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

NAME
     arp -- Address Resolution Protocol

SYNOPSIS
     pseudo-device ether

DESCRIPTION
     The Address Resolution Protocol (ARP) is used to dynamically map between
     Internet host addresses and Ethernet addresses.  It is used by all of the
     Ethernet interface drivers.  It is not specific to Internet protocols or
     to Ethernet, but this implementation currently supports only that combi-
     nation.

     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 tar-
     get host does not respond after several requests, the host is considered
     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, static 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.  A static entry will
     not time out, but may be overwritten by network traffic, while a perma-
     nent entry will not time out and can not be overwritten.

     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
     address).

DIAGNOSTICS
     duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x
     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 info overwritten for %x!! by %x:%x:%x:%x:%x:%x on %x  An existing
     route has been overwritten with a new Ethernet address, for example when
     the other host has changed Ethernet cards.  If the route previously was
     static/non-expiring, the new route will expire normally.

     arp: attempt to overwrite permanent entry for %x!! by %x:%x:%x:%x:%x:%x
     on %x  As above, but the existing route had been manually set up as per-
     manent.  The routing information is not modified.

     arp: attempt to overwrite entry for %x!! on %x by %x:%x:%x:%x:%x:%x on
     %x  ARP has noticed an attempt to overwrite a host's routing entry on one
     interface with a routing entry for a different interface.  The routing
     information is not modified.

     arp: received reply to broadcast or multicast address  ARP received a
     response which is a broadcast or multicast address.  This might indicate
     an ARP spoofing attempt.

     arp: ether address is broadcast for IP address %s!  ARP requested infor-
     mation for a host, and received an answer indicating that the host's Eth-
     ernet address is the Ethernet broadcast address.  This indicates a mis-
     configured or broken device.

     arp: ether address is multicast for IP address %s!  ARP requested infor-
     mation for a host, and received an answer indicating that the host's Eth-
     ernet address is the Ethernet multicast address.  This indicates a mis-
     configured or broken device.

     arp: attempt to add entry for %s on %s by %s on %s  This usually indi-
     cates there is more than one interface connected to the same hub, or that
     the networks have somehow been short-circuited (e.g. IPs that should have
     been present on interface one are present on interface two).

     arplookup: unable to enter address for %s  An IP received on the inter-
     face does not match the network/netmask of the interface.  This indicates
     a netmask problem.

SEE ALSO
     inet(4), route(4), arp(8), ifconfig(8), route(8)

STANDARDS
     David C. Plummer, An Ethernet Address Resolution Protocol, RFC 826,
     November 1982.

     Samuel J. Leffler and Michael J. Karels, Trailer Encapsulations, RFC 893,
     April 1984.

BSD                             April 29, 2017                             BSD