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ARP(4)                       Device Drivers Manual                      ARP(4)

NAME
     arp - Address Resolution Protocol

SYNOPSIS
     #include <netinet/if_ether.h>

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.

     Each ARP cache entry is stored in a network interface which a response of
     ARP comes in.  ARP cache entires 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.

DIAGNOSTICS
     ARP implements Address Conflict Detection.  When an address is first
     added to the host, it is marked tentative and ARP probes the network to
     discover if another host has the address.  If another host replies with
     the same address, then the local address is marked duplicate and the host
     will not use it.  Otherwise the tentative mark is removed and the host
     can start using the address.

     ARP will defend the host's active address when a conflicting message is
     received.  However, if another conflicting message for the address is
     found within a 10 second period, then the address is marked duplicate and
     the host will stop using it.

     For some systems such as a router or a server, it is desirable never to
     give up an assigned address.  This can be achieved by setting the
     sysctl(7) variable net.inet.ip_dad_count to 0.

     In all of the above cases, ARP will log diagnostic messages which include
     the hardware address of the conflicting host.

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.

     Cheshire, S., "RFC 5227", IPv4 Address Conflict Detection.

HISTORY
     Since NetBSD 8.0, the ARP cache was not stored in the routing table.

     Address Conflict Detection was added in NetBSD 8.0.

NetBSD 8.0                     October 12, 2016                     NetBSD 8.0