191 lines
9.6 KiB
HTML
191 lines
9.6 KiB
HTML
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>The Clock Mini-HOWTO: Software</TITLE>
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<LINK HREF="Clock-4.html" REL=next>
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<LINK HREF="Clock-2.html" REL=previous>
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<LINK HREF="Clock.html#toc3" REL=contents>
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<A HREF="Clock-4.html">Next</A>
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<A HREF="Clock.html#toc3">Contents</A>
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<HR>
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<H2><A NAME="s3">3. Software</A></H2>
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<H2><A NAME="ss3.1">3.1 Clock(8) and Hwclock(8)</A>
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</H2>
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<P>All linux distributions install either the old <CODE>clock(8)</CODE>
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or the newer <CODE>hwclock(8)</CODE>, but without a correction
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factor. Some may also install <CODE>adjtimex(8)</CODE>, or they may
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include it on the CD as optional (or you can download it from
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the usual Linux archive sites). Some distributions also include
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a graphical clock setting program that runs in an X-window, but
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those are designed for interactive use, and the system will still
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install <CODE>clock(8)</CODE> or <CODE>hwclock(8)</CODE> for use in the
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startup scripts.
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<P><CODE>Clock(8)</CODE> requires you to calculate the correction factor
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by hand, but <CODE>hwclock(8)</CODE> calculates it automatically
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whenever you use it to reset the RTC (using another program to
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set the RTC will interfere with the drift correction, so always
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use the same program if you're using a correction factor). If you
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have an older system that still uses <CODE>clock(8)</CODE> and you
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want to upgrade, you can find <CODE>hwclock(8)</CODE> in the
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"<CODE>util-linux</CODE>" package, version 2.7 or later.
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See the man page for more information.
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<P>
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<DL>
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<DT><B>Note</B><DD><P>The man page for <CODE>hwclock(8)</CODE> may be called
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"<CODE>clock</CODE>" for backward compatibility, so
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try both names. <CODE>Hwclock(8)</CODE> will respond to commands
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written for <CODE>clock(8)</CODE>, but the result may not be the
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same-- in particular, "<CODE>hwclock -a</CODE>" is
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not quite the same as "<CODE>clock -a</CODE>", so if
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you're upgrading to <CODE>hwclock</CODE> I'd suggest replacing
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all references to "<CODE>clock</CODE>" in your
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startup scripts to use <CODE>hwclock</CODE>'s native commands
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instead.
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</DL>
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<P>The startup scripts vary from one distribution to another, so you
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may have to search a bit to find where it sets the clock. Typical
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locations are <CODE>/etc/rc.local</CODE>,
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<CODE>/etc/rc.d/rc.sysinit</CODE>,
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<CODE>/etc/rc.d/boot</CODE>, or some
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similar place.
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<P>The correction factor for the RTC is stored in
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<CODE>/etc/adjtime</CODE>.
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Red Hat has a script in
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<CODE>/etc/sysconfig/clock</CODE> that controls the
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options to <CODE>hwclock</CODE>.
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<P>When you're setting the clock to determine the drift rate, keep
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in mind that your local telephone time announcement may or may
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not be accurate. If you don't have a shortwave radio or GPS
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receiver, you can hear the audio feed from WWV by calling
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(303)499-7111 (this is a toll call to Boulder, Colorado). It
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will cut you off after three minutes, but that should be long
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enough to set the clock. USNO and Canada's CHU also have
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telephone time services, but I prefer WWV's because there is
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more time between the announcement and the "beep".
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You can also get the time from a network time server using the
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<CODE>ntpdate</CODE> program that comes with <CODE>ntpd</CODE>, and
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there's a javaclock at
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<A HREF="www.time.gov">www.time.gov</A>.
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<P>In any case, what you're setting is the system clock, not the RTC
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(see the man page for the <CODE>date</CODE> command for the formats
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to use). Then use <CODE>hwclock</CODE> to set the RTC and calculate
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the drift rate. If you're doing this by hand, you should be able
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to set it within a second or two, and get a reasonable
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approximation of the drift rate after a few weeks. Then you can
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run <CODE>adjtimex(8)</CODE> to fine-tune the system clock.
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<H2><A NAME="ss3.2">3.2 Adjtimex(8)</A>
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</H2>
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<P><CODE>Adjtimex(8)</CODE> allows the user to adjust the kernel's time
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variables, and therefore change the speed of the system clock
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(you must be logged in as "<B>root</B>" to do
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this). It is cleverly designed to compare the system clock to the
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RTC using the same correction factor used by <CODE>clock(8)</CODE> or
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<CODE>hwclock(8)</CODE>, as stored in <CODE>/etc/adjtime</CODE>.
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So, once you've established the drift rate of the RTC, it's fairly simple
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to correct the system clock as well. Once you've got it running
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at the right speed, you can add a line to your startup scripts to
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set the correct kernel variables at boot time. Since
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<CODE>adjtimex(8)</CODE> was designed to work with <CODE>clock(8)</CODE>
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or <CODE>hwclock(8)</CODE>, it includes a work-around for the
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"every 11 minutes" bug.
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<P>After you've installed <CODE>adjtimex(8)</CODE> you can get more
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information on setting it up by typing "<CODE>man 8
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adjtimex</CODE>" (there is also an <CODE>adjtimex(2)</CODE> man
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page, which is not what you want) and by reading the
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<CODE>README</CODE> file in <CODE>/usr/doc/adjtimex-1.3/README</CODE>
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(where the version number in the path will be the current version
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of <CODE>adjtimex(8)</CODE>).
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<H2><A NAME="ss3.3">3.3 Xntpd and ntpd: the Network Time Protocol</A>
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</H2>
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<P><CODE>Xntpd</CODE> (NTPv3) has been replaced by <CODE>ntpd</CODE>
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(NTPv4); the earlier version is no longer being maintained.
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<P><CODE>Ntpd</CODE> is the standard program for synchronizing clocks
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across a network, and it comes with a list of public time servers
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you can connect to. It can be a little more complicated to set up
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than the other programs described here, but if you're interested
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in this kind of thing I highly recommend that you take a look at
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it anyway.
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<P>The "home base" for information on <CODE>ntpd</CODE>
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is the NTP website at
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<A HREF="http://www.eecis.udel.edu/~ntp/">http://www.eecis.udel.edu/~ntp/</A>
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which also includes links to all kinds of interesting time-related
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stuff (including software for other OS's). Some linux
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distributions include <CODE>ntpd</CODE> on the CD. There is a list of
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public time servers at
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<A HREF="http://www.eecis.udel.edu/~mills/ntp/clock2.html">http://www.eecis.udel.edu/~mills/ntp/clock2.html</A>.
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<P>A relatively new feature in <CODE>ntpd</CODE> is a "burst mode"
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which is designed for machines that have only intermittent
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dial-up access to the internet.
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<P><CODE>Ntpd</CODE> includes drivers for quite a few radio clocks
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(although some appear to be better supported than others). Most
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radio clocks are designed for commercial use and cost thousands
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of dollars, but there are some cheaper alternatives (discussed in
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later sections). In the past most were WWV or WWVB receivers, but
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now most of them seem to be GPS receivers. NIST has a PDF file
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that lists manufacturers of radio clocks on their website at
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<A HREF="http://www.boulder.nist.gov/timefreq/links.htm">http://www.boulder.nist.gov/timefreq/links.htm</A> (near
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the bottom of the page). The NTP website also includes many links
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to manufacturers of radio clocks at
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<A HREF="http://www.eecis.udel.edu/~ntp/hardware.htm">http://www.eecis.udel.edu/~ntp/hardware.htm</A> and
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<A HREF="http://www.eecis.udel.edu/~mills/ntp/refclock.htm">http://www.eecis.udel.edu/~mills/ntp/refclock.htm</A>.
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Either list may or may not be up to date at any given time <CODE>:-)</CODE>.
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The list of drivers for <CODE>ntpd</CODE> is at
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<A HREF="http://www.eecis.udel.edu/~ntp/ntp_spool/html/refclock.htm">http://www.eecis.udel.edu/~ntp/ntp_spool/html/refclock.htm</A>.
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<P><CODE>Ntpd</CODE> also includes drivers for several dial-up time
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services. These are all long-distance (toll) calls, so be sure to
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calculate the effect on your phone bill before using them.
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<H2><A NAME="ss3.4">3.4 Chrony</A>
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</H2>
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<P><CODE>Xntpd</CODE> was originally written for machines that have a
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full-time connection to a network time server or radio clock. In
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theory it can also be used with machines that are only connected
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intermittently, but Richard Curnow couldn't get it to work the
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way he wanted it to, so he wrote "<CODE>chrony</CODE>" as an
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alternative for those of us who only have network access when
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we're dialed in to an ISP (this is the same problem that
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<CODE>ntpd</CODE>'s new "burst mode" was designed to solve).
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The current version of <CODE>chrony</CODE> includes drift correction
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for the RTC, for machines that are turned off for long periods of
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time.
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<P>You can get more information from Richard Curnow's website at
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<A HREF="http://www.rrbcurnow.freeuk.com/chrony">http://www.rrbcurnow.freeuk.com/chrony</A> or
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<A HREF="http://go.to/chrony">http://go.to/chrony</A>.
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There are also two <CODE>chrony</CODE> mailing lists, one for
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announcements and one for discussion by users. For information
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send email to
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<CODE>
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<A HREF="mailto:chrony-users-subscribe@egroups.com">chrony-users-subscribe@egroups.com</A></CODE>
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or
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<CODE>
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<A HREF="mailto:chrony-announce-subscribe@egroups.com">chrony-announce-subscribe@egroups.com</A></CODE>
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<P>Chrony is normally distributed as source code only, but Debian
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has been including a binary in their "unstable"
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collection. The source file is also available at the usual Linux
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archive sites.
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<H2><A NAME="ss3.5">3.5 Clockspeed</A>
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</H2>
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<P>Another option is the <CODE>clockspeed</CODE> program by DJ
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Bernstein. It gets the time from a network time server and simply
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resets the system clock every three seconds. It can also be used
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to synchronize several machines on a LAN.
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<P>I've sometimes had trouble reaching his website at
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<A HREF="http://Cr.yp.to/clockspeed.html">http://Cr.yp.to/clockspeed.html</A>, so if you get a DNS
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error try again on another day. I'll try to update this
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section if I get some better information.
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<HR>
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<A HREF="Clock-4.html">Next</A>
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