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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 PF_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
       behaviour can be tuned by the sysctls defined below.

       When there is no positive feedback for an existing mapping  after  some
       time  (see  the  sysctls  below)  a neighbour 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 an unicast probe is send ucast_solicit
       times. If that fails too it will broadcast a new  ARP  request  to  the
       network.  Requests are only send when there is data queued for sending.

       Linux will automatically add a non-permanent 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 PF_INET sockets.  They take a pointer
       to a struct arpreq as their parameter.

       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 & 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 socket 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.

SYSCTLS

       ARP supports a sysctl interface to configure parameters on a global  or
       per-interface basis.  The sysctls can be accessed by reading or writing
       the /proc/sys/net/ipv4/neigh/*/* files or with the sysctl(2) interface.
       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 sysctls are specified in seconds.

       anycast_delay
              The maximum number of jiffies to delay before replying to a IPv6
              neighbour  solicitation  message.   Anycast  support  is not yet
              implemented.  Defaults to 1 second.

       app_solicit
              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
              Once a neighbour 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.

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

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

       gc_stale_time
              Determines how often to check for stale neighbour entries.  When
              a  neighbour  entry  is  considered  stale  it is resolved again
              before sending data to it.  Defaults to 60 seconds.

       gc_thresh1
              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
              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
              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
              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
              The  maximum  number  of  attempts  to  resolve  an  address  by
              multicast/broadcast  before  marking  the  entry as unreachable.
              Defaults to 3.

       proxy_delay
              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
              The  maximum  number of packets which may be queued to proxy-ARP
              addresses.  Defaults to 64.

       retrans_time
              The number of jiffies to delay before retransmitting a  request.
              Defaults to 1 second.

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

       unres_qlen
              The maximum number of packets  which  may  be  queued  for  each
              unresolved address by other network layers.  Defaults to 3.

BUGS

       Some  timer  settings  are  specified in jiffies, which is architecture
       related.  On the Alpha a jiffy is 1/1024 of a  second,  on  most  other
       architectures it is 1/100s.

       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.

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/* sysctls did not exist before Linux 2.2.

SEE ALSO

       capabilities(7), ip(7)

       RFC 826 for a description of ARP.
       RFC 2461  for  a  description  of IPv6 neighbour discovery and the base
       algorithms used.

       Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.