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28 .\" @(#)arp4.4 6.5 (Berkeley) 4/18/94
36 .Nd Address Resolution Protocol
40 The Address Resolution Protocol (ARP) is used to dynamically
41 map between Protocol Addresses (such as IP addresses) and
42 Local Network Addresses (such as Ethernet addresses).
43 This implementation maps IP addresses to Ethernet,
45 or Token Ring addresses.
46 It is used by all the Ethernet interface drivers.
48 ARP caches Internet-Ethernet address mappings.
49 When an interface requests a mapping for an address not in the cache,
50 ARP queues the message which requires the mapping and broadcasts
51 a message on the associated network requesting the address mapping.
52 If a response is provided, the new mapping is cached and any pending
53 message is transmitted.
54 ARP will queue at most one packet while waiting for a response to a
56 only the most recently ``transmitted'' packet is kept.
57 If the target host does not respond after several requests,
58 the host is considered to be down allowing an error to be returned to
59 transmission attempts.
60 Further demand for this mapping causes ARP request retransmissions, that
61 are ratelimited to one packet per second.
64 for a non-responding destination host, and
66 for a non-responding router.
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
75 causing routes to individual hosts on that network to be created on
77 These routes time out periodically (normally 20 minutes after validated;
78 entries are not validated when not in use).
80 ARP entries may be added, deleted or changed with the
83 Manually-added entries may be temporary or permanent,
86 in which case the system will respond to ARP requests for that host
87 as if it were the target of the request.
90 ARP was used to negotiate the use of a trailer encapsulation.
91 This is no longer supported.
93 ARP watches passively for hosts impersonating the local host (i.e., a host
94 which responds to an ARP mapping request for the local host's address).
96 Proxy ARP is a feature whereby the local host will respond to requests
97 for addresses other than itself, with its own address.
98 Normally, proxy ARP in
100 is set up on a host-by-host basis using the
102 utility, by adding an entry for each host inside a given subnet for
103 which proxying of ARP requests is desired.
106 feature causes the local host to act as a proxy for
108 hosts reachable through some other network interface,
109 different from the one the request came in from.
110 It may be enabled by setting the
113 .Va net.link.ether.inet.proxyall
116 The ARP protocol implements a number of configurable variables in
117 .Va net.link.ether.inet
122 .Bl -tag -width "log_arp_permanent_modify"
123 .It Va allow_multicast
124 Should the kernel install ARP entries with multicast bit set in
125 the hardware address.
126 Installing such entries is RFC 1812 violation, but some prorietary
127 load balancing techniques require routers on network to do so.
128 Turned off by default.
129 .It Va log_arp_movements
130 Should the kernel log movements of IP addresses from one hardware
135 Turned on by default.
136 .It Va log_arp_permanent_modify
137 Should the kernel log attempts of remote host on network to modify a
142 Turned on by default.
143 .It Va log_arp_wrong_iface
144 Should the kernel log attempts to insert an ARP entry on an interface
145 when the IP network the address belongs to is connected to an other
150 Turned on by default.
151 .It Va max_log_per_second
152 Limit number of remotely triggered logging events to a configured value
154 Default is 1 log message per second.
156 How long an ARP entry is held in the cache until it needs to be refreshed.
157 Default is 1200 seconds.
159 How many packets hold in the per-entry output queue while the entry
161 Default is one packet.
163 Number of retransmits before host is considered down and error is returned.
166 Enables ARP proxying for all hosts on net.
167 Turned off by default.
169 If an ARP entry is added for local address, force the traffic to go through
170 the loopback interface.
171 Turned on by default.
173 Lifetime of an incomplete ARP entry.
174 Default is 20 seconds.
178 .It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!"
179 ARP has discovered another host on the local network which responds to
180 mapping requests for its own Internet address with a different Ethernet
181 address, generally indicating that two hosts are attempting to use the
182 same Internet address.
183 .It "arp: link address is broadcast for IP address %d.%d.%d.%d!"
184 ARP requested information for a host, and received an answer indicating
185 that the host's ethernet address is the ethernet broadcast address.
186 This indicates a misconfigured or broken device.
187 .It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s"
188 ARP had a cached value for the ethernet address of the referenced host,
189 but received a reply indicating that the host is at a new address.
190 This can happen normally when host hardware addresses change,
191 or when a mobile node arrives or leaves the local subnet.
192 It can also indicate a problem with proxy ARP.
193 This message can only be issued if the sysctl
194 .Va net.link.ether.inet.log_arp_movements
195 is set to 1, which is the system's default behaviour.
196 .It "arpresolve: can't allocate llinfo for %d.%d.%d.%d"
197 The route for the referenced host points to a device upon which ARP is
198 required, but ARP was unable to allocate a routing table entry in which
199 to store the host's MAC address.
200 This usually points to a misconfigured routing table.
201 It can also occur if the kernel cannot allocate memory.
202 .It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1"
203 Physical connections exist to the same logical IP network on both if0 and if1.
204 It can also occur if an entry already exists in the ARP cache for the IP
205 address above, and the cable has been disconnected from if0, then reconnected
207 This message can only be issued if the sysctl
208 .Va net.link.ether.inet.log_arp_wrong_iface
209 is set to 1, which is the system's default behaviour.
210 .It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s"
211 ARP has received an ARP reply that attempts to overwrite a permanent
212 entry in the local ARP table.
213 This error will only be logged if the sysctl
214 .Va net.link.ether.inet.log_arp_permanent_modify
215 is set to 1, which is the system's default behaviour.
216 .It "arp: %x:%x:%x:%x:%x:%x is multicast"
217 Kernel refused to install an entry with multicast hardware address.
218 If you really want such addresses being installed, set the sysctl
219 .Va net.link.ether.inet.allow_multicast
231 .%B "An Ethernet Address Resolution Protocol"
237 .%B "Trailer Encapsulations"