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## 3. Software

## 3.1 Clock(8) and Hwclock(8)

All linux distributions install either the old `clock(8)` or the newer `hwclock(8)`, but without a correction factor. Some may also install `adjtimex(8)`, or they may include it on the CD as optional (or you can download it from the usual Linux archive sites). Some distributions also include a graphical clock setting program that runs in an X-window, but those are designed for interactive use, and the system will still install `clock(8)` or `hwclock(8)` for use in the startup scripts.

`Clock(8)` requires you to calculate the correction factor by hand, but `hwclock(8)` calculates it automatically whenever you use it to reset the RTC (using another program to set the RTC will interfere with the drift correction, so always use the same program if you're using a correction factor). If you have an older system that still uses `clock(8)` and you want to upgrade, you can find `hwclock(8)` in the "`util-linux`" package, version 2.7 or later. See the man page for more information.

Note

  The man page for `hwclock(8)` may be called "`clock`" for backward compatibility, so try both names. `Hwclock(8)` will respond to commands written for `clock(8)`, but the result may not be the same-- in particular, "`hwclock -a`" is not quite the same as "`clock -a`", so if you're upgrading to `hwclock` I'd suggest replacing all references to "`clock`" in your startup scripts to use `hwclock`'s native commands instead.

The startup scripts vary from one distribution to another, so you may have to search a bit to find where it sets the clock. Typical locations are `/etc/rc.local`, `/etc/rc.d/rc.sysinit`, `/etc/rc.d/boot`, or some similar place.

The correction factor for the RTC is stored in `/etc/adjtime`. Red Hat has a script in `/etc/sysconfig/clock` that controls the options to `hwclock`.

When you're setting the clock to determine the drift rate, keep in mind that your local telephone time announcement may or may not be accurate. If you don't have a shortwave radio or GPS receiver, you can hear the audio feed from WWV by calling (303)499-7111 (this is a toll call to Boulder, Colorado). It will cut you off after three minutes, but that should be long enough to set the clock. USNO and Canada's CHU also have telephone time services, but I prefer WWV's because there is more time between the announcement and the "beep". You can also get the time from a network time server using the `ntpdate` program that comes with `ntpd`, and there's a javaclock at www.time.gov.

In any case, what you're setting is the system clock, not the RTC (see the man page for the `date` command for the formats to use). Then use `hwclock` to set the RTC and calculate the drift rate. If you're doing this by hand, you should be able to set it within a second or two, and get a reasonable approximation of the drift rate after a few weeks. Then you can run `adjtimex(8)` to fine-tune the system clock.

## 3.2 Adjtimex(8)

`Adjtimex(8)` allows the user to adjust the kernel's time variables, and therefore change the speed of the system clock (you must be logged in as "root" to do this). It is cleverly designed to compare the system clock to the RTC using the same correction factor used by `clock(8)` or `hwclock(8)`, as stored in `/etc/adjtime`. So, once you've established the drift rate of the RTC, it's fairly simple to correct the system clock as well. Once you've got it running at the right speed, you can add a line to your startup scripts to set the correct kernel variables at boot time. Since `adjtimex(8)` was designed to work with `clock(8)` or `hwclock(8)`, it includes a work-around for the "every 11 minutes" bug.

After you've installed `adjtimex(8)` you can get more information on setting it up by typing "`man 8 adjtimex`" (there is also an `adjtimex(2)` man page, which is not what you want) and by reading the `README` file in `/usr/doc/adjtimex-1.3/README` (where the version number in the path will be the current version of `adjtimex(8)`).

## 3.3 Xntpd and ntpd: the Network Time Protocol

`Xntpd` (NTPv3) has been replaced by `ntpd` (NTPv4); the earlier version is no longer being maintained.

`Ntpd` is the standard program for synchronizing clocks across a network, and it comes with a list of public time servers you can connect to. It can be a little more complicated to set up than the other programs described here, but if you're interested in this kind of thing I highly recommend that you take a look at it anyway.

The "home base" for information on `ntpd` is the NTP website at http://www.eecis.udel.edu/~ntp/ which also includes links to all kinds of interesting time-related stuff (including software for other OS's). Some linux distributions include `ntpd` on the CD. There is a list of public time servers at http://www.eecis.udel.edu/~mills/ntp/clock2.html.

A relatively new feature in `ntpd` is a "burst mode" which is designed for machines that have only intermittent dial-up access to the internet.

`Ntpd` includes drivers for quite a few radio clocks (although some appear to be better supported than others). Most radio clocks are designed for commercial use and cost thousands of dollars, but there are some cheaper alternatives (discussed in later sections). In the past most were WWV or WWVB receivers, but now most of them seem to be GPS receivers. NIST has a PDF file that lists manufacturers of radio clocks on their website at http://www.boulder.nist.gov/timefreq/links.htm (near the bottom of the page). The NTP website also includes many links to manufacturers of radio clocks at http://www.eecis.udel.edu/~ntp/hardware.htm and http://www.eecis.udel.edu/~mills/ntp/refclock.htm. Either list may or may not be up to date at any given time `:-)`. The list of drivers for `ntpd` is at http://www.eecis.udel.edu/~ntp/ntp_spool/html/refclock.htm.

`Ntpd` also includes drivers for several dial-up time services. These are all long-distance (toll) calls, so be sure to calculate the effect on your phone bill before using them.

## 3.4 Chrony

`Xntpd` was originally written for machines that have a full-time connection to a network time server or radio clock. In theory it can also be used with machines that are only connected intermittently, but Richard Curnow couldn't get it to work the way he wanted it to, so he wrote "`chrony`" as an alternative for those of us who only have network access when we're dialed in to an ISP (this is the same problem that `ntpd`'s new "burst mode" was designed to solve). The current version of `chrony` includes drift correction for the RTC, for machines that are turned off for long periods of time.

You can get more information from Richard Curnow's website at http://www.rrbcurnow.freeuk.com/chrony or http://go.to/chrony. There are also two `chrony` mailing lists, one for announcements and one for discussion by users. For information send email to `chrony-users-subscribe@egroups.com` or `chrony-announce-subscribe@egroups.com`

Chrony is normally distributed as source code only, but Debian has been including a binary in their "unstable" collection. The source file is also available at the usual Linux archive sites.

## 3.5 Clockspeed

Another option is the `clockspeed` program by DJ Bernstein. It gets the time from a network time server and simply resets the system clock every three seconds. It can also be used to synchronize several machines on a LAN.

I've sometimes had trouble reaching his website at http://Cr.yp.to/clockspeed.html, so if you get a DNS error try again on another day. I'll try to update this section if I get some better information.

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