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ARP(4)                    NetBSD Programmer's 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 re-
     quests 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 re-
     quest; only the most recently ``transmitted'' packet is kept.  If the
     target host does not respond after several requests, the host is consid-
     ered to be down for a short period (normally 20 seconds), allowing an er-
     ror 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-creat-
     ed host routes.  The route to a directly-attached Ethernet network is in-
     stalled as a ``cloning'' route (one with the RTF_CLONING flag set), caus-
     ing 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 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 at-
     tempting to use the same Internet address.


SEE ALSO

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


     Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.

     Leffler, S.J. and Karels, M.J., "RFC893", Trailer Encapsulations.

NetBSD 1.6.2                    April 18, 1994                               1