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1 | .\" Copyright (c) 1985, 1986, 1988, 1994 |
2 | .\" The Regents of the University of California. All rights reserved. | |
d3eca8d4 | 3 | .\" |
ef449921 | 4 | .\" %sccs.include.redist.roff% |
d3eca8d4 | 5 | .\" |
7153f1cf | 6 | .\" @(#)arp4.4 6.5 (Berkeley) %G% |
54b3b5b7 | 7 | .\" |
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8 | .Dd |
9 | .Dt ARP 4 | |
10 | .Os BSD 4 | |
11 | .Sh NAME | |
12 | .Nm arp | |
13 | .Nd Address Resolution Protocol | |
14 | .Sh SYNOPSIS | |
15 | .Em "pseudo-device ether" | |
16 | .Sh DESCRIPTION | |
17 | The Address Resolution Protocol (ARP) is a protocol used to dynamically | |
18 | map between Internet host addresses and 10Mb/s Ethernet addresses. | |
19 | It is used by all the 10Mb/s Ethernet interface drivers. | |
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20 | It is not specific to Internet protocols or to 10Mb/s Ethernet, |
21 | but this implementation currently supports only that combination. | |
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22 | .Pp |
23 | ARP caches Internet-Ethernet address mappings. | |
24 | When an interface requests a mapping for an address not in the cache, | |
25 | ARP queues the message which requires the mapping and broadcasts | |
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26 | a message on the associated network requesting the address mapping. |
27 | If a response is provided, the new mapping is cached and any pending | |
e17e7b88 | 28 | message is transmitted. |
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29 | ARP will queue at most one packet while waiting for a response to a |
30 | mapping request; | |
d3eca8d4 | 31 | only the most recently ``transmitted'' packet is kept. |
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32 | If the target host does not respond after several requests, |
33 | the host is considered to be down for a short period (normally 20 seconds), | |
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34 | allowing an error to be returned to transmission attempts during this |
35 | interval. | |
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36 | The error is |
37 | .Li EHOSTDOWN | |
38 | for a non-responding destination host, and | |
39 | .Li EHOSTUNREACH | |
40 | for a non-responding router. | |
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41 | .Pp |
42 | The ARP cache is stored in the system routing table as | |
43 | dynamically-created host routes. | |
44 | The route to a directly-attached Ethernet network is installed as a | |
45 | .Dq cloning | |
46 | route (one with the | |
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47 | .Li RTF_CLONING |
48 | flag set), | |
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49 | causing routes to individual hosts on that network to be created on |
50 | demand. | |
51 | These routes time out periodically (normally 20 minutes after validated; | |
52 | entries are not validated when not in use). | |
ef449921 | 53 | An entry for a host which is not responding is a |
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54 | .Dq reject |
55 | route (one with the | |
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56 | .Li RTF_REJECT |
57 | flag set). | |
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58 | .Pp |
59 | ARP entries may be added, deleted or changed with the | |
60 | .Xr arp 8 | |
61 | utility. | |
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62 | Manually-added entries may be temporary or permanent, |
63 | and may be | |
7153f1cf | 64 | .Dq published , |
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65 | in which case the system will respond to ARP requests for that host |
66 | as if it were the target of the request. | |
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67 | .Pp |
68 | In the past, | |
69 | ARP was used to negotiate the use of a trailer encapsulation. | |
ef449921 | 70 | This is no longer supported. |
7153f1cf | 71 | .Pp |
d3eca8d4 | 72 | ARP watches passively for hosts impersonating the local host (i.e. a host |
2bfcba9f | 73 | which responds to an ARP mapping request for the local host's address). |
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74 | .Sh DIAGNOSTICS |
75 | .Em "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x." | |
d3eca8d4 | 76 | ARP has discovered another host on the local network which responds to |
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77 | mapping requests for its own Internet address with a different Ethernet |
78 | address, generally indicating that two hosts are attempting to use the | |
79 | same Internet address. | |
80 | .Sh SEE ALSO | |
81 | .Xr inet 4 , | |
82 | .Xr route 4 , | |
83 | .Xr arp 8 , | |
84 | .Xr ifconfig 8 , | |
85 | .Xr route 8 | |
86 | .sp | |
87 | .Rs | |
88 | .%A Plummer, D. | |
89 | .%B "An Ethernet Address Resolution Protocol" | |
90 | .%T RFC826 | |
91 | .Re | |
92 | .Rs | |
93 | .%A Leffler, S.J. | |
94 | .%A Karels, M.J. | |
95 | .%B "Trailer Encapsulations | |
96 | .%T RFC893 | |
97 | .Re | |
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