195 lines
6.0 KiB
Groff
195 lines
6.0 KiB
Groff
.\" $OpenBSD: dhclient.8,v 1.3 2004/04/09 18:30:15 jmc Exp $
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.\"
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.\" Copyright (c) 1997 The Internet Software Consortium.
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.\" 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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.\"
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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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.\" 3. Neither the name of The Internet Software Consortium nor the names
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.\" of its contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
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.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
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.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
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.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
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.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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.\" 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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.\" This software has been written for the Internet Software Consortium
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.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
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.\" Enterprises. To learn more about the Internet Software Consortium,
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.\" see ``http://www.isc.org/isc''. To learn more about Vixie
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.\" Enterprises, see ``http://www.vix.com''.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd July 22, 2005
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.Dt DHCLIENT 8
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.Os
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.Sh NAME
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.Nm dhclient
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.Nd "Dynamic Host Configuration Protocol (DHCP) client"
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.Sh SYNOPSIS
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.Nm
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.Op Fl dqu
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.Op Fl c Ar file
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.Op Fl l Ar file
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.Ar interface
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.Sh DESCRIPTION
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The
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.Nm
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utility provides a means for configuring network interfaces using DHCP, BOOTP,
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or if these protocols fail, by statically assigning an address.
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.Pp
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The name of the network interface that
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.Nm
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should attempt to
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configure must be specified on the command line.
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.Pp
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The options are as follows:
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.Bl -tag -width ".Fl c Ar file"
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.It Fl b
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Forces
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.Nm
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to immediately move to the background.
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.It Fl c Ar file
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Specify an alternate location,
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.Ar file ,
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for the configuration file.
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.It Fl d
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Forces
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.Nm
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to always run as a foreground process.
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By default,
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.Nm
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runs in the foreground until it has configured the interface, and then
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will revert to running in the background.
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.It Fl l Ar file
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Specify an alternate location,
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.Ar file ,
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for the leases file.
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.It Fl q
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Forces
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.Nm
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to be less verbose on startup.
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.It Fl u
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Forces
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.Nm
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to reject leases with unknown options in them.
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The default behaviour is to accept such lease offers.
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.El
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.Pp
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The DHCP protocol allows a host to contact a central server which
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maintains a list of IP addresses which may be assigned on one or more
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subnets.
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A DHCP client may request an address from this pool, and
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then use it on a temporary basis for communication on the network.
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The DHCP protocol also provides a mechanism whereby a client can learn
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important details about the network to which it is attached, such as
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the location of a default router, the location of a name server, and
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so on.
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.Pp
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On startup,
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.Nm
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reads
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.Pa /etc/dhclient.conf
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for configuration instructions.
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It then gets a list of all the
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network interfaces that are configured in the current system.
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It then attempts to configure each interface with DHCP.
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.Pp
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In order to keep track of leases across system reboots and server
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restarts,
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.Nm
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keeps a list of leases it has been assigned in the
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.Pa /var/db/dhclient.leases. Ns Ar IFNAME
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file.
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.Ar IFNAME
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represents the network interface of the DHCP client
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(e.g.,
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.Li em0 ) ,
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one for each interface.
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On startup, after reading the
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.Xr dhclient.conf 5
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file,
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.Nm
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reads the leases file to refresh its memory about what leases it has been
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assigned.
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.Pp
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Old leases are kept around in case the DHCP server is unavailable when
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.Nm
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is first invoked (generally during the initial system boot
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process).
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In that event, old leases from the
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.Pa dhclient.leases. Ns Ar IFNAME
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file which have not yet expired are tested, and if they are determined to
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be valid, they are used until either they expire or the DHCP server
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becomes available.
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.Pp
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A mobile host which may sometimes need to access a network on which no
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DHCP server exists may be preloaded with a lease for a fixed
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address on that network.
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When all attempts to contact a DHCP server have failed,
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.Nm
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will try to validate the static lease, and if it
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succeeds, it will use that lease until it is restarted.
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.Pp
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A mobile host may also travel to some networks on which DHCP is not
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available but BOOTP is.
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In that case, it may be advantageous to
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arrange with the network administrator for an entry on the BOOTP
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database, so that the host can boot quickly on that network rather
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than cycling through the list of old leases.
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.Sh NOTES
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You must have the Berkeley Packet Filter (BPF) configured in your kernel.
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The
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.Nm
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utility
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requires at least one
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.Pa /dev/bpf*
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device for each broadcast network interface that is attached to your system.
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See
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.Xr bpf 4
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for more information.
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.Sh FILES
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.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact
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.It Pa /etc/dhclient.conf
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DHCP client configuration file
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.It Pa /var/db/dhclient.leases. Ns Ar IFNAME
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database of acquired leases
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.El
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.Sh SEE ALSO
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.Xr dhclient.conf 5 ,
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.Xr dhclient.leases 5 ,
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.Xr dhclient-script 8 ,
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.Xr dhcp 8 ,
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.Xr dhcpd 8 ,
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.Xr dhcrelay 8
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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utility
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was written by
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.An Ted Lemon Aq mellon@fugue.com
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and
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.An Elliot Poger Aq elliot@poger.com .
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.Pp
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The current implementation was reworked by
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.An Henning Brauer Aq henning@openbsd.org .
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