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