arp(7) — Linux manual page


ARP(7)                    Linux Programmer's 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 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 capa‐
       bility 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-ter‐
       minated 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 in‐
       terface 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_reach‐
              able_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 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 ex‐
              ceed 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 miscon‐
              figuration).  Defaults to 1 second.

       mcast_solicit (since Linux 2.2)
              The maximum number of attempts to resolve an address by multi‐
              cast/broadcast before marking the entry as unreachable.  De‐
              faults 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

       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 re‐
              quest.  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 un‐
              resolved address by other network layers.  Defaults to 3.

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

       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.

COLOPHON         top

       This page is part of release 5.09 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)

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