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ARP(4)                DragonFly Kernel Interfaces Manual                ARP(4)


NAME

     arp - Address Resolution Protocol


SYNOPSIS

     device ether


DESCRIPTION

     The Address Resolution Protocol (ARP) is used to dynamically map between
     Protocol Addresses (such as IP addresses) and Local Network Addresses
     (such as Ethernet addresses).  This implementation maps IP addresses to
     Ethernet, ARCnet, or Token Ring addresses.  It is used by all the
     Ethernet interface drivers.

     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

     arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!: 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: ether address is broadcast for IP address %d.%d.%d.%d!: ARP
     requested information for a host, and received an answer indicating that
     the host's ethernet address is the ethernet broadcast address.  This
     indicates a misconfigured or broken device.

     arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x: ARP
     had a cached value for the ethernet address of the referenced host, but
     received a reply indicating that the host is at a new address.  This can
     happen normally when host hardware addresses change, or when a mobile
     node arrives or leaves the local subnet.  It can also indicate a problem
     with proxy ARP.

     arpresolve: can't allocate llinfo for %d.%d.%d.%d: The route for the
     referenced host points to a device upon which ARP is required, but ARP
     was unable to allocate a routing table entry in which to store the host's
     MAC address.  This usually points to a misconfigured routing table.  It
     can also occur if the kernel cannot allocate memory.


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.

DragonFly 5.9-DEVELOPMENT       April 18, 1994       DragonFly 5.9-DEVELOPMENT

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