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ARP(4)				   BSD Kernel Interfaces Manual 			   ARP(4)

NAME
     arp -- Address Resolution Protocol

SYNOPSIS
     device ether

DESCRIPTION
     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 Ethernet interface drivers.

     ARP caches Internet-Ethernet address mappings.  When an interface requests 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 pro-
     vided, 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 considered to be down allowing an error to be returned to transmission attempts.
     Further demand for this mapping causes ARP request retransmissions, 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-created 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 encapsulation.  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 address).

     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) utility, 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 inter-
     face, different from the one the request came in from.  It may be enabled by setting the
     sysctl(8) MIB variable net.link.ether.inet.proxyall to 1.

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

     allow_multicast	       Should the kernel install ARP entries with multicast bit set in
			       the hardware address.  Installing such entries is RFC 1812 viola-
			       tion, but some prorietary load balancing techniques require
			       routers on network to do so.  Turned off by default.

     log_arp_movements	       Should the kernel log movements of IP addresses from one hardware
			       address to an other.  See DIAGNOSTICS below.  Turned on by
			       default.

     log_arp_permanent_modify  Should the kernel log attempts of remote host on network to modify
			       a permanent ARP entry.  See DIAGNOSTICS below.  Turned on by
			       default.

     log_arp_wrong_iface       Should the kernel log attempts to insert an ARP entry on an inter-
			       face when the IP network the address belongs to is connected to an
			       other interface.  See DIAGNOSTICS below.  Turned on by default.

     max_age		       How long an ARP entry is held in the cache until it needs to be
			       refreshed.  Default is 1200 seconds.

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

     maxtries		       Number of retransmits before host is considered down and error is
			       returned.  Default is 5 tries.

     proxyall		       Enables ARP proxying for all hosts on net.  Turned off by default.

     useloopback	       If an ARP entry is added for local address, force the traffic to
			       go through the loopback interface.  Turned on by default.

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

DIAGNOSTICS
     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 Internet 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
     net.link.ether.inet.log_arp_movements 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 referenced 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 discon-
     nected from if0, then reconnected to if1.	This message can only be issued if the sysctl
     net.link.ether.inet.log_arp_wrong_iface is set to 1, which is the system's default behav-
     iour.

     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 permanent entry in the local ARP table.
     This error will only be logged if the sysctl net.link.ether.inet.log_arp_permanent_modify 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 hard-
     ware address.  If you really want such addresses being installed, set the sysctl
     net.link.ether.inet.allow_multicast to a positive value.

SEE ALSO
     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					September 3, 2012				      BSD
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