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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 configrable 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.
152 How long an ARP entry is held in the cache until it needs to be refreshed.
153 Default is 1200 seconds.
155 How many packets hold in the per-entry output queue while the entry
157 Default is one packet.
159 Number of retransmits before host is considered down and error is returned.
162 Enables ARP proxying for all hosts on net.
163 Turned off by default.
165 If an ARP entry is added for local address, force the traffic to go through
166 the loopback interface.
167 Turned on by default.
169 Lifetime of an incomplete ARP entry.
170 Default is 20 seconds.
174 .It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!"
175 ARP has discovered another host on the local network which responds to
176 mapping requests for its own Internet address with a different Ethernet
177 address, generally indicating that two hosts are attempting to use the
178 same Internet address.
179 .It "arp: link address is broadcast for IP address %d.%d.%d.%d!"
180 ARP requested information for a host, and received an answer indicating
181 that the host's ethernet address is the ethernet broadcast address.
182 This indicates a misconfigured or broken device.
183 .It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s"
184 ARP had a cached value for the ethernet address of the referenced host,
185 but received a reply indicating that the host is at a new address.
186 This can happen normally when host hardware addresses change,
187 or when a mobile node arrives or leaves the local subnet.
188 It can also indicate a problem with proxy ARP.
189 This message can only be issued if the sysctl
190 .Va net.link.ether.inet.log_arp_movements
191 is set to 1, which is the system's default behaviour.
192 .It "arpresolve: can't allocate llinfo for %d.%d.%d.%d"
193 The route for the referenced host points to a device upon which ARP is
194 required, but ARP was unable to allocate a routing table entry in which
195 to store the host's MAC address.
196 This usually points to a misconfigured routing table.
197 It can also occur if the kernel cannot allocate memory.
198 .It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1"
199 Physical connections exist to the same logical IP network on both if0 and if1.
200 It can also occur if an entry already exists in the ARP cache for the IP
201 address above, and the cable has been disconnected from if0, then reconnected
203 This message can only be issued if the sysctl
204 .Va net.link.ether.inet.log_arp_wrong_iface
205 is set to 1, which is the system's default behaviour.
206 .It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s"
207 ARP has received an ARP reply that attempts to overwrite a permanent
208 entry in the local ARP table.
209 This error will only be logged if the sysctl
210 .Va net.link.ether.inet.log_arp_permanent_modify
211 is set to 1, which is the system's default behaviour.
212 .It "arp: %x:%x:%x:%x:%x:%x is multicast"
213 Kernel refused to install an entry with multicast hardware address.
214 If you really want such addresses being installed, set the sysctl
215 .Va net.link.ether.inet.allow_multicast
227 .%B "An Ethernet Address Resolution Protocol"
233 .%B "Trailer Encapsulations