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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 kernel.

       A user process can receive ARP packets by using packet(7) sockets.  There is also a mecha-
       nism  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 be-
       haviour 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 got-
       ten 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.

       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 sup-
       port 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.

       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

	      The maximum number of jiffies to delay before replying to a IPv6 neighbour  solici-
	      tation 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 proto-
	      cols.  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

	      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

	      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  net-
	      work 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 sec-

	      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.

       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 ori-
       ented 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

       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.


       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)
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