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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 for a short period	(normally 20 seconds), allowing	an
     error to be returned to transmission attempts during this interval.  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).  An	entry for a
     host which	is not responding is a ``reject'' route	(one with the
     RTF_REJECT	flag set).

     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.  It	may be enabled
     by	setting	the sysctl(8) MIB variable	to 1.

     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: ether	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: 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.

     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	5.4			 June 16, 2004			   FreeBSD 5.4


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