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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94 33.\" $FreeBSD$ 34.\" 35.Dd June 16, 2004 36.Dt ARP 4 37.Os 38.Sh NAME 39.Nm arp 40.Nd Address Resolution Protocol 41.Sh SYNOPSIS 42.Cd "device ether" 43.Sh DESCRIPTION 44The Address Resolution Protocol (ARP) is used to dynamically 45map between Protocol Addresses (such as IP addresses) and 46Local Network Addresses (such as Ethernet addresses). 47This implementation maps IP addresses to Ethernet, 48ARCnet, 49or Token Ring addresses. 50It is used by all the Ethernet interface drivers. 51.Pp 52ARP caches Internet-Ethernet address mappings. 53When an interface requests a mapping for an address not in the cache, 54ARP queues the message which requires the mapping and broadcasts 55a message on the associated network requesting the address mapping. 56If a response is provided, the new mapping is cached and any pending 57message is transmitted. 58ARP will queue at most one packet while waiting for a response to a 59mapping request; 60only the most recently ``transmitted'' packet is kept. 61If the target host does not respond after several requests, 62the host is considered to be down for a short period (normally 20 seconds), 63allowing an error to be returned to transmission attempts during this 64interval. 65The error is 66.Er EHOSTDOWN 67for a non-responding destination host, and 68.Er EHOSTUNREACH 69for a non-responding router. 70.Pp 71The ARP cache is stored in the system routing table as 72dynamically-created host routes. 73The route to a directly-attached Ethernet network is installed as a 74.Dq cloning 75route (one with the 76.Li RTF_CLONING 77flag set), 78causing routes to individual hosts on that network to be created on 79demand. 80These routes time out periodically (normally 20 minutes after validated; 81entries are not validated when not in use). 82An entry for a host which is not responding is a 83.Dq reject 84route (one with the 85.Li RTF_REJECT 86flag set). 87.Pp 88ARP entries may be added, deleted or changed with the 89.Xr arp 8 90utility. 91Manually-added entries may be temporary or permanent, 92and may be 93.Dq published , 94in which case the system will respond to ARP requests for that host 95as if it were the target of the request. 96.Pp 97In the past, 98ARP was used to negotiate the use of a trailer encapsulation. 99This is no longer supported. 100.Pp 101ARP watches passively for hosts impersonating the local host (i.e. a host 102which responds to an ARP mapping request for the local host's address). 103.Pp 104Proxy ARP is a feature whereby the local host will respond to requests 105for addresses other than itself, with its own address. 106Normally, proxy ARP in 107.Fx 108is set up on a host-by-host basis using the 109.Xr arp 8 110utility, by adding an entry for each host inside a given subnet for 111which proxying of ARP requests is desired. 112However, the 113.Dq "proxy all" 114feature causes the local host to act as a proxy for 115.Em all 116hosts. 117It may be enabled by setting the 118.Xr sysctl 8 119MIB variable 120.Va net.link.ether.inet.proxyall 121to 1. 122.Sh DIAGNOSTICS 123.Em "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!" : 124ARP has discovered another host on the local network which responds to 125mapping requests for its own Internet address with a different Ethernet 126address, generally indicating that two hosts are attempting to use the 127same Internet address. 128.Pp 129.Em "arp: ether address is broadcast for IP address %d.%d.%d.%d!" : 130ARP requested information for a host, and received an answer indicating 131that the host's ethernet address is the ethernet broadcast address. 132This indicates a misconfigured or broken device. 133.Pp 134.Em "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x" : 135ARP had a cached value for the ethernet address of the referenced host, 136but received a reply indicating that the host is at a new address. 137This can happen normally when host hardware addresses change, 138or when a mobile node arrives or leaves the local subnet. 139It can also indicate a problem with proxy ARP. 140This message can only be issued if the sysctl 141.Va net.link.ether.inet.log_arp_movements 142is set to 1, which is the system's default behaviour. 143.Pp 144.Em "arpresolve: can't allocate llinfo for %d.%d.%d.%d" : 145The route for the referenced host points to a device upon which ARP is 146required, but ARP was unable to allocate a routing table entry in which 147to store the host's MAC address. 148This usually points to a misconfigured routing table. 149It can also occur if the kernel cannot allocate memory. 150.Pp 151.Em "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" : 152Physical connections exist to the same logical IP network on both if0 and if1. 153It can also occur if an entry already exists in the ARP cache for the IP 154address above, and the cable has been disconnected from if0, then reconnected 155to if1. 156This message can only be issued if the sysctl 157.Va net.link.ether.inet.log_arp_wrong_iface 158is set to 1, which is the system's default behaviour. 159.Sh SEE ALSO 160.Xr inet 4 , 161.Xr route 4 , 162.Xr arp 8 , 163.Xr ifconfig 8 , 164.Xr route 8 , 165.Xr sysctl 8 166.Rs 167.%A Plummer, D. 168.%B "An Ethernet Address Resolution Protocol" 169.%T RFC826 170.Re 171.Rs 172.%A Leffler, S.J. 173.%A Karels, M.J. 174.%B "Trailer Encapsulations 175.%T RFC893 176.Re 177