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ARP(4)                     Kernel Interfaces Manual                     ARP(4)

NAME
     arp -- Address Resolution Protocol

SYNOPSIS
     #include <&lt;netinet/if_ether.h>&gt;

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

     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
     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 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-
     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).  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
     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).

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.

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


     Plummer, D., "RFC 826", An Ethernet Address Resolution Protocol.

     Leffler, S.J. and Karels, M.J., "RFC 893", Trailer Encapsulations.

NetBSD 6.1.5                    April 18, 1994                    NetBSD 6.1.5