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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 .\" SUCH DAMAGE. 27 .\" 28 .Dd October 7, 2016 29 .Dt ARP 4 30 .Os 31 .Sh NAME 32 .Nm arp 33 .Nd Address Resolution Protocol 34 .Sh SYNOPSIS 35 .Cd "device ether" 36 .Sh DESCRIPTION 37 The Address Resolution Protocol (ARP) is used to dynamically 38 map between Protocol Addresses (such as IP addresses) and 39 Local Network Addresses (such as Ethernet addresses). 40 This implementation maps IP addresses to Ethernet addresses. 41 It is used by all the Ethernet interface drivers. 42 .Pp 43 ARP caches Internet-Ethernet address mappings. 44 When an interface requests a mapping for an address not in the cache, 45 ARP queues the message which requires the mapping and broadcasts 46 a message on the associated network requesting the address mapping. 47 If a response is provided, the new mapping is cached and any pending 48 message is transmitted. 49 ARP will queue at most 50 .Va net.link.ether.inet.maxhold 51 packets while waiting for a response to a mapping request; 52 only the most recently ``transmitted'' packets are kept. 53 If the target host does not respond after several requests, 54 the host is considered to be down allowing an error to be returned to 55 transmission attempts. 56 Further demand for this mapping causes ARP request retransmissions, that 57 are ratelimited to one packet per second. 58 The error is 59 .Er EHOSTDOWN 60 for a non-responding destination host, and 61 .Er EHOSTUNREACH 62 for a non-responding router. 63 .Pp 64 The ARP cache is stored in per-interface link-level table. 65 .Pp 66 ARP entries may be added, deleted or changed with the 67 .Xr arp 8 68 utility. 69 Manually-added entries may be temporary or permanent, 70 and may be 71 .Dq published , 72 in which case the system will respond to ARP requests for that host 73 as if it were the target of the request. 74 .Pp 75 In the past, 76 ARP was used to negotiate the use of a trailer encapsulation. 77 This is no longer supported. 78 .Pp 79 ARP watches passively for hosts impersonating the local host (i.e., a host 80 which responds to an ARP mapping request for the local host's address). 81 .Pp 82 Proxy ARP is a feature whereby the local host will respond to requests 83 for addresses other than itself, with its own address. 84 Normally, proxy ARP in 85 .Fx 86 is set up on a host-by-host basis using the 87 .Xr arp 8 88 utility, by adding an entry for each host inside a given subnet for 89 which proxying of ARP requests is desired. 90 However, the 91 .Dq "proxy all" 92 feature causes the local host to act as a proxy for 93 .Em all 94 hosts reachable through some other network interface, 95 different from the one the request came in from. 96 It may be enabled by setting the 97 .Xr sysctl 8 98 MIB variable 99 .Va net.link.ether.inet.proxyall 100 to 1. 101 .Sh MIB Variables 102 The ARP protocol implements a number of configurable variables in 103 .Va net.link.ether.inet 104 branch 105 of the 106 .Xr sysctl 3 107 MIB. 108 .Bl -tag -width "log_arp_permanent_modify" 109 .It Va allow_multicast 110 Install ARP entries with the multicast bit set in the hardware address. 111 Installing such entries is an RFC 1812 violation, but some proprietary load 112 balancing techniques require routers to do so. 113 Turned off by default. 114 .It Va garp_rexmit_count 115 Retransmit gratuitous ARP (GARP) packets when an IPv4 address is added to an 116 interface. 117 A GARP is always transmitted when an IPv4 address is added to an interface. 118 A non-zero value causes the GARP packet to be retransmitted the stated number 119 of times. 120 The interval between retransmissions is doubled each time, so the 121 retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds). 122 The default value of zero means only the initial GARP is sent; no 123 additional GARP packets are retransmitted. 124 The maximum value is sixteen. 125 .Pp 126 The default behavior of a single GARP packet is usually sufficient. 127 However, a single GARP might be dropped or lost in some circumstances. 128 This is particularly harmful when a shared address is passed between cluster 129 nodes. 130 Neighbors on the network link might then work with a stale ARP cache and send 131 packets destined for that address to the node that previously owned the 132 address, which might not respond. 133 .It Va log_arp_movements 134 Log movements of IP addresses from one hardware address to another. 135 See 136 .Sx DIAGNOSTICS 137 below. 138 Turned on by default. 139 .It Va log_arp_permanent_modify 140 Log attempts by a remote host to modify a permanent ARP entry. 141 See 142 .Sx DIAGNOSTICS 143 below. 144 Turned on by default. 145 .It Va log_arp_wrong_iface 146 Log attempts to insert an ARP entry on an interface when the IP network to 147 which the address belongs is connected to another interface. 148 See 149 .Sx DIAGNOSTICS 150 below. 151 Turned on by default. 152 .It Va max_log_per_second 153 Limit the number of remotely triggered logging events to a configured value per 154 second. 155 Default is 1 log message per second. 156 .It Va max_age 157 How long an ARP entry is held in the cache until it needs to be refreshed. 158 Default is 1200 seconds. 159 .It Va maxhold 160 How many packets to hold in the per-entry output queue while the entry 161 is being resolved. 162 Default is 16 packets. 163 .It Va maxtries 164 Number of retransmits before a host is considered down and an error is 165 returned. 166 Default is 5 tries. 167 .It Va proxyall 168 Enables ARP proxying. 169 Turned off by default. 170 .It Va wait 171 Lifetime of an incomplete ARP entry. 172 Default is 20 seconds. 173 .El 174 .Sh DIAGNOSTICS 175 .Bl -diag 176 .It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!" 177 ARP has discovered another host on the local network which responds to 178 mapping requests for its own Internet address with a different Ethernet 179 address, generally indicating that two hosts are attempting to use the 180 same Internet address. 181 .It "arp: link address is broadcast for IP address %d.%d.%d.%d!" 182 ARP requested information for a host, and received an answer indicating 183 that the host's ethernet address is the ethernet broadcast address. 184 This indicates a misconfigured or broken device. 185 .It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s" 186 ARP had a cached value for the ethernet address of the referenced host, 187 but received a reply indicating that the host is at a new address. 188 This can happen normally when host hardware addresses change, 189 or when a mobile node arrives or leaves the local subnet. 190 It can also indicate a problem with proxy ARP. 191 This message can only be issued if the sysctl 192 .Va net.link.ether.inet.log_arp_movements 193 is set to 1, which is the system's default behaviour. 194 .It "arpresolve: can't allocate llinfo for %d.%d.%d.%d" 195 The route for the referenced host points to a device upon which ARP is 196 required, but ARP was unable to allocate a routing table entry in which 197 to store the host's MAC address. 198 This usually points to a misconfigured routing table. 199 It can also occur if the kernel cannot allocate memory. 200 .It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" 201 Physical connections exist to the same logical IP network on both if0 and if1. 202 It can also occur if an entry already exists in the ARP cache for the IP 203 address above, and the cable has been disconnected from if0, then reconnected 204 to if1. 205 This message can only be issued if the sysctl 206 .Va net.link.ether.inet.log_arp_wrong_iface 207 is set to 1, which is the system's default behaviour. 208 .It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s" 209 ARP has received an ARP reply that attempts to overwrite a permanent 210 entry in the local ARP table. 211 This error will only be logged if the sysctl 212 .Va net.link.ether.inet.log_arp_permanent_modify 213 is set to 1, which is the system's default behaviour. 214 .It "arp: %x:%x:%x:%x:%x:%x is multicast" 215 Kernel refused to install an entry with multicast hardware address. 216 If you really want such addresses being installed, set the sysctl 217 .Va net.link.ether.inet.allow_multicast 218 to a positive value. 219 .El 220 .Sh SEE ALSO 221 .Xr inet 4 , 222 .Xr route 4 , 223 .Xr arp 8 , 224 .Xr ifconfig 8 , 225 .Xr route 8 , 226 .Xr sysctl 8 227 .Rs 228 .%A Plummer, D. 229 .%B "An Ethernet Address Resolution Protocol" 230 .%T RFC826 231 .Re 232 .Rs 233 .%A Leffler, S.J. 234 .%A Karels, M.J. 235 .%B "Trailer Encapsulations" 236 .%T RFC893 237 .Re 238