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ARP(4)		       FreeBSD Kernel Interfaces Manual			ARP(4)

     arp -- Address Resolution Protocol

     device ether

     The Address Resolution Protocol (ARP) is used to dynamically map between
     Protocol Addresses	(such as IP addresses) and Local Network Addresses
     (such as Ethernet addresses).  This implementation	maps IP	addresses to
     Ethernet, ARCnet, or Token	Ring addresses.	 It is used by all the Ether-
     net interface drivers.

     ARP caches	Internet-Ethernet address mappings.  When an interface
     requests a	mapping	for an address not in the cache, ARP queues the	mes-
     sage which	requires the mapping and broadcasts a message on the associ-
     ated network requesting the address mapping.  If a	response is provided,
     the new mapping is	cached and any pending message is transmitted.	ARP
     will queue	at most	one packet while waiting for a response	to a mapping
     request; only the most recently ``transmitted'' packet is kept.  If the
     target host does not respond after	several	requests, the host is consid-
     ered to be	down allowing an error to be returned to transmission
     attempts.	Further	demand for this	mapping	causes ARP request retransmis-
     sions, that are ratelimited to one	packet per second.  The	error is
     EHOSTDOWN for a non-responding destination	host, and EHOSTUNREACH for a
     non-responding router.

     The ARP cache is stored in	the system routing table as dynamically-cre-
     ated host routes.	The route to a directly-attached Ethernet network is
     installed as a ``cloning''	route (one with	the RTF_CLONING	flag set),
     causing routes to individual hosts	on that	network	to be created on
     demand.  These routes time	out periodically (normally 20 minutes after
     validated;	entries	are not	validated when not in use).

     ARP entries may be	added, deleted or changed with the arp(8) utility.
     Manually-added entries may	be temporary or	permanent, and may be
     ``published'', in which case the system will respond to ARP requests for
     that host as if it	were the target	of the request.

     In	the past, ARP was used to negotiate the	use of a trailer encapsula-
     tion.  This is no longer supported.

     ARP watches passively for hosts impersonating the local host (i.e., a
     host which	responds to an ARP mapping request for the local host's

     Proxy ARP is a feature whereby the	local host will	respond	to requests
     for addresses other than itself, with its own address.  Normally, proxy
     ARP in FreeBSD is set up on a host-by-host	basis using the	arp(8) util-
     ity, by adding an entry for each host inside a given subnet for which
     proxying of ARP requests is desired.  However, the	``proxy	all'' feature
     causes the	local host to act as a proxy for all hosts reachable through
     some other	network	interface, different from the one the request came in
     from.  It may be enabled by setting the sysctl(8) MIB variable to 1.

MIB Variables
     The ARP protocol implements a number of configrable variables in branch	of the sysctl(3) MIB.

     max_age	   How long an ARP entry is held in the	cache until it needs
		   to be refreshed.

     maxtries	   Number of retransmits before	host is	considered down	and
		   error is returned.

     useloopback   If an ARP entry is added for	local address, force the traf-
		   fic to go through the loopback interface.

     proxyall	   Enables ARP proxying	for all	hosts on net.

     arp: %x:%x:%x:%x:%x:%x is using my	IP address %d.%d.%d.%d!	 ARP has dis-
     covered another host on the local network which responds to mapping
     requests for its own Internet address with	a different Ethernet address,
     generally indicating that two hosts are attempting	to use the same	Inter-
     net address.

     arp: link address is broadcast for	IP address %d.%d.%d.%d!	 ARP requested
     information for a host, and received an answer indicating that the	host's
     ethernet address is the ethernet broadcast	address.  This indicates a
     misconfigured or broken device.

     arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x	on %s
     ARP had a cached value for	the ethernet address of	the referenced host,
     but received a reply indicating that the host is at a new address.	 This
     can happen	normally when host hardware addresses change, or when a	mobile
     node arrives or leaves the	local subnet.  It can also indicate a problem
     with proxy	ARP.  This message can only be issued if the sysctl is set to 1,	which is the system's
     default behaviour.

     arpresolve: can't allocate	llinfo for %d.%d.%d.%d	The route for the ref-
     erenced host points to a device upon which	ARP is required, but ARP was
     unable to allocate	a routing table	entry in which to store	the host's MAC
     address.  This usually points to a	misconfigured routing table.  It can
     also occur	if the kernel cannot allocate memory.

     arp: %d.%d.%d.%d is on if0	but got	reply from %x:%x:%x:%x:%x:%x on	if1
     Physical connections exist	to the same logical IP network on both if0 and
     if1.  It can also occur if	an entry already exists	in the ARP cache for
     the IP address above, and the cable has been disconnected from if0, then
     reconnected to if1.  This message can only	be issued if the sysctl is	set to 1, which	is the sys-
     tem's default behaviour.

     arp: %x:%x:%x:%x:%x:%x attempts to	modify permanent entry for %d.%d.%d.%d
     on	%s  ARP	has received an	ARP reply that attempts	to overwrite a perma-
     nent entry	in the local ARP table.	 This error will only be logged	if the
     sysctl is set	to 1, which is
     the system's default behaviour.

     inet(4), route(4),	arp(8),	ifconfig(8), route(8), sysctl(8)

     Plummer, D., "RFC826", An Ethernet	Address	Resolution Protocol.

     Leffler, S.J.  and	Karels,	M.J., "RFC893",	Trailer	Encapsulations.

FreeBSD	7.0			March 28, 2007			   FreeBSD 7.0


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