arp(7) — Linux manual page


arp(7)              Miscellaneous Information Manual              arp(7)

NAME         top

       arp - Linux ARP kernel module.

DESCRIPTION         top

       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 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

       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

       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 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

VERSIONS         top

       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         top

       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.

SEE ALSO         top

       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.

Linux man-pages (unreleased)     (date)                           arp(7)

Pages that refer to this page: send(2)ip(7)arp(8)