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

       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 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 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.

       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 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 (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

       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 (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 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.

       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).

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

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

       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 5.13 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

Linux                              2020-08-13                             ARP(7)