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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. 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.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94 29.\" $FreeBSD$ 30.\" 31.Dd October 7, 2016 32.Dt ARP 4 33.Os 34.Sh NAME 35.Nm arp 36.Nd Address Resolution Protocol 37.Sh SYNOPSIS 38.Cd "device ether" 39.Sh DESCRIPTION 40The Address Resolution Protocol (ARP) is used to dynamically 41map between Protocol Addresses (such as IP addresses) and 42Local Network Addresses (such as Ethernet addresses). 43This implementation maps IP addresses to Ethernet addresses. 44It is used by all the Ethernet interface drivers. 45.Pp 46ARP caches Internet-Ethernet address mappings. 47When an interface requests a mapping for an address not in the cache, 48ARP queues the message which requires the mapping and broadcasts 49a message on the associated network requesting the address mapping. 50If a response is provided, the new mapping is cached and any pending 51message is transmitted. 52ARP will queue at most 53.Va net.link.ether.inet.maxhold 54packets while waiting for a response to a mapping request; 55only the most recently ``transmitted'' packets are kept. 56If the target host does not respond after several requests, 57the host is considered to be down allowing an error to be returned to 58transmission attempts. 59Further demand for this mapping causes ARP request retransmissions, that 60are ratelimited to one packet per second. 61The error is 62.Er EHOSTDOWN 63for a non-responding destination host, and 64.Er EHOSTUNREACH 65for a non-responding router. 66.Pp 67The ARP cache is stored in per-interface link-level table. 68.Pp 69ARP entries may be added, deleted or changed with the 70.Xr arp 8 71utility. 72Manually-added entries may be temporary or permanent, 73and may be 74.Dq published , 75in which case the system will respond to ARP requests for that host 76as if it were the target of the request. 77.Pp 78In the past, 79ARP was used to negotiate the use of a trailer encapsulation. 80This is no longer supported. 81.Pp 82ARP watches passively for hosts impersonating the local host (i.e., a host 83which responds to an ARP mapping request for the local host's address). 84.Pp 85Proxy ARP is a feature whereby the local host will respond to requests 86for addresses other than itself, with its own address. 87Normally, proxy ARP in 88.Fx 89is set up on a host-by-host basis using the 90.Xr arp 8 91utility, by adding an entry for each host inside a given subnet for 92which proxying of ARP requests is desired. 93However, the 94.Dq "proxy all" 95feature causes the local host to act as a proxy for 96.Em all 97hosts reachable through some other network interface, 98different from the one the request came in from. 99It may be enabled by setting the 100.Xr sysctl 8 101MIB variable 102.Va net.link.ether.inet.proxyall 103to 1. 104.Sh MIB Variables 105The ARP protocol implements a number of configurable variables in 106.Va net.link.ether.inet 107branch 108of the 109.Xr sysctl 3 110MIB. 111.Bl -tag -width "log_arp_permanent_modify" 112.It Va allow_multicast 113Install ARP entries with the multicast bit set in the hardware address. 114Installing such entries is an RFC 1812 violation, but some proprietary load 115balancing techniques require routers to do so. 116Turned off by default. 117.It Va garp_rexmit_count 118Retransmit gratuitous ARP (GARP) packets when an IPv4 address is added to an 119interface. 120A GARP is always transmitted when an IPv4 address is added to an interface. 121A non-zero value causes the GARP packet to be retransmitted the stated number 122of times. 123The interval between retransmissions is doubled each time, so the 124retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds). 125The default value of zero means only the initial GARP is sent; no 126additional GARP packets are retransmitted. 127The maximum value is sixteen. 128.Pp 129The default behavior of a single GARP packet is usually sufficient. 130However, a single GARP might be dropped or lost in some circumstances. 131This is particularly harmful when a shared address is passed between cluster 132nodes. 133Neighbors on the network link might then work with a stale ARP cache and send 134packets destined for that address to the node that previously owned the 135address, which might not respond. 136.It Va log_arp_movements 137Log movements of IP addresses from one hardware address to another. 138See 139.Sx DIAGNOSTICS 140below. 141Turned on by default. 142.It Va log_arp_permanent_modify 143Log attempts by a remote host to modify a permanent ARP entry. 144See 145.Sx DIAGNOSTICS 146below. 147Turned on by default. 148.It Va log_arp_wrong_iface 149Log attempts to insert an ARP entry on an interface when the IP network to 150which the address belongs is connected to another interface. 151See 152.Sx DIAGNOSTICS 153below. 154Turned on by default. 155.It Va max_log_per_second 156Limit the number of remotely triggered logging events to a configured value per 157second. 158Default is 1 log message per second. 159.It Va max_age 160How long an ARP entry is held in the cache until it needs to be refreshed. 161Default is 1200 seconds. 162.It Va maxhold 163How many packets to hold in the per-entry output queue while the entry 164is being resolved. 165Default is 16 packets. 166.It Va maxtries 167Number of retransmits before a host is considered down and an error is 168returned. 169Default is 5 tries. 170.It Va proxyall 171Enables ARP proxying. 172Turned off by default. 173.It Va wait 174Lifetime of an incomplete ARP entry. 175Default is 20 seconds. 176.El 177.Sh DIAGNOSTICS 178.Bl -diag 179.It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!" 180ARP has discovered another host on the local network which responds to 181mapping requests for its own Internet address with a different Ethernet 182address, generally indicating that two hosts are attempting to use the 183same Internet address. 184.It "arp: link address is broadcast for IP address %d.%d.%d.%d!" 185ARP requested information for a host, and received an answer indicating 186that the host's ethernet address is the ethernet broadcast address. 187This indicates a misconfigured or broken device. 188.It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s" 189ARP had a cached value for the ethernet address of the referenced host, 190but received a reply indicating that the host is at a new address. 191This can happen normally when host hardware addresses change, 192or when a mobile node arrives or leaves the local subnet. 193It can also indicate a problem with proxy ARP. 194This message can only be issued if the sysctl 195.Va net.link.ether.inet.log_arp_movements 196is set to 1, which is the system's default behaviour. 197.It "arpresolve: can't allocate llinfo for %d.%d.%d.%d" 198The route for the referenced host points to a device upon which ARP is 199required, but ARP was unable to allocate a routing table entry in which 200to store the host's MAC address. 201This usually points to a misconfigured routing table. 202It can also occur if the kernel cannot allocate memory. 203.It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" 204Physical connections exist to the same logical IP network on both if0 and if1. 205It can also occur if an entry already exists in the ARP cache for the IP 206address above, and the cable has been disconnected from if0, then reconnected 207to if1. 208This message can only be issued if the sysctl 209.Va net.link.ether.inet.log_arp_wrong_iface 210is set to 1, which is the system's default behaviour. 211.It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s" 212ARP has received an ARP reply that attempts to overwrite a permanent 213entry in the local ARP table. 214This error will only be logged if the sysctl 215.Va net.link.ether.inet.log_arp_permanent_modify 216is set to 1, which is the system's default behaviour. 217.It "arp: %x:%x:%x:%x:%x:%x is multicast" 218Kernel refused to install an entry with multicast hardware address. 219If you really want such addresses being installed, set the sysctl 220.Va net.link.ether.inet.allow_multicast 221to a positive value. 222.El 223.Sh SEE ALSO 224.Xr inet 4 , 225.Xr route 4 , 226.Xr arp 8 , 227.Xr ifconfig 8 , 228.Xr route 8 , 229.Xr sysctl 8 230.Rs 231.%A Plummer, D. 232.%B "An Ethernet Address Resolution Protocol" 233.%T RFC826 234.Re 235.Rs 236.%A Leffler, S.J. 237.%A Karels, M.J. 238.%B "Trailer Encapsulations" 239.%T RFC893 240.Re 241