Commit | Line | Data |
---|---|---|
ef449921 KB |
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 | .\" |
ef449921 | 6 | .\" @(#)arp4.4 6.4 (Berkeley) %G% |
54b3b5b7 | 7 | .\" |
54b3b5b7 | 8 | .TH ARP 4 "" |
d3eca8d4 KM |
9 | .UC 5 |
10 | .SH NAME | |
11 | arp \- Address Resolution Protocol | |
12 | .SH SYNOPSIS | |
2bfcba9f | 13 | .B "pseudo-device ether" |
d3eca8d4 | 14 | .SH DESCRIPTION |
ef449921 KB |
15 | The Address Resolution Protocol |
16 | (ARP) is a protocol used to dynamically map between Internet host addresses | |
2bfcba9f KM |
17 | and 10Mb/s Ethernet addresses. It is |
18 | used by all the 10Mb/s Ethernet interface drivers. | |
e17e7b88 MK |
19 | It is not specific to Internet protocols or to 10Mb/s Ethernet, |
20 | but this implementation currently supports only that combination. | |
d3eca8d4 KM |
21 | .PP |
22 | ARP caches Internet-Ethernet address mappings. When an interface | |
23 | requests a mapping for an address not in the cache, ARP queues the | |
24 | message which requires the mapping and broadcasts | |
25 | a message on the associated network requesting the address mapping. | |
26 | If a response is provided, the new mapping is cached and any pending | |
e17e7b88 | 27 | message is transmitted. |
2bfcba9f | 28 | ARP will queue |
ef449921 | 29 | at most one packet while waiting for a response to a mapping request; |
d3eca8d4 | 30 | only the most recently ``transmitted'' packet is kept. |
ef449921 KB |
31 | If the target host does not respond after several requests, |
32 | the host is considered to be down for a short period (normally 20 seconds), | |
33 | allowing an error to be returned to transmission attempts | |
34 | during this interval. | |
35 | The error is | |
36 | .Li EHOSTDOWN | |
37 | for a non-responding destination host, and | |
38 | .Li EHOSTUNREACH | |
39 | for a non-responding router. | |
d3eca8d4 | 40 | .PP |
ef449921 KB |
41 | The ARP cache is stored in the system routing table |
42 | as dynamically-created host routes. | |
43 | The route to a directly-attached Ethernet network is installed | |
44 | as a | |
45 | .\"XXX want macro for "quoted string" | |
46 | .quote cloning | |
47 | route | |
48 | (one with the | |
49 | .Li RTF_CLONING | |
50 | flag set), | |
51 | causing routes to individual hosts on that network to be created on demand. | |
52 | These routes time out periodically (normally 20 minutes after | |
53 | validated; entries are not validated when not in use). | |
54 | An entry for a host which is not responding is a | |
55 | .\"XXX want macro for "quoted string" | |
56 | .quote reject | |
57 | route | |
58 | (one with the | |
59 | .Li RTF_REJECT | |
60 | flag set). | |
61 | .PP | |
62 | ARP entries may be added, deleted or changed with | |
63 | .Xr arp 8 . | |
64 | Manually-added entries may be temporary or permanent, | |
65 | and may be | |
66 | .\" XXX | |
67 | .quote published , | |
68 | in which case the system will respond to ARP requests for that host | |
69 | as if it were the target of the request. | |
2bfcba9f | 70 | .PP |
ef449921 KB |
71 | In the past, ARP was used to negotiate the use of a trailer encapsulation. |
72 | This is no longer supported. | |
e17e7b88 | 73 | .PP |
d3eca8d4 | 74 | ARP watches passively for hosts impersonating the local host (i.e. a host |
2bfcba9f | 75 | which responds to an ARP mapping request for the local host's address). |
d3eca8d4 | 76 | .SH DIAGNOSTICS |
ef449921 | 77 | .B "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x." |
d3eca8d4 | 78 | ARP has discovered another host on the local network which responds to |
ef449921 KB |
79 | mapping requests for its own Internet address with a different |
80 | Ethernet address, generally indicating that two hosts are attempting | |
81 | to use the same Internet address. | |
d3eca8d4 | 82 | .SH SEE ALSO |
ef449921 | 83 | inet(4), route(4), arp(8), ifconfig(8), route(8) |
2bfcba9f | 84 | .br |
e17e7b88 | 85 | ``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer, |
ef449921 | 86 | Network Information Center. |
e17e7b88 MK |
87 | .br |
88 | ``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels, | |
ef449921 KB |
89 | Network Information Center. |
90 |