arp - Linux ARP kernel module.


   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

   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 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.

          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

   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 (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.


   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.


   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.


   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.


   This  page  is  part of release 4.09 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


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.