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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 Ether‐
      net interface drivers.
 
      ARP caches Internet-Ethernet address mappings.  When an interface
      requests a mapping for an address not in the cache, ARP queues the mes‐
      sage which requires the mapping and broadcasts a message on the associ‐
      ated 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 consid‐
      ered 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-cre‐
      ated host routes.  The route to a directly-attached Ethernet network is
      installed 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(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 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
      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(8) util‐
      ity, 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.  It may be enabled
sysctl(8) MIB variable net.link.ether.inet.proxyall to 1.
 

DIAGNOSTICS

      arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!: ARP has dis‐
      covered 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 Inter‐
      net 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.  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 ref‐
      erenced 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 sys‐
      tem’s default behaviour.
sysctl(8)
 
      Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.
 
      Leffler, S.J.  and Karels, M.J., "RFC893", Trailer Encapsulations.
 

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