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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  ad-
       dresses	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 lo-
       cal 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 re-
       ceives 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  tar-
       get, 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  de-
       vices.	Unless otherwise specified, time-related interfaces are	speci-
       fied 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 im-
	      plemented.  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  ex-
	      tended if	it receives positive feedback from higher level	proto-
	      cols.  Defaults to 30 seconds.  This file	is now obsolete	in fa-
	      vor 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 be-
	      fore 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.   De-
	      faults 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  po-
	      tential  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.  De-
	      faults 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 re-

       retrans_time_ms (since Linux 2.6.12)
	      The number of milliseconds to delay before retransmitting	a  re-
	      quest.  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 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 3.74 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

Linux				  2008-11-25				ARP(7)


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