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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, 44ARCnet, 45or Token Ring addresses. 46It is used by all the Ethernet interface drivers. 47.Pp 48ARP caches Internet-Ethernet address mappings. 49When an interface requests a mapping for an address not in the cache, 50ARP queues the message which requires the mapping and broadcasts 51a message on the associated network requesting the address mapping. 52If a response is provided, the new mapping is cached and any pending 53message is transmitted. 54ARP will queue at most one packet while waiting for a response to a 55mapping request; 56only the most recently ``transmitted'' packet is kept. 57If the target host does not respond after several requests, 58the host is considered to be down allowing an error to be returned to 59transmission attempts. 60Further demand for this mapping causes ARP request retransmissions, that 61are ratelimited to one packet per second. 62The error is 63.Er EHOSTDOWN 64for a non-responding destination host, and 65.Er EHOSTUNREACH 66for a non-responding router. 67.Pp 68The ARP cache is stored in the system routing table as 69dynamically-created host routes. 70The route to a directly-attached Ethernet network is installed as a 71.Dq cloning 72route (one with the 73.Li RTF_CLONING 74flag set), 75causing routes to individual hosts on that network to be created on 76demand. 77These routes time out periodically (normally 20 minutes after validated; 78entries are not validated when not in use). 79.Pp 80ARP entries may be added, deleted or changed with the 81.Xr arp 8 82utility. 83Manually-added entries may be temporary or permanent, 84and may be 85.Dq published , 86in which case the system will respond to ARP requests for that host 87as if it were the target of the request. 88.Pp 89In the past, 90ARP was used to negotiate the use of a trailer encapsulation. 91This is no longer supported. 92.Pp 93ARP watches passively for hosts impersonating the local host (i.e., a host 94which responds to an ARP mapping request for the local host's address). 95.Pp 96Proxy ARP is a feature whereby the local host will respond to requests 97for addresses other than itself, with its own address. 98Normally, proxy ARP in 99.Fx 100is set up on a host-by-host basis using the 101.Xr arp 8 102utility, by adding an entry for each host inside a given subnet for 103which proxying of ARP requests is desired. 104However, the 105.Dq "proxy all" 106feature causes the local host to act as a proxy for 107.Em all 108hosts reachable through some other network interface, 109different from the one the request came in from. 110It may be enabled by setting the 111.Xr sysctl 8 112MIB variable 113.Va net.link.ether.inet.proxyall 114to 1. 115.Sh MIB Variables 116The ARP protocol implements a number of configurable variables in 117.Va net.link.ether.inet 118branch 119of the 120.Xr sysctl 3 121MIB. 122.Bl -tag -width "log_arp_permanent_modify" 123.It Va allow_multicast 124Install ARP entries with the multicast bit set in the hardware address. 125Installing such entries is an RFC 1812 violation, but some proprietary load 126balancing techniques require routers to do so. 127Turned off by default. 128.It Va garp_rexmit_count 129Retransmit gratuitous ARP (GARP) packets when an IPv4 address is added to an 130interface. 131A GARP is always transmitted when an IPv4 address is added to an interface. 132A non-zero value causes the GARP packet to be retransmitted the stated number 133of times. 134The interval between retransmissions is doubled each time, so the 135retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds). 136The default value of zero means only the initial GARP is sent; no 137additional GARP packets are retransmitted. 138The maximum value is sixteen. 139.Pp 140The default behavior of a single GARP packet is usually sufficient. 141However, a single GARP might be dropped or lost in some circumstances. 142This is particularly harmful when a shared address is passed between cluster 143nodes. 144Neighbors on the network link might then work with a stale ARP cache and send 145packets destined for that address to the node that previously owned the 146address, which might not respond. 147.It Va log_arp_movements 148Log movements of IP addresses from one hardware address to another. 149See 150.Sx DIAGNOSTICS 151below. 152Turned on by default. 153.It Va log_arp_permanent_modify 154Log attempts by a remote host to modify a permanent ARP entry. 155See 156.Sx DIAGNOSTICS 157below. 158Turned on by default. 159.It Va log_arp_wrong_iface 160Log attempts to insert an ARP entry on an interface when the IP network to 161which the address belongs is connected to another interface. 162See 163.Sx DIAGNOSTICS 164below. 165Turned on by default. 166.It Va max_log_per_second 167Limit the number of remotely triggered logging events to a configured value per 168second. 169Default is 1 log message per second. 170.It Va max_age 171How long an ARP entry is held in the cache until it needs to be refreshed. 172Default is 1200 seconds. 173.It Va maxhold 174How many packets to hold in the per-entry output queue while the entry 175is being resolved. 176Default is one packet. 177.It Va maxtries 178Number of retransmits before a host is considered down and an error is 179returned. 180Default is 5 tries. 181.It Va proxyall 182Enables ARP proxying. 183Turned off by default. 184.It Va wait 185Lifetime of an incomplete ARP entry. 186Default is 20 seconds. 187.El 188.Sh DIAGNOSTICS 189.Bl -diag 190.It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!" 191ARP has discovered another host on the local network which responds to 192mapping requests for its own Internet address with a different Ethernet 193address, generally indicating that two hosts are attempting to use the 194same Internet address. 195.It "arp: link address is broadcast for IP address %d.%d.%d.%d!" 196ARP requested information for a host, and received an answer indicating 197that the host's ethernet address is the ethernet broadcast address. 198This indicates a misconfigured or broken device. 199.It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s" 200ARP had a cached value for the ethernet address of the referenced host, 201but received a reply indicating that the host is at a new address. 202This can happen normally when host hardware addresses change, 203or when a mobile node arrives or leaves the local subnet. 204It can also indicate a problem with proxy ARP. 205This message can only be issued if the sysctl 206.Va net.link.ether.inet.log_arp_movements 207is set to 1, which is the system's default behaviour. 208.It "arpresolve: can't allocate llinfo for %d.%d.%d.%d" 209The route for the referenced host points to a device upon which ARP is 210required, but ARP was unable to allocate a routing table entry in which 211to store the host's MAC address. 212This usually points to a misconfigured routing table. 213It can also occur if the kernel cannot allocate memory. 214.It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" 215Physical connections exist to the same logical IP network on both if0 and if1. 216It can also occur if an entry already exists in the ARP cache for the IP 217address above, and the cable has been disconnected from if0, then reconnected 218to if1. 219This message can only be issued if the sysctl 220.Va net.link.ether.inet.log_arp_wrong_iface 221is set to 1, which is the system's default behaviour. 222.It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s" 223ARP has received an ARP reply that attempts to overwrite a permanent 224entry in the local ARP table. 225This error will only be logged if the sysctl 226.Va net.link.ether.inet.log_arp_permanent_modify 227is set to 1, which is the system's default behaviour. 228.It "arp: %x:%x:%x:%x:%x:%x is multicast" 229Kernel refused to install an entry with multicast hardware address. 230If you really want such addresses being installed, set the sysctl 231.Va net.link.ether.inet.allow_multicast 232to a positive value. 233.El 234.Sh SEE ALSO 235.Xr inet 4 , 236.Xr route 4 , 237.Xr arp 8 , 238.Xr ifconfig 8 , 239.Xr route 8 , 240.Xr sysctl 8 241.Rs 242.%A Plummer, D. 243.%B "An Ethernet Address Resolution Protocol" 244.%T RFC826 245.Re 246.Rs 247.%A Leffler, S.J. 248.%A Karels, M.J. 249.%B "Trailer Encapsulations" 250.%T RFC893 251.Re 252