arp

ARP(4)                      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 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
     net.link.ether.inet.proxyall to 1.

MIB Variables
     The ARP protocol implements a number of configurable variables in
     net.link.ether.inet 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 sixteen.

                               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.

DIAGNOSTICS
     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
     net.link.ether.inet.log_arp_movements 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
     net.link.ether.inet.log_arp_wrong_iface 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 net.link.ether.inet.log_arp_permanent_modify 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 net.link.ether.inet.allow_multicast to a
     positive value.

SEE ALSO
     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.

                                October 7, 2016