network_cmds/arp.tproj/arp4.4

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  32 .\"     @(#)arp4.4      6.5 (Berkeley) 4/18/94
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  34 .Dd April 18, 1994
  35 .Dt ARP 4
  36 .Os BSD 4
  37 .Sh NAME
  38 .Nm arp
  39 .Nd Address Resolution Protocol
  40 .Sh SYNOPSIS
  41 .Em "pseudo-device ether"
  42 .Sh DESCRIPTION
  43 The Address Resolution Protocol (ARP) is a protocol used to dynamically
  44 map between Internet host addresses and 10Mb/s Ethernet addresses.
  45 It is used by all the 10Mb/s Ethernet interface drivers.
  46 It is not specific to Internet protocols or to 10Mb/s Ethernet,
  47 but this implementation currently supports only that combination.
  48 .Pp
  49 ARP caches Internet-Ethernet address mappings.
  50 When an interface requests a mapping for an address not in the cache,
  51 ARP queues the message which requires the mapping and broadcasts
  52 a message on the associated network requesting the address mapping.
  53 If a response is provided, the new mapping is cached and any pending
  54 message is transmitted.
  55 ARP will queue at most one packet while waiting for a response to a
  56 mapping request;
  57 only the most recently ``transmitted'' packet is kept.
  58 If the target host does not respond after several requests,
  59 the host is considered to be down for a short period (normally 20 seconds),
  60 allowing an error to be returned to transmission attempts during this
  61 interval.
  62 The error is
  63 .Li EHOSTDOWN
  64 for a non-responding destination host, and
  65 .Li EHOSTUNREACH
  66 for a non-responding router.
  67 .Pp
  68 The ARP cache is stored in the system routing table as
  69 dynamically-created host routes.
  70 The route to a directly-attached Ethernet network is installed as a
  71 .Dq cloning
  72 route (one with the
  73 .Li RTF_CLONING
  74 flag set),
  75 causing routes to individual hosts on that network to be created on
  76 demand.
  77 These routes time out periodically (normally 20 minutes after validated;
  78 entries are not validated when not in use).
  79 An entry for a host which is not responding is a
  80 .Dq reject
  81 route (one with the
  82 .Li RTF_REJECT
  83 flag set).
  84 .Pp
  85 ARP entries may be added, deleted or changed with the
  86 .Xr arp 8
  87 utility.
  88 Manually-added entries may be temporary or permanent,
  89 and may be
  90 .Dq published ,
  91 in which case the system will respond to ARP requests for that host
  92 as if it were the target of the request.
  93 .Pp
  94 In the past,
  95 ARP was used to negotiate the use of a trailer encapsulation.
  96 This is no longer supported.
  97 .Pp
  98 ARP watches passively for hosts impersonating the local host (i.e. a host
  99 which responds to an ARP mapping request for the local host's address).
 100 .Sh DIAGNOSTICS
 101 .Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
 102 ARP has discovered another host on the local network which responds to
 103 mapping requests for its own Internet address with a different Ethernet
 104 address, generally indicating that two hosts are attempting to use the
 105 same Internet address.
 106 .Sh SEE ALSO
 107 .Xr inet 4 ,
 108 .Xr route 4 ,
 109 .Xr arp 8 ,
 110 .Xr ifconfig 8 ,
 111 .Xr route 8
 112 .sp
 113 .Rs
 114 .%A Plummer, D.
 115 .%B "An Ethernet Address Resolution Protocol"
 116 .%T RFC826
 117 .Re
 118 .Rs
 119 .%A Leffler, S.J.
 120 .%A Karels, M.J.
 121 .%B "Trailer Encapsulations
 122 .%T RFC893
 123 .Re