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arp - Address Resolution Protocol

 

NAME

      arp - Address Resolution Protocol
 

SYNOPSIS

      pseudo-device ether
 

DESCRIPTION

      The Address Resolution Protocol (ARP) is a protocol used to dynamically
      map between Internet host addresses and 10Mb/s Ethernet addresses.  It is
      used by all the 10Mb/s Ethernet interface drivers.  It is not specific to
      Internet protocols or to 10Mb/s Ethernet, but this implementation cur‐
      rently supports only that combination.
 
      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 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 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).
 

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.
 
      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.
      inet(4), route(4), arp(8), ifconfig(8), route(8)
 
      Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.
 
      Leffler, S.J.  and Karels, M.J., "RFC893", Trailer Encapsulations.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.
 

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