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32 .\" @(#)arp4.4 6.5 (Berkeley) 4/18/94
40 .Nd Address Resolution Protocol
42 .Cd "pseudo-device ether"
44 The Address Resolution Protocol (ARP) is a protocol used to dynamically
45 map between Internet host addresses and 10Mb/s Ethernet addresses.
46 It is used by all the 10Mb/s Ethernet interface drivers.
47 It is not specific to Internet protocols or to 10Mb/s Ethernet,
48 but this implementation currently supports only that combination.
50 ARP caches Internet-Ethernet address mappings.
51 When an interface requests a mapping for an address not in the cache,
52 ARP queues the message which requires the mapping and broadcasts
53 a message on the associated network requesting the address mapping.
54 If a response is provided, the new mapping is cached and any pending
55 message is transmitted.
56 ARP will queue at most one packet while waiting for a response to a
58 only the most recently ``transmitted'' packet is kept.
59 If the target host does not respond after several requests,
60 the host is considered to be down for a short period (normally 20 seconds),
61 allowing an error to be returned to transmission attempts during this
65 for a non-responding destination host, and
67 for a non-responding router.
69 The ARP cache is stored in the system routing table as
70 dynamically-created host routes.
71 The route to a directly-attached Ethernet network is installed as a
76 causing routes to individual hosts on that network to be created on
78 These routes time out periodically (normally 20 minutes after validated;
79 entries are not validated when not in use).
80 An entry for a host which is not responding is a
86 ARP entries may be added, deleted or changed with the
89 Manually-added entries may be temporary or permanent,
92 in which case the system will respond to ARP requests for that host
93 as if it were the target of the request.
96 ARP was used to negotiate the use of a trailer encapsulation.
97 This is no longer supported.
99 ARP watches passively for hosts impersonating the local host (i.e. a host
100 which responds to an ARP mapping request for the local host's address).
102 .Em "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" :
103 ARP has discovered another host on the local network which responds to
104 mapping requests for its own Internet address with a different Ethernet
105 address, generally indicating that two hosts are attempting to use the
106 same Internet address.
108 .Em "arp: ether address is broadcast for IP address %d.%d.%d.%d!" :
109 ARP requested information for a host, and received an answer indicating
110 that the host's ethernet address is the ethernet broadcast address.
111 This indicates a misconfigured or broken device.
113 .Em "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x" :
114 ARP had a cached value for the ethernet address of the referenced host,
115 but received a reply indicating that the host is at a new address. This
116 can happen normally when host hardware addresses change, or when a mobile
117 node arrives or leaves the local subnet. It can also indicate a problem
120 .Em "arpresolve: can't allocate llinfo for %d.%d.%d.%d" :
121 The route for the referenced host points to a device upon which ARP is
122 required, but ARP was unable to allocate a routing table entry in which
123 to store the host's MAC address. This usually points to a misconfigured
124 routing table. It can also occur if the kernel cannot allocate memory.
133 .%B "An Ethernet Address Resolution Protocol"
139 .%B "Trailer Encapsulations