ARP(4)                    OpenBSD Programmer's 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 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 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 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, 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 ad-
     dress).

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 setup as perma-
     nent.  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.

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


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

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


OpenBSD 3.0                     April 18, 1994                               2
