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ARP(4)			 BSD 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 re-
     quests a mapping for an address not in the	cache, ARP queues the message
     which requires the	mapping	and broadcasts a message on the	associated
     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	re-
     quest; 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 at-
     tempts.  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), caus-
     ing 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	ad-

     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 configurable variables in branch	of the sysctl(3) MIB.

     allow_multicast	       Install ARP entries with	the multicast bit set
			       in the hardware address.	 Installing such en-
			       tries is	an RFC 1812 violation, but some	pro-
			       prietary	load balancing techniques require
			       routers to do so.  Turned off by	default.

     garp_rexmit_count	       Retransmit gratuitous ARP (GARP)	packets	when
			       an IPv4 address is added	to an interface.  A
			       GARP is always transmitted when an IPv4 address
			       is added	to an interface.  A non-zero value
			       causes the GARP packet to be retransmitted the
			       stated number of	times.	The interval between
			       retransmissions is doubled each time, so	the
			       retransmission intervals	are: {1, 2, 4, 8, 16,
			       ...} (seconds).	The default value of zero
			       means only the initial GARP is sent; no addi-
			       tional GARP packets are retransmitted.  The
			       maximum value is	sixteen.

			       The default behavior of a single	GARP packet is
			       usually sufficient.  However, a single GARP
			       might be	dropped	or lost	in some	circumstances.
			       This is particularly harmful when a shared ad-
			       dress is	passed between cluster nodes.  Neigh-
			       bors on the network link	might then work	with a
			       stale ARP cache and send	packets	destined for
			       that address to the node	that previously	owned
			       the address, which might	not respond.

     log_arp_movements	       Log movements of	IP addresses from one hardware
			       address to another.  See	DIAGNOSTICS below.
			       Turned on by default.

     log_arp_permanent_modify  Log attempts by a remote	host to	modify a per-
			       manent ARP entry.  See DIAGNOSTICS below.
			       Turned on by default.

     log_arp_wrong_iface       Log attempts to insert an ARP entry on an in-
			       terface when the	IP network to which the	ad-
			       dress belongs is	connected to another inter-
			       face.  See DIAGNOSTICS below.  Turned on	by de-

     max_log_per_second	       Limit the number	of remotely triggered logging
			       events to a configured value per	second.	 De-
			       fault is	1 log message per second.

     max_age		       How long	an ARP entry is	held in	the cache un-
			       til it needs to be refreshed.  Default is 1200

     maxhold		       How many	packets	to hold	in the per-entry out-
			       put queue while the entry is being resolved.
			       Default is one packet.

     maxtries		       Number of retransmits before a host is consid-
			       ered down and an	error is returned.  Default is
			       5 tries.

     proxyall		       Enables ARP proxying.  Turned off by default.

     wait		       Lifetime	of an incomplete ARP entry.  Default
			       is 20 seconds.

     arp: %x:%x:%x:%x:%x:%x is using my	IP address %d.%d.%d.%d on %s!  ARP has
     discovered	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.

     arp: %x:%x:%x:%x:%x:%x is multicast  Kernel refused to install an entry
     with multicast hardware address.  If you really want such addresses being
     installed,	set the	sysctl to a	posi-
     tive value.

     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.

BSD				October	7, 2016				   BSD


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