204 lines
5.6 KiB
Groff
204 lines
5.6 KiB
Groff
.\" $NetBSD: ntpdate.8,v 1.4 2001/04/27 00:10:07 wiz Exp $
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.\" Converted from HTML to mandoc by ntp-html2mdoc.pl
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.\"
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.Dd March 29, 2000
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.Dt NTPDATE 8
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.Os
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.Sh NAME
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.Nm ntpdate
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.Nd set the date and time via NTP
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.Sh SYNOPSIS
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.Nm
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.Op Fl bBdoqsuv
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.Op Fl a Ar key
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.Op Fl e Ar authdelay
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.Op Fl k Ar keyfile
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.Op Fl o Ar version
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.Op Fl p Ar samples
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.Op Fl t Ar timeout
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.Op Ar server ...
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.Sh DESCRIPTION
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.Nm
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sets the local date and time by polling the Network Time
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Protocol (NTP) server(s) given as the
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.Ar server
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arguments to determine
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the correct time. It must be run as root on the local host. A number of
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samples are obtained from each of the servers specified and a subset of
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the NTP clock filter and selection algorithms are applied to select the
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best of these. Note that the accuracy and reliability of
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.Nm
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depends on the number of servers, the number of polls each time it is run
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and the interval between runs.
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.Pp
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.Nm
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can be run manually as necessary to set the host clock,
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or it can be run from the host startup script to set the clock at boot
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time. This is useful in some cases to set the clock initially before starting
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the NTP daemon
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.Pa ntpd .
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It is also possible to run
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.Nm
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from a
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.Pa cron
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script. However, it is important to note that
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.Nm
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with contrived
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.Pa cron
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scripts is no substitute for the NTP daemon,
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which uses sophisticated algorithms to maximize accuracy and reliability
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while minimizing resource use. Finally, since
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.Nm
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does not
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discipline the host clock frequency as does
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.Pa ntpd ,
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the accuracy
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using
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.Nm
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is limited.
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.Pp
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Time adjustments are made by
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.Nm
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in one of two ways. If
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.Nm
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determines the clock is in error more than 0.5 second
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it will simply step the time by calling the system
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.Pa settimeofday()
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routine. If the error is less than 0.5 seconds, it will slew the time by
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calling the system
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.Pa adjtime()
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routine. The latter technique is
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less disruptive and more accurate when the error is small, and works quite
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well when
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.Nm
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is run by
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.Pa cron
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every hour or two.
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.Pp
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.Nm
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will decline to set the date if an NTP server daemon
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(e.g.,
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.Pa ntpd
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) is running on the same host. When running
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.Nm
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on a regular basis from
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.Pa cron
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as an alternative to running a daemon,
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doing so once every hour or two will result in precise enough timekeeping
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to avoid stepping the clock.
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.Pp
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If NetInfo support is compiled into
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.Nm "" ,
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then the
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.Pa server
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argument is optional if
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.Nm
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can find a time
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server in the NetInfo configuration for
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.Pa ntpd
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.
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.Sh COMMAND LINE OPTIONS
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.Bl -tag -width indent
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.It Fl a Ar key
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Enable the authentication function and specify the key identifier to be
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used for authentication as the argument
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.Ar key
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.Nm "" .
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The
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keys and key identifiers must match in both the client and server key files.
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The default is to disable the authentication function.
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.It Fl B
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Force the time to always be slewed using the adjtime() system call, even
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if the measured offset is greater than +-128 ms. The default is to step
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the time using settimeofday() if the offset is greater than +-128 ms. Note
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that, if the offset is much greater than +-128 ms in this case, that it
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can take a long time (hours) to slew the clock to the correct value. During
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this time. the host should not be used to synchronize clients.
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.It Fl b
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Force the time to be stepped using the settimeofday() system call, rather
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than slewed (default) using the adjtime() system call. This option should
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be used when called from a startup file at boot time.
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.It Fl d
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Enable the debugging mode, in which
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.Nm
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will go through all
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the steps, but not adjust the local clock. Information useful for general
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debugging will also be printed.
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.It Fl e Ar authdelay
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Specify the processing delay to perform an authentication function as the
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value
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.Ar authdelay
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, in seconds and fraction (see
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.Pa ntpd
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for
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details). This number is usually small enough to be negligible for most
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purposes, though specifying a value may improve timekeeping on very slow
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CPU's.
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.It Fl k Ar keyfile
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Specify the path for the authentication key file as the string
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.Ar keyfile
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.
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The default is
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.Pa /etc/ntp.keys .
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This file should be in the format
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described in
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.Pa ntpd
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.
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.It Fl o Ar version
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Specify the NTP version for outgoint packets as the integer
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.Ar version
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,
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which can be 1 or 2. The default is 3. This allows
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.Nm
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to
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be used with older NTP versions.
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.It Fl p Ar samples
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Specify the number of samples to be acquired from each server as the integer
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.Ar samples
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, with values from 1 to 8 inclusive. The default is 4.
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.It Fl q
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Query only - don't set the clock.
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.It Fl s
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Divert logging output from the standard output (default) to the system
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.Pa syslog
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facility. This is designed primarily for convenience of
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.Pa cron
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scripts.
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.It Fl t Ar timeout
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Specify the maximum time waiting for a server response as the value
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.Ar timeout
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,
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in seconds and fraction. The value is is rounded to a multiple of 0.2 seconds.
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The default is 1 second, a value suitable for polling across a LAN.
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.It Fl u
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Direct
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.Nm
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to use an unprivileged port or outgoing packets.
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This is most useful when behind a firewall that blocks incoming traffic
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to privileged ports, and you want to synchronise with hosts beyond the
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firewall. Note that the
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.Fl d
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option always uses unprivileged ports.
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.It Fl v
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Be verbose. This option will cause
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.Nm
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's version identification
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string to be logged.
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.El
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.Sh FILES
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.Bl -tag -width /etc/ntp.keys -compact
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.It Pa /etc/ntp.keys
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encryption keys used by
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.Nm "" .
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.El
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.Sh BUGS
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The slew adjustment is actually 50% larger than the measured offset, since
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this (it is argued) will tend to keep a badly drifting clock more accurate.
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This is probably not a good idea and may cause a troubling hunt for some
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values of the kernel variables
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.Pa tick
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and
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.Pa tickadj .
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.Sh AUTHOR
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David L. Mills (mills@udel.edu)
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