194 lines
7.5 KiB
Groff
194 lines
7.5 KiB
Groff
.\" Copyright (c) 1985, 1986, 1988, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94
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.\" $FreeBSD$
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.\"
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.Dd December 18, 2005
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.Dt ARP 4
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.Os
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.Sh NAME
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.Nm arp
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.Nd Address Resolution Protocol
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.Sh SYNOPSIS
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.Cd "device ether"
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.Sh DESCRIPTION
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The Address Resolution Protocol (ARP) is used to dynamically
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map between Protocol Addresses (such as IP addresses) and
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Local Network Addresses (such as Ethernet addresses).
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This implementation maps IP addresses to Ethernet,
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ARCnet,
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or Token Ring addresses.
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It is used by all the Ethernet interface drivers.
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.Pp
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ARP caches Internet-Ethernet address mappings.
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When an interface requests a mapping for an address not in the cache,
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ARP queues the message which requires the mapping and broadcasts
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a message on the associated network requesting the address mapping.
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If a response is provided, the new mapping is cached and any pending
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message is transmitted.
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ARP will queue at most one packet while waiting for a response to a
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mapping request;
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only the most recently ``transmitted'' packet is kept.
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If the target host does not respond after several requests,
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the host is considered to be down allowing an error to be returned to
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transmission attempts.
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Further demand for this mapping causes ARP request retransmissions, that
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are ratelimited to one packet per second.
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The error is
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.Er EHOSTDOWN
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for a non-responding destination host, and
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.Er EHOSTUNREACH
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for a non-responding router.
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.Pp
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The ARP cache is stored in the system routing table as
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dynamically-created host routes.
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The route to a directly-attached Ethernet network is installed as a
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.Dq cloning
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route (one with the
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.Li RTF_CLONING
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flag set),
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causing routes to individual hosts on that network to be created on
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demand.
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These routes time out periodically (normally 20 minutes after validated;
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entries are not validated when not in use).
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.Pp
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ARP entries may be added, deleted or changed with the
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.Xr arp 8
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utility.
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Manually-added entries may be temporary or permanent,
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and may be
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.Dq published ,
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in which case the system will respond to ARP requests for that host
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as if it were the target of the request.
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.Pp
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In the past,
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ARP was used to negotiate the use of a trailer encapsulation.
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This is no longer supported.
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.Pp
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ARP watches passively for hosts impersonating the local host (i.e., a host
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which responds to an ARP mapping request for the local host's address).
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.Pp
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Proxy ARP is a feature whereby the local host will respond to requests
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for addresses other than itself, with its own address.
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Normally, proxy ARP in
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.Fx
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is set up on a host-by-host basis using the
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.Xr arp 8
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utility, by adding an entry for each host inside a given subnet for
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which proxying of ARP requests is desired.
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However, the
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.Dq "proxy all"
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feature causes the local host to act as a proxy for
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.Em all
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hosts reachable through some other network interface,
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different from the one the request came in from.
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It may be enabled by setting the
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.Xr sysctl 8
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MIB variable
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.Va net.link.ether.inet.proxyall
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to 1.
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.Sh MIB Variables
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The ARP protocol implements a number of configrable variables in
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.Va net.link.ether.inet
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branch
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of the
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.Xr sysctl 3
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MIB.
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.Bl -tag
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.It Va prune_intvl
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How frequently the ARP cache is cleaned from expired entries.
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.It Va max_age
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How long an ARP entry is held in the cache until it needs to be refreshed.
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.It Va maxtries
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Number of retransmits before host is considered down and error is returned.
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.It Va useloopback
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If an ARP entry is added for local address, force the traffic to go through
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the loopback interface.
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.It Va proxyall
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Enables ARP proxying for all hosts on net.
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.El
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.Sh DIAGNOSTICS
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.Bl -diag
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.It "arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!"
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ARP has discovered another host on the local network which responds to
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mapping requests for its own Internet address with a different Ethernet
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address, generally indicating that two hosts are attempting to use the
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same Internet address.
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.It "arp: link address is broadcast for IP address %d.%d.%d.%d!"
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ARP requested information for a host, and received an answer indicating
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that the host's ethernet address is the ethernet broadcast address.
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This indicates a misconfigured or broken device.
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.It "arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s"
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ARP had a cached value for the ethernet address of the referenced host,
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but received a reply indicating that the host is at a new address.
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This can happen normally when host hardware addresses change,
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or when a mobile node arrives or leaves the local subnet.
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It can also indicate a problem with proxy ARP.
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This message can only be issued if the sysctl
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.Va net.link.ether.inet.log_arp_movements
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is set to 1, which is the system's default behaviour.
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.It "arpresolve: can't allocate llinfo for %d.%d.%d.%d"
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The route for the referenced host points to a device upon which ARP is
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required, but ARP was unable to allocate a routing table entry in which
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to store the host's MAC address.
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This usually points to a misconfigured routing table.
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It can also occur if the kernel cannot allocate memory.
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.It "arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1"
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Physical connections exist to the same logical IP network on both if0 and if1.
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It can also occur if an entry already exists in the ARP cache for the IP
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address above, and the cable has been disconnected from if0, then reconnected
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to if1.
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This message can only be issued if the sysctl
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.Va net.link.ether.inet.log_arp_wrong_iface
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is set to 1, which is the system's default behaviour.
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.It "arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s"
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ARP has received an ARP reply that attempts to overwrite a permanent
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entry in the local ARP table.
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This error will only be logged if the sysctl
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.Va net.link.ether.inet.log_arp_permanent_modify
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is set to 1, which is the system's default behaviour.
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.El
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.Sh SEE ALSO
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.Xr inet 4 ,
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.Xr route 4 ,
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.Xr arp 8 ,
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.Xr ifconfig 8 ,
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.Xr route 8 ,
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.Xr sysctl 8
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.Rs
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.%A Plummer, D.
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.%B "An Ethernet Address Resolution Protocol"
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.%T RFC826
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.Re
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.Rs
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.%A Leffler, S.J.
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.%A Karels, M.J.
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.%B "Trailer Encapsulations
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.%T RFC893
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.Re
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