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howtos:network_services:ntp [2013/03/01 16:36 (UTC)] – Added a short explanation about problems with NFS and NIS when clients are out of sync kikinovakhowtos:network_services:ntp [2024/06/19 14:51 (UTC)] (current) – old revision restored (2024/06/17 17:22 (UTC)) revert, sorry I didn't read the thread to the end before my last edit j12i
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 ====== Synchronize your network with NTP ====== ====== Synchronize your network with NTP ======
-NTP (//Network Time Protocol//) allows clock synchronization between computer systemsThe following HOWTO describes:+NTP is the //Network Time Protocol//, used to synchronize host clocks to one 
 +another. Your Slackware distribution comes with NTP preinstalled.
-  * configuring an NTP server on Slackware Linux; +Your reasons for running NTP might include:
-  * synchronizing client PCs with your local NTP server.+
-===== Introduction =====+  * 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//
-When several users manipulate shared data on different client PCs on a networkit's important that these machines are all synchronized. This is especially true if you share files over NFS, or if you use NIS for centralized authentication. You'll get all sorts of weird errors if your clocks are out of sync. Unfortunately, the clients' onboard clocks aren't sufficiently precise.+Using NTP, your Slackware host can become any of the following:
-That's where NTP (//Network Time Protocol//) comes in handy. It allows networked machines to adjust their clocks so as to be perfectly synchronized. series of public time servers on the Internet allow the reception of the exact time. From this pointwe can use NTP in several ways.+  * An NTP //client//, that corrects its own host clock to match that of another host 
 +  * //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//)
-  * The ''ntpdate'' command makes an initial correction of the BIOS clock. +It is very common to operate NTP as both a server and a client. In other 
-  * This one-time-adjustment isn't sufficient for a server that is supposed to be up 24/7, since its clock will drift away gradually from the exact time. In that casewe have to configure the ''ntpd'' daemon (shipping with the ''ntp'' package). This daemon contacts public time servers at regular intervals and proceeds with incremental corrections of the local clock. +wordsyour Slackware host can get accurate time from another host on the 
-  * The ''ntpd'' daemon can in its turn be configured as a time server for the local client machines.+Internet and then serve that time to hosts on your local network.
-It's considered good practice to use ''ntpdate'' for the initial adjustment and ''ntpd'' for regular time synchronization.+<note important> 
 +This HOWTO is based on Slackware version 15.0 and the NTP reference 
 +implementation that comes with it. As of this writing, Slackware 15.0 is patched 
 +to NTP version ntp-4.2.8p18. To check your Slackware version see: 
 +"[[howtos:misc:checking_a_slackware_version|Checking a Slackware Version]]". To 
 +check your NTP software version, use the ntpd command:
-===== Firewall considerations =====+  $ ntpd --version 
 +  ntpd 4.2.8p18@1.4062-o Tue Jun  4 19:56:25 UTC 2024 (1) 
-The NTP services uses UDP port 123. Open this port if you want to allow remote machines to connect to your NTP server. 
-===== Synchronize a LAN server or a public root server with an NTP server on the Internet ===== 
-Create an empty log file:+===== Access control =====
-<code> +NTP uses UDP port 123. Open outbound port 123 if you want access to Internet 
-# touch /var/log/ntp.log +time
-</code>+Open inbound port 123 if you want other hosts to get time from you.
-Visit [[http://www.pool.ntp.org|http://www.pool.ntp.org]] and choose a list of servers according to your country.+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.
-Configure the NTP service by editing ''/etc/ntp.conf''. You might backup the existing ''ntp.conf'' file and start from scratch. +Please turn your attention to the ACL pre-supplied by Slackware in 
-In the example below, the list of four servers is chosen for my company's location (France):+  restrict default limited kod nomodify notrap nopeer noquery 
 +  restrict -6 default limited kod nomodify notrap nopeer noquery 
 +  restrict 
 +  restrict ::1
-<code> +This ACL prevents NTP from running as either a client (nopeer) or a server 
-# /etc/ntp.conf+(noquery). 
 +Lines one and two drop all packets except requests for basic information. 
 +Lines three and four make an exception for packets that originate from your own 
 +host, so you can control your own NTP service using the ''ntpq'' command.
-driftfile /etc/ntp/drift +Line two is redundant. It should be deleted.
-logfile /var/log/ntp.log+
-server 0.fr.pool.ntp.org +If you want to use the public NTP server pool, you must add a line to relax the 
-server 1.fr.pool.ntp.org +ACL restrictions enough to allow peering with associations
-server 2.fr.pool.ntp.org +Add a ''restrict source'' command without the ''nopeer'' flag, such as:
-server 3.fr.pool.ntp.org+
-server +  restrict source limited kod nomodify notrap noquery
-fudge stratum 10+
-restrict default nomodify nopeer notrap +If you want to allow clients to get time from this host, you must add a line to 
-restrict mask +further relax these restrictions. 
-</code>+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:
-Here's a little explanation for some options:+  restrict mask limited kod nomodify notrap nopeer
-  * The ''fudge stratum 10'' directive is a "dummy" server acting as fallback IP in case the external time source becomes momentarily unreachable. When this happens, NTP will continue to work and base itself on this "internal" server. +To let you control your NTP service from your maintenance VLAN and not just from 
-  * NTP has its own arsenal of rules to limit access to the service, which can be used independently from a firewall. The ''restrict'' directives in the above configuration prevent distant computers from changing the servers' configuration (first ''restrict'' statement)and the machine is configured to trust itself (second ''restrict'' statement).  +the host itselfyou might want to add the VLAN with no restrictions
-  * A ''restrict'' statement without any argument but followed by the hostname boils down to an ''allow all''.+Supposing the maintenance VLAN is '''', you could add:
-==== Manage the NTP service ====+  restrict mask
-Before starting the serviceproceed to an initial adjustment of your system clock:+If you want more sophisticated access control than what's described herefor 
 +example to encrypt traffic or let you authenticate for administration tasks 
 +from any host, look into the secure authentication features of NTP. See: 
 +Support]]" at //[[http://doc.ntp.org|The NTP Project]]//.
-# ntpdate pool.ntp.org 
-<note>The ''ntpdate'' command is normally considered obsolete, but it still comes in handy when performing important time adjustments. The "orthodox" way would be to use the ''ntpd -g'' command - the official replacement for ''ntpdate'' - but its use will fail if your system clock is off for more than half an hour.</note> 
-Activate the NTP service:+===== Diagnostic logging =====
-<code> +NTP prefers to use SYSLOG for logging. 
-# chmod +x /etc/rc.d/rc.ntpd +There is an alternate logging feature in NTP itself that can be used instead. 
-</code>+Looking again at the preinstalled Slackware ''/etc/ntp.conf'', the alternate 
 +logging feature has already been turned on:
-Manage the NTP service:+<note important> 
 +  logfile /var/log/ntp 
 +Not recommended. 
-<code> +It is simpler to delete the ''logfile'' line and use Slackware's preinstalled 
-/etc/rc.d/rc.ntpd start|stop|restart|status +SYSLOG package. 
-</code>+Using SYSLOG, NTP logs warnings and errors to ''/var/log/syslog'', and routine 
 +status messages to ''/var/log/messages''.
-Now display the list of servers your machine is actually connected to: 
-# ntpq -p 
-remote           refid      st t when poll reach   delay   offset jitter 
-*panopea.unstabl  2 u   30   64  377   56.136  -249.48 80.680 
-+88-190-17-126.r   2 u   29   64  377   77.571  -205.94 94.278 
-+      2 u   29   64  377   77.097  -249.57 85.641 
--ntp.univ-poitie   3 u   29   64  377   57.747  -191.58 107.002 
-LOCAL(0)        .LOCL.          10 l  164   64  374    0.000    0.000 0.001 
-The little ''*'' asterisk preceding one of the above lines means your machine is effectively synchronized with the respective NTP server.+==== Filtering the log ====
-<note tip>The first synchronization can take a few minutessometimes up to half an hour.</note>+NTP lets you filter certain messages out of the logbased on the message'
 +''class'' and ''type''Currently there are four classes defined:
-===== Synchronize your client PC(s) with your local NTP server =====+  clock peer sync sys
-In a LAN, it is considered good practice to synchronize only one machine - the server - with a public NTP server, and the client PCs with the local server. This saves bandwidth and takes some load off the public NTP servers.+and four types defined:
-As above, proceed to an initial adjustment of the system clock:+  info events status statistics
-<code> +Because the preinstalled Slackware ''/etc/ntp.conf'' does not customize the 
-# ntpdate pool.ntp.org +filter, you get out-of-box behavior. 
-</code>+NTP will pass messages that are tagged with the ''sync'' class and drop all 
 +messages that are tagged with any other class.
-Create an empty logfile:+<note important> 
 +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). 
-<code> +A better choice is to log all available diagnostic messages. 
-# touch /var/log/ntp.log +To do this, disable all filtering by class or type in ''/etc/ntp.conf'':
-Now configure NTP to synchronize with the LAN server. Replace the example's IP ('''') with your real server's IP:+  logconfig =allall
-# /etc/ntp.conf 
-driftfile /etc/ntp/drift 
-logfile /var/log/ntp.log 
-server Statistics gathering =====
-server +NTP can keep a statistical record of its performance, that you can analyze to 
-fudge stratum 10+check the health of your NTP-managed clock. 
 +The preinstalled Slackware ''/etc/ntp.conf'' already configures the directory 
 +path for these statistics:
-restrict default ignore +  statsdir /var/lib/ntp/stats
-restrict mask +
-restrict mask +
-  * The three ''restrict'' statements mean we're blocking all NTP traffic except for the client itself and the server.+But to actually collect statistics, you must create the empty directory and make 
 +it writable by the NTP daemon:
-Activate and start the NTP service:+  # mkdir /var/lib/ntp/stats 
 +  # chown ntp:ntp /var/lib/ntp/stats
-<code+and add a command to ''/etc/ntp.conf'' that identifies the statistics you want 
-chmod +/etc/rc.d/rc.ntpd +collected. 
-# /etc/rc.d/rc.ntpd start +The most commonly analyzed record is NTP's system clock updates in the 
-</code>+''loopstats'' file: 
 +  statistics loopstats 
 +There are a total of eight recordtypes that NTP will keep. 
 +For information, see: 
 +"[[https://doc.ntp.org/documentation/4.2.8-series/monopt/|Monitoring Options]]" 
 +at //[[http://doc.ntp.org|The NTP Project]]//
 +At the end of this HOWTO, there is an example of charting the loopstats using 
 +the preinstalled Slackware **gnuplot** package. 
 +<note important
 +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: 
 +<note important> 
 +  #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. 
 +<note important> 
 +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. 
 +<note important> 
 +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 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'': 
 +<note important> 
 +  server 
 +  fudge stratum 10 
 +Use of this clock driver is no longer recommended. 
 +The local clock driver is now deprecated, having 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: 
 +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. 
 +===== Startup ===== 
 +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 
 +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 
 +  fi 
 +===== Monitoring NTP ===== 
 +You can review how the pool discovery process is working with the command 
 +  # ntpq -n -p
-As above, use the ''ntpq -p'' command to check if the synchronization went well:+Here is some sample output:
 <code> <code>
-# ntpq -p 
      remote           refid      st t when poll reach   delay   offset  jitter      remote           refid      st t when poll reach   delay   offset  jitter
 ============================================================================== ==============================================================================
-*     3 u  916 1024  377    0.367    7.897   2.552 + pool.ntp.org    .POOL.          16 p    -   64    0    0.000   +0.000   0.000 
- LOCAL(0)        .LOCL         10 l  10h   64    0    0.000    0.000   0.000+-    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 
 +*      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
 </code> </code>
-<note tip>Againyou might have to wait few minutes before the first synchronization takes place.</note>+In a nutshellthis 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 
 +Options]]" at //[[http://doc.ntp.org|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 
 +^ 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. 
 +<file gnuplot> 
 +#! /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: 
 +{{ :howtos:network_services:ntp-gnuplot-example.png |}}
-=== Synchronizing at boot time === 
-If instead of using a daemon, all you want to do is to synchronize time once per boot, it is possible to add the command ''ntpd -qg &'' in the bottom of the file ''/etc/rc.d/rc.local''. This will synchronize the time once in the background and will exit and it needs ''/etc/rc.d/rc.ntpd'' to not be an executable. 
-  ~# cat /etc/rc.d/rc.local 
-  #!/bin/sh 
-  # 
-  # /etc/rc.d/rc.local:  Local system initialization script. 
-  # 
-  # Put any local startup commands in here.  Also, if you have 
-  # anything that needs to be run at shutdown time you can 
-  # make an /etc/rc.d/rc.local_shutdown script and put those 
-  # commands in there. 
-  . 
-  . 
-  . 
-  ntpd -qg & 
 ====== Sources ====== ====== Sources ======
   * Originally written by [[wiki:user:kikinovak|Niki Kovacs]]   * Originally written by [[wiki:user:kikinovak|Niki Kovacs]]
 +  * Performance monitoring section contributed by Dominik Drobek
 +  * Rewritten and updated to current best practice by [[wiki:user:metaed|Edward McGuire]].
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-{{tag>howtos time clock synchronization author_kikinovak}}+{{tag>howtos time clock synchronization author_kikinovak slackware_15.0 author_metaed}}
 howtos:network_services:ntp ()