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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 address).

     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 configurable variables in branch
     of the sysctl(3) MIB.

     allow_multicast           Install ARP entries with the multicast bit set in the hardware
                               address.  Installing such entries is an RFC 1812 violation, but
                               some proprietary load balancing techniques require routers to do
                               so.  Turned off by default.

     garp_rexmit_count         Retransmit gratuitous ARP (GARP) packets when an IPv4 address is
                               added to an interface.  A GARP is always transmitted when an IPv4
                               address is added to an interface.  A non-zero value causes the
                               GARP packet to be retransmitted the stated number of times.  The
                               interval between retransmissions is doubled each time, so the
                               retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds).
                               The default value of zero means only the initial GARP is sent; no
                               additional GARP packets are retransmitted.  The maximum value is

                               The default behavior of a single GARP packet is usually
                               sufficient.  However, a single GARP might be dropped or lost in
                               some circumstances.  This is particularly harmful when a shared
                               address is passed between cluster nodes.  Neighbors on the network
                               link might then work with a stale ARP cache and send packets
                               destined for that address to the node that previously owned the
                               address, which might not respond.

     log_arp_movements         Log movements of IP addresses from one hardware address to
                               another.  See DIAGNOSTICS below.  Turned on by default.

     log_arp_permanent_modify  Log attempts by a remote host to modify a permanent ARP entry.
                               See DIAGNOSTICS below.  Turned on by default.

     log_arp_wrong_iface       Log attempts to insert an ARP entry on an interface when the IP
                               network to which the address belongs is connected to another
                               interface.  See DIAGNOSTICS below.  Turned on by default.

     max_log_per_second        Limit the number of remotely triggered logging events to a
                               configured value per second.  Default is 1 log message per second.

     max_age                   How long an ARP entry is held in the cache until it needs to be
                               refreshed.  Default is 1200 seconds.

     maxhold                   How many packets to hold in the per-entry output queue while the
                               entry is being resolved.  Default is one packet.

     maxtries                  Number of retransmits before a host is considered down and an
                               error is returned.  Default is 5 tries.

     proxyall                  Enables ARP proxying.  Turned off by default.

     wait                      Lifetime of an incomplete ARP entry.  Default is 20 seconds.


     arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!  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

     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.

     arp: %x:%x:%x:%x:%x:%x is multicast  Kernel refused to install an entry with multicast
     hardware address.  If you really want such addresses being installed, set the sysctl to a positive value.


     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.