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Welcome to the Slackware Documentation Project

Synchronize your network with NTP

NTP is the Network Time Protocol, used to synchronize host clocks to one another. Your Slackware distribution comes with NTP preinstalled.

Your reasons for running NTP might include:

  • make timestamps in system logs agree with one other, to make sense of events recorded in multiple system logs
  • enable software protocols and encryption that depend on accurate time, e.g., Kerberos or PCI
  • prevent software build issues caused when your remote filesystem says your source file was modified in the future
  • prevent issues with database software that cannot tolerate setting the host clock back in time
  • know when to stop hacking and turn on the new Star Trek: Picard

Using NTP, your Slackware host can become any of the following:

  • An NTP client, that corrects its own host clock to match that of another host
  • A standalone NTP client, that does not match its own clock to another host but does take advantage of NTP's ability to make frequency corrections when the host clock gains or loses time too quickly
  • An NTP server, that shares its system time with other hosts
  • A primary NTP server, that gets its system time not from another host but from a hardware clock that has direct access to the Coordinated Universal Time timekeeping process (also known as a stratum 1 host)

It is very common to operate NTP as both a server and a client. In other words, your Slackware host can get accurate time from another host on the Internet and then serve that time to hosts on your local network.

This HOWTO is based on Slackware version 15.0 and the NTP reference implementation version 4.2.8 that comes with it. To check your Slackware version see: “Checking a Slackware Version”. To check your NTP software version, use the ntpd command:
$ ntpd --version
ntpd 4.2.8p15@1.3728-o Fri May 21 19:02:16 UTC 2021 (1)

Access control

NTP uses UDP port 123. Open port 123 in your host firewall if you want to allow other hosts to connect to your host. Open port 123 in your Internet firewall if you want access to Internet time.

NTP uses the restrict command in /etc/ntp.conf to impose additional restrictions by creating an ACL (access control list). The ACL is used by a mini-firewall within NTP itself that drops inbound packets based on options you choose.

Please turn your attention to the ACL pre-supplied by Slackware in /etc/ntp.conf:

restrict default limited kod nomodify notrap nopeer noquery
restrict -6 default limited kod nomodify notrap nopeer noquery
restrict ::1

This ACL prevents NTP from running as either a client (nopeer) or a server (noquery). It drops all packets except requests for basic information (first two lines). It makes an exception for packets that originate from your Slackware host itself (last two lines). This exception is what lets you control your own NTP service using the ntpq command.

The second line is redundant and should be deleted.

If you want to use the public NTP server pool, you must add a line to relax the ACL restrictions enough to allow peering with associations. Add a restrict source command without the nopeer flag, such as:

restrict source limited kod nomodify notrap noquery

If you want to allow devices on your network (or anywhere you like) to get time from this host, you must add a line to relax these restrictions to permit clients. Add a restrict address command that identifies the device(s) and/or network(s) that are allowed to get time. If your local network is, you could add:

restrict mask limited kod nomodify notrap nopeer

To let you control your NTP service from your maintenance VLAN and not just the host itself, you might want to add the VLAN with no restrictions. Supposing the maintenance VLAN is, you could add:

restrict mask

If you want more sophisticated access control than what's described here, for example to encrypt traffic or let you authenticate for administration tasks from any host, look into the secure authentication features of NTP. See: “Authentication Support” at The NTP Project.

Diagnostic logging

NTP prefers to use SYSLOG for logging. There is an alternate logging feature in NTP itself that can be used instead. Looking again at the preinstalled Slackware /etc/ntp.conf, the alternate logging feature has already been turned on:

logfile /var/log/ntp

Not recommended. If you use the alternate logging feature, you are also responsible for managing the logfile so that it does not eventually consume all the available space in the filesystem. Not having to do this is one of the big advantages of using SYSLOG.

It is simpler to delete the logfile line and use Slackware's preinstalled SYSLOG package. Using SYSLOG, NTP logs warnings and errors to /var/log/syslog, and routine status messages to /var/log/messages.

If you still want to use the alternate logging feature, be sure to create the empty file and make it writable by the NTP daemon:

# touch /var/log/ntp.log
# chown ntp:ntp /var/log/ntp.log

Filtering the log

NTP lets you filter certain messages out of the log, based on the message's class and type. Currently there are four classes defined:

clock peer sync sys

and four types defined:

info events status statistics

Because the preinstalled Slackware /etc/ntp.conf does not customize the filter, you get out-of-box behavior. NTP will pass messages that are tagged with the sync class and drop all messages that are tagged with any other class.

The out-of-box behavior unfortunately filters the message associated with at least one common fatal condition (termination of the ntpd process when the clock offset exceeds its panic threshold).

If you want all available diagnostic messages logged, you should disable all filtering by class or type in /etc/ntp.conf:

logconfig =allall

Statistics gathering

NTP can keep a statistical record of its performance, that you can analyze to check the health of your NTP-managed clock. The preinstalled Slackware /etc/ntp.conf already configures the directory path for these statistics:

statsdir /var/lib/ntp/stats

But to actually collect statistics, you must create the empty directory and make it writable by the NTP daemon:

# mkdir /var/lib/ntp/stats
# chown ntp:ntp /var/lib/ntp/stats

and add a command to /etc/ntp.conf that identifies the statistics you want collected. The most commonly analyzed record is NTP's system clock updates in the loopstats file:

statistics loopstats

There are a total of eight recordtypes that NTP will keep. For information, see: “Monitoring Options” at The NTP Project.

At the end of this HOWTO, there is an example of charting the loopstats using the preinstalled Slackware gnuplot package.

As with the logfile, if you collect statistics, you are responsible for managing the statistics files so that they do not eventually consume all the available space in the filesystem.

Operating NTP as a client

The preinstalled Slackware /etc/ntp.conf already has commands in it that would make NTP a client of the public NTP server pool, just commented out. Here are the relevant lines:

#server 0.pool.ntp.org iburst
#server 1.pool.ntp.org iburst
#server 2.pool.ntp.org iburst
#server 3.pool.ntp.org iburst

As of NTP 4, this is no longer the recommended way to use the public NTP pool.

You should replace the multiple server commands with a single pool command. The command that is equivalent to the lines above is:

pool pool.ntp.org

Remember that you must also add the restrict source command to the ACL as described in an earlier section for this to work.

The NTP Pool Project formerly recommended using country-specific pools in the server commands: “you get a bit better result if you use the continental zones … and even better time if you use the country zone”. This is no longer true. They now recommend looking up the global pool pool.ntp.org, stating that the global pool “will usually return IP addresses for servers in or close to your country … for most users this will give the best results”.

It's not safe to trust specific individual clocks in the public NTP pool. This is why NTP looks at multiple clocks and compares them before it selects a clock to synchronize with. It's important to configure the clock selection process. Current best practice is to wait until at 3 of 4 public clocks contacted agree about what time it is. Add the command:

tos minclock 4 minsane 3

It's recommended to set NTP to associate with an odd number of pool clocks, equal to at least minclock + 2. If your chosen minclock is 4, you can calculate your target number of pool clocks as:

minclock + 2 + 1 = 7

(You can use a larger odd number if you wish, but 7 is adequate).

NTP counts every clock you declare explicitly in /etc/ntp.conf, plus the pool clocks it discovers, against its maxclock parameter. So to come up with the right limit, take the number you just calculated, and add 1 for each explicit clock declaration you have in /etc/ntp.conf, including your pool command, and use that number to set maxclock. For example, if you just have the one pool command and no other clocks declared, then

maxclock = 7 + 1

and you should add the command:

tos maxclock 8

You can easily double-check your clock associations using the command

# ntpq -n -p

and verify that the number of pool clocks is what you expected.

The risk that you run if you don't set minclock, minsane, and maxclock properly is that the NTP clock selection algorithm will get it wrong at boot time and give you inaccurate time, or even panic and exit.

Correcting for a fast or slow hardware clock

Any hardware clock runs a few parts per million too fast or too slow. Over time, NTP automatically calculates what this error is and compensates for it. It can also store its calculation in a file that it re-reads when restarted.

The preinstalled Slackware /etc/ntp.conf already has the necessary command in it to enable this feature:

driftfile /var/lib/ntp/drift

Operating NTP as a server

Beyond access control, there is no configuration needed to let your NTP host operate as a server and supply time to your other devices.

In fact, it is a good idea to make one host on your network the primary time server, and configure your other devices to get time from it. This reduces bandwidth on your uplink. Plus it reduces the load on the public NTP pool if you are using it.

If you have client devices that are Slackware hosts, they should not use the pool command. Intead they can use the server command and identify your primary local time host by IP address. Otherwise, they are configured much like your primary time host.

You might want your other devices to stay synchronized with your primary time host even when your uplink goes down. The way this used to work was by adding your own hardware clock as a sort of “emergency” reference clock that will keep your devices in synch with one another even without an uplink. This is the approach taken by the preinstalled Slackware /etc/ntp.conf:

fudge stratum 10

Use of this clock driver is no longer recommended.

The local clock driver has been replaced by Orphan Mode. The commands above should be changed to:

tos orphan 10

Orphan mode is also useful in a closed environment, say a high-security installation, where you will not use the NTP pool or an actual UTC hardware clock and only want the devices on a network to agree on the time. It's less less helpful when you want a timely reminder to watch Star Trek: Picard.

If you do run in a closed environment, NTP will have no way to calculate drift of your hardware clock against a true clock. You can make your own observations of how fast or slow your hardware clock tends to drift, and manually compensate using the command:

tinker freq NNN

where NNN is the observed frequency error of your hardware clock in parts per million. This is mutually exclusive with the driftfile command so you will also have to take that command out.


At one time it was standard practice to use the ntpdate command to make an quick rough adjustment to the system clock and then start ntpd, but in this version of NTP that's no longer recommended. It is recommended instead to start ntpd as early as possible in the boot sequence and use the -g option to set the time.

This is already what Slackware 15.0 is preconfigured to do. To have NTP run at startup, make /etc/rc.d/rc.ntpd an executable script:

# chmod 755 /etc/rc.d/rc.ntpd

and then either reboot or start it manually:

# /etc/rc.d/rc.ntpd start

The script /etc/rc.d/rc.ntpd looks for the file /run/ntpd.pid when it is passed the stop or status option. The command to create this file is already present in /etc/ntp.conf:

pidfile /var/run/ntpd.pid

You may have noticed that the pathnames disagree, and that's mildly infuriating but makes no actual difference because /var/run is a symbolic link to /run.

System services that should wait to start until the clock is stable can be preceded by the ntp-wait command, for example databases. For example, you could conceivably edit the MariaDB section of /etc/rc.d/rc.M to read:

# Start the MariaDB database:
if [ -x /etc/rc.d/rc.mysqld ]; then
  /usr/sbin/ntp-wait -v
  /etc/rc.d/rc.mysqld start

Monitoring NTP

You can review how the pool discovery process is working with the command

# ntpq -n -p

Here is some sample output:

     remote           refid      st t when poll reach   delay   offset  jitter
 pool.ntp.org    .POOL.          16 p    -   64    0    0.000   +0.000   0.000
-    2 u  665 1024  377   20.524   -1.211   0.030
-   .SOCK.           1 u 1000 1024  377   51.379   -4.051   0.166
+      3 u  966 1024  377    1.644   +0.731   0.197
*      2 u  748 1024  377   40.568   +0.372   0.119
-       2 u  905 1024  377   16.545   -1.035   0.147
+      3 u  556 1024  377    1.565   +0.827   0.076
+       2 u  220 1024  377    0.325   -1.269   0.865

In a nutshell, this tells us we have 1 master pool source (type=“p”) plus 7 peers (type “u”). It also tells us:

  • Our system peer (that we are getting our time from) is (“*”).
  • We have three more peer candidates (“+”) in case our system peer goes away.
  • NTP is considering pruning three outliers (“-”).

The loopstats file and other statistics files are described in detail at “Monitoring Options” at The NTP Project. Here's a sample and a brief explanation:

59760 66558.710 0.000038135 4.882 0.000265925 0.027674 10
59760 68002.750 0.000538317 4.885 0.000305204 0.025906 10
59760 69002.727 0.000199760 4.885 0.000309570 0.024235 10
59760 69437.711 0.000656689 4.886 0.000331590 0.022673 10
59760 71213.750 0.000522794 4.890 0.000313766 0.021244 10
59760 73341.750 0.000484582 4.951 0.000293812 0.029264 10
59760 75485.750 0.000486984 5.011 0.000274837 0.034780 10
59760 76374.727 -0.000069057 5.011 0.000323638 0.032534 10
59760 77579.750 0.000202697 5.012 0.000317617 0.030434 10
59760 79162.710 -0.000183575 5.011 0.000326988 0.028471 10
Column 1 Modified Julian Date of the observation
Column 2 Time since midnight (seconds)

You can combine and convert these to a UNIX system value for plotting, as shown below.

Column 3 Difference observed between your system clock and your time source (seconds)
Column 5 Column 3's jitter
Column 4 Difference between your system clock frequency and the time source frequency (parts per million)
Column 6 Column 4's jitter

Here is a sample gnuplot program that charts recent loop statistics.

#! /usr/bin/gnuplot -ps
# Input - four most recent loopstats files
filelist=system("ls -rt /var/lib/ntp/stats/loopstats.* | tail -4")
# Output - X server
set terminal x11
# Multiplot layout
set multiplot layout 2,1
# Settings common to both graphs
set xlabel "Modified Julian Days (d)"
set xdata time
set xtics format "%F\n%T"
set xzeroaxis linetype 1
set grid
set errorbars small linecolor "dark-gray"
set key left bottom box
# Time Offset graph
set title 'local NTP clock: Time Offset' font ',20'
set ylabel "Time Offset +/- RMS Jitter (ms)"
plot [] [] for [filename in filelist] filename \
        using (86400.0*($1-40587)+$2):(1000.0*$3):(1000.0*$5) \
        title filename \
        with yerrorbars pointtype 1
# Frequency Offset graph
set title 'local NTP clock: Frequency Offset' font ',20'
set ylabel "Frequency Offset +/- Allan deviation (PPM)"
plot [] [] for [filename in filelist] filename \
        using (86400.0*($1-40587)+$2):($4):($6) \
        title filename \
        with yerrorbars pointtype 1

Sample output:


  • Originally written by Niki Kovacs
  • Performance monitoring section contributed by Dominik Drobek
  • Rewritten and updated to current best practice by Edward McGuire.

 howtos:network_services:ntp ()