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     arp — Address Resolution Protocol


     device ether


     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 allowing an error to be
     returned to transmission attempts.  Further demand for this mapping
     causes ARP request retransmissions, that are ratelimited to one packet
     per second.  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).

     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

     Proxy ARP is a feature whereby the local host will respond to requests
     for addresses other than itself, with its own address.  Normally, proxy
     ARP in FreeBSD is set up on a host-by-host basis using the arp(8)
     utility, by adding an entry for each host inside a given subnet for which
     proxying of ARP requests is desired.  However, the “proxy all” feature
     causes the local host to act as a proxy for all hosts reachable through
     some other network interface, different from the one the request came in
     from.  It may be enabled by setting the sysctl(8) MIB variable to 1.

MIB Variables

     The ARP protocol implements a number of configrable variables in branch of the sysctl(3) MIB.

     max_age       How long an ARP entry is held in the cache until it needs
                   to be refreshed.

     maxtries      Number of retransmits before host is considered down and
                   error is returned.

     useloopback   If an ARP entry is added for local address, force the
                   traffic to go through the loopback interface.

     proxyall      Enables ARP proxying for all hosts on net.


     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: link 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 on %s
     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.  This message can only be issued if the sysctl is set to 1, which is the system's
     default behaviour.

     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.

     arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1
     Physical connections exist to the same logical IP network on both if0 and
     if1.  It can also occur if an entry already exists in the ARP cache for
     the IP address above, and the cable has been disconnected from if0, then
     reconnected to if1.  This message can only be issued if the sysctl is set to 1, which is the
     system's default behaviour.

     arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d
     on %s  ARP has received an ARP reply that attempts to overwrite a
     permanent entry in the local ARP table.  This error will only be logged
     if the sysctl is set to 1,
     which is the system's default behaviour.


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

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

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