffies before reply-
               ing.  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 retransmitting a
               request.  Defaults to 1 second.  This file is now obso-
               lete in favor of retrans_time_ms.

               The number of milliseconds to delay before retransmit-
               ting a request.  Defaults to 1000 milliseconds.

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

               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-oriented 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).

     Page 4                        Linux             (printed 5/24/22)

     ARP(7)                    (2020-08-13)                     ARP(7)

          This man page mashes together functionality that is IPv4-
          specific with functionality that is shared between IPv4 and
          IPv6.

     SEE ALSO
          capabilities(7), ip(7), arpd(8)

          RFC 826 for a description of ARP.  RFC 2461 for a descrip-
          tion 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 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 5                        Linux             (printed 5/24/22)