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

       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 con‐
       sidered stale.  Positive feedback can be gotten from  a	higher	layer;
       for example from a successful TCP ACK.  Other protocols can signal for‐
       ward 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.

	      │		    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 read‐
       ing 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 spec‐
       ified 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  dae‐
	      mon  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 pro‐
	      tocols.  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 neigh‐
	      bor 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 num‐
	      ber 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	multi‐
	      cast/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 sec‐

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

       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 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.

       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 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2008-11-25				ARP(7)

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