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	32	.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94
	33	.\"
	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