ARP(4)							   BSD Kernel Interfaces 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 sup-
     ports 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 con-
     sidered 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.

BSD
								  April 18, 1994							       BSD