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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  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 only sent 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.

   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 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.

       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 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).

       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

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.