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NAME    [Toc]    [Back]

       arp - Linux ARP kernel module.

DESCRIPTION    [Toc]    [Back]

       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 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    [Toc]    [Back]

       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    [Toc]    [Back]

       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.

	      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.

	      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.

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

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

	      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.

	      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.

	      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.

	      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.

	      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.

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

	      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.

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

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

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

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

BUGS    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]


       RFC826 for a description of ARP.
       RFC2461	for  a	description  of  IPv6 neighbour discovery and the base
       algorithms used.

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

Linux Man Page			  1999-06-03				ARP(7)
[ Back ]
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