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ARP(7)                     Linux Programmer's Manual                    ARP(7)



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 behavior
       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  neighbor cache entry is considered
       stale.  Positive feedback can be gotten from a higher layer; for  exam-
       ple  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 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 & 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.

              tab(:) allbox; c s l l.  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
              neighbor  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  dae-
              mon  via  netlink  before dropping back to multicast probes (see
              mcast_solicit).  Defaults to 0.

       base_reachable_time
              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 pro-
              tocols.  Defaults to 30 seconds.

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

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

       gc_stale_time
              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
              The minimum number of entries to keep in  the  ARP  cache.   The
              garbage collector will not run if there are fewer than this num-
              ber 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  multi-
              cast/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 sec-
              onds.

       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 unre-
              solved 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 inter-
       faces (when forwarding and proxy arp is enabled for the interface).

       The neigh/* sysctls 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 gen-
       erate  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
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-07-27                            ARP(7)