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Diffstat (limited to 'usr.sbin/arp/arp.4')
| -rw-r--r-- | usr.sbin/arp/arp.4 | 235 |
1 files changed, 235 insertions, 0 deletions
diff --git a/usr.sbin/arp/arp.4 b/usr.sbin/arp/arp.4 new file mode 100644 index 0000000..0ff0f2c --- /dev/null +++ b/usr.sbin/arp/arp.4 @@ -0,0 +1,235 @@ +.\" Copyright (c) 1985, 1986, 1988, 1994 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94 +.\" $FreeBSD$ +.\" +.Dd September 3, 2012 +.Dt ARP 4 +.Os +.Sh NAME +.Nm arp +.Nd Address Resolution Protocol +.Sh SYNOPSIS +.Cd "device ether" +.Sh DESCRIPTION +The Address Resolution Protocol (ARP) is used to dynamically +map between Protocol Addresses (such as IP addresses) and +Local Network Addresses (such as Ethernet addresses). +This implementation maps IP addresses to Ethernet, +ARCnet, +or Token Ring addresses. +It is used by all the Ethernet interface drivers. +.Pp +ARP caches Internet-Ethernet address mappings. +When an interface requests a mapping for an address not in the cache, +ARP queues the message which requires the mapping and broadcasts +a message on the associated network requesting the address mapping. +If a response is provided, the new mapping is cached and any pending +message is transmitted. +ARP will queue at most one packet while waiting for a response to a +mapping request; +only the most recently ``transmitted'' packet is kept. +If the target host does not respond after several requests, +the host is considered to be down allowing an error to be returned to +transmission attempts. +Further demand for this mapping causes ARP request retransmissions, that +are ratelimited to one packet per second. +The error is +.Er EHOSTDOWN +for a non-responding destination host, and +.Er EHOSTUNREACH +for a non-responding router. +.Pp +The ARP cache is stored in the system routing table as +dynamically-created host routes. +The route to a directly-attached Ethernet network is installed as a +.Dq cloning +route (one with the +.Li RTF_CLONING +flag set), +causing routes to individual hosts on that network to be created on +demand. +These routes time out periodically (normally 20 minutes after validated; +entries are not validated when not in use). +.Pp +ARP entries may be added, deleted or changed with the +.Xr arp 8 +utility. +Manually-added entries may be temporary or permanent, +and may be +.Dq published , +in which case the system will respond to ARP requests for that host +as if it were the target of the request. +.Pp +In the past, +ARP was used to negotiate the use of a trailer encapsulation. +This is no longer supported. +.Pp +ARP watches passively for hosts impersonating the local host (i.e., a host +which responds to an ARP mapping request for the local host's address). +.Pp +Proxy ARP is a feature whereby the local host will respond to requests +for addresses other than itself, with its own address. +Normally, proxy ARP in +.Fx +is set up on a host-by-host basis using the +.Xr arp 8 +utility, by adding an entry for each host inside a given subnet for +which proxying of ARP requests is desired. +However, the +.Dq "proxy all" +feature causes the local host to act as a proxy for +.Em all +hosts reachable through some other network interface, +different from the one the request came in from. +It may be enabled by setting the +.Xr sysctl 8 +MIB variable +.Va net.link.ether.inet.proxyall +to 1. +.Sh MIB Variables +The ARP protocol implements a number of configurable variables in +.Va net.link.ether.inet +branch +of the +.Xr sysctl 3 +MIB. +.Bl -tag -width "log_arp_permanent_modify" +.It Va allow_multicast +Should the kernel install ARP entries with multicast bit set in +the hardware address. +Installing such entries is RFC 1812 violation, but some prorietary +load balancing techniques require routers on network to do so. +Turned off by default. +.It Va log_arp_movements +Should the kernel log movements of IP addresses from one hardware +address to an other. +See +.Sx DIAGNOSTICS +below. +Turned on by default. +.It Va log_arp_permanent_modify +Should the kernel log attempts of remote host on network to modify a +permanent ARP entry. +See +.Sx DIAGNOSTICS +below. +Turned on by default. +.It Va log_arp_wrong_iface +Should the kernel log attempts to insert an ARP entry on an interface +when the IP network the address belongs to is connected to an other +interface. +See +.Sx DIAGNOSTICS +below. +Turned on by default. +.It Va max_age +How long an ARP entry is held in the cache until it needs to be refreshed. +Default is 1200 seconds. +.It Va maxhold +How many packets hold in the per-entry output queue while the entry +is being resolved. +Default is one packet. +.It Va maxtries +Number of retransmits before host is considered down and error is returned. +Default is 5 tries. +.It Va proxyall +Enables ARP proxying for all hosts on net. +Turned off by default. +.It Va useloopback +If an ARP entry is added for local address, force the traffic to go through +the loopback interface. +Turned on by default. +.It Va wait +Lifetime of an incomplete ARP entry. +Default is 20 seconds. +.El +.Sh DIAGNOSTICS +.Bl -diag +.It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!" +ARP has discovered another host on the local network which responds to +mapping requests for its own Internet address with a different Ethernet +address, generally indicating that two hosts are attempting to use the +same Internet address. +.It "arp: link address is broadcast for IP address %d.%d.%d.%d!" +ARP requested information for a host, and received an answer indicating +that the host's ethernet address is the ethernet broadcast address. +This indicates a misconfigured or broken device. +.It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s" +ARP had a cached value for the ethernet address of the referenced host, +but received a reply indicating that the host is at a new address. +This can happen normally when host hardware addresses change, +or when a mobile node arrives or leaves the local subnet. +It can also indicate a problem with proxy ARP. +This message can only be issued if the sysctl +.Va net.link.ether.inet.log_arp_movements +is set to 1, which is the system's default behaviour. +.It "arpresolve: can't allocate llinfo for %d.%d.%d.%d" +The route for the referenced host points to a device upon which ARP is +required, but ARP was unable to allocate a routing table entry in which +to store the host's MAC address. +This usually points to a misconfigured routing table. +It can also occur if the kernel cannot allocate memory. +.It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1" +Physical connections exist to the same logical IP network on both if0 and if1. +It can also occur if an entry already exists in the ARP cache for the IP +address above, and the cable has been disconnected from if0, then reconnected +to if1. +This message can only be issued if the sysctl +.Va net.link.ether.inet.log_arp_wrong_iface +is set to 1, which is the system's default behaviour. +.It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s" +ARP has received an ARP reply that attempts to overwrite a permanent +entry in the local ARP table. +This error will only be logged if the sysctl +.Va net.link.ether.inet.log_arp_permanent_modify +is set to 1, which is the system's default behaviour. +.It "arp: %x:%x:%x:%x:%x:%x is multicast" +Kernel refused to install an entry with multicast hardware address. +If you really want such addresses being installed, set the sysctl +.Va net.link.ether.inet.allow_multicast +to a positive value. +.El +.Sh SEE ALSO +.Xr inet 4 , +.Xr route 4 , +.Xr arp 8 , +.Xr ifconfig 8 , +.Xr route 8 , +.Xr sysctl 8 +.Rs +.%A Plummer, D. +.%B "An Ethernet Address Resolution Protocol" +.%T RFC826 +.Re +.Rs +.%A Leffler, S.J. +.%A Karels, M.J. +.%B "Trailer Encapsulations" +.%T RFC893 +.Re |
