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Troubleshooting NTP on Red Hat Linux

News NTP -- Network Time Protocol Recommended Links RHEL NTP configuration Changing timezone in RHEL6 from the command line Redhat Network Configuration RHEL handling of DST change
Configuration of Solaris NTP server Troubleshooting NTP on Solaris   ILO 3 NTP configuration   Humor Etc

Note: Some versions of Red Hat Linux do not propagate NTP setting you put during installation to Anaconda. In this case you need to set up NTP manually after first reboot.

Preliminary Steps

  1. Check if you have a correct timezone How do I change my system timezone from the command line without using redhat-config-date?

    In order to change the timezone of your system you need to edit /etc/sysconfig/clock:


    Note: If your system's BIOS has UTC set to true, then set UTC to true.UTC setting in the configuration file must always reflect your actual BIOS settings.

    ... ... ...

    Finally save the file /etc/sysconfig/clock and on next reboot the system time will be set to the defined timezone.

    For the time on the machine to reflect the change timezone we need to link the zoneinfo file to /etc/localtime. This can be done as follows :

    # ln -sf /usr/share/zoneinfo/Brazil/East /etc/localtime
    Now by typing the date command to display the time you should see if reflect the newly linked timezone :
    # date
    Thu Sep 30 10:06:23 BRT 2004
  2. Check if /etc/ntp/ntpservers contains NTP servers that you need and correct them if necessary (the default set are Red Hat NTP servers that might be not accessible because of firewall or proxy):

    vi /etc/ntp/ntpservers
  3. Run ntpdate to check if it will synchronize your clock with a time server. ntpd will not synchronize your clock with a time server if your system clock is significantly off NTP time.
    ntpdate -u
  4. Check if ntpdis configured to start on the runlevels you work (typically 3 and 5).
    chkconfig ntpd on
  5. Try to start the ntpd daemon manually.
    service ntpd start

Note: The graphical user interface utility redhat-config-date (Red Hat Linux) or system-config-date (Fedora Core) may also be used to administer NTP.

Using ntpq -p command

The most useful command for querying any time server's status is "ntpq -p". You can query a remote time server for the same kind of information with "ntpq -p <hostname>".

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
*      2 u  781 1024  377    2.003   -0.466   0.434      2 u  984 1024  377   19.834    1.209   0.228
The first position in the output lines is a quick indicator of the status of the remote server. A "*" indicates the server to which you are currently synchronized. It may take some part of an hour before your server considers itself synchronized. A "+" is a candidate to take the leading role if the current server becomes unsuitable for some reason.

The labeled columns for this are:

The IP address or DNS name of the remote server
An identification of the type of the reference clock.
The "stratum" or level of the server: for almost all systems, 2 is great. Your local system will have a higher number.
The type of service. Your setup will show "l" for local on your local system, or "u" for "unicast" for communicating with remote servers.
This is the number of seconds since the server was last heard from. After a couple of minutes of operation your server should start to report numeric values here.
Current polling interval in seconds. When remote servers are responding, "when" should be no greater than "poll".
This and the remaining fields are important indicators of the health of your local server, your remote servers, and their communication. This field is really a bit array indicating whether responses have been received to your local server's eight most recent requests. The value starts at 0. If your local server is receiving responses to all its requests, it will go to 1, then 3, then 7. The display is in octal, so 377 is the maximum value. Anything less indicates that either your local server recently started or some requests did not receive responses.
Recent average roundtrip time in milliseconds from request to response.
Estimated differential between your system clock and this time server's clock, in milliseconds. You may consider this the "bottom line" on the accuracy of your system clock. NTP can usually drive this down to the level of the jitter or less.
A measure of the variability of the delays between request and receipt of a response, in milliseconds. High jitter tends to limit your server's ability to synchronize accurately.

Using tcpdump

You can easily check if packets are going out from your system and coming back to your box using tcpdump. NTP uses port 123, so you might issue a command like this as root:

  # tcpdump udp port 123

Allow it to monitor traffic for up to an hour. You should see packets going out to your time servers port 123 (ntp) and replies coming back from each of them. Your local UDP port will probably also be number 123. Kernel firewalling (ipchains or iptables) can prevent synchronization from working even if the servers are replying to your system's requests.

If you see packets coming in from the remote time servers, but ntpq indicates your server is not seeing them, then be sure to look at your ipchains/iptables configuration

Also check the system log for configuration messages from ntpd.

Using syslog

Your ntpd will log various events through your system's syslog facility. This typically means the messages will show up in /var/log/messages, in lines containing the string "ntpd". Configuration problems are reported here. If your local server steps the time ahead or back suddenly, it will report that here also with a message such as: time reset -6.394626 s.

Be careful if your clock is ahead of time

You should be careful adjusting your clock if it is significantly ahead of the actual time. You can use it to prevent ntpd from making large sudden adjustments to your system clock while the system is running, and to make sure the system clock never jumps backward in time. The NTP FAQ suggests doing the following steps if you can take your server down for a little while:

  1. Shutdown;
  2. Set the BIOS clock;
  3. Restart.

For systems with clocks ahead of real time, plan to leave the system off until real time catches up with its internal time when it was shut down. Since the system clock is initialized from the BIOS clock, this prevents the system clock from jumping back while your system is running.

For systems with clocks ahead of real time, plan to leave the system off until real time catches up with its internal time when it was shut down.

Recall that Linux can be configured to interpret the BIOS time as either local time or GMT (UTC). You can infer the configuration by comparing output from the "date" command with the BIOS clock time.

If case BIOS is configured with UTC time, Linux can automatically adjust time to summer time in spring and winter time in autumn.

If case BIOS is configured with UTC time, Linux can automatically adjust time to summer time in spring and winter time in autumn.

See RHEL handling of DST change

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Old News ;-)

[Dec 01, 2015] HowTo Verify My NTP Working Or Not

by Vivek Gite on March 25, 2010 last updated November 29, 2015

... You can use any one of the following program to verify ntp client configuration:

  1. ntpq - standard NTP query program
  2. ntpstat - show network time synchronisation status
  3. timedatectl - show or set info about ntp using systemd

ntpstat command

The ntpstat command will report the synchronisation state of the NTP daemon running on the local machine. If the local system is found to be synchronised to a reference time source, ntpstat will report the approximate time accuracy.

You can use the exit status (return values) to verify its operations from a shell script or command line itself:

Type the command as follows:
$ ntpstat
Sample outputs:

synchronised to NTP server ( at stratum 3
   time correct to within 42 ms
   polling server every 1024 s

Use the echo command to display exit status of ntp client:
$ echo $?
Sample outputs:


ntpq command

The ntpq utility program is used to monitor NTP daemon ntpd operations and determine performance. The program can be run either in interactive mode or controlled using command line arguments. Type the following command
$ ntpq -pn
$ ntpq -p
Sample outputs:

     remote           refid      st t when poll reach   delay   offset  jitter
*      2 u  509 1024  377   51.661   -3.343   0.279
+ns1.your-site.c   3 u  899 1024  377   48.395    2.047   1.006       2 u  930 1024  377    0.693    1.035   0.241
 LOCAL(0)        .LOCL.          10 l   45   64  377    0.000    0.000   0.001

The above is an example of working ntp client. Where,

  1. -p : Print a list of the peers known to the server as well as a summary of their state.
  2. -n : Output all host addresses in dotted-quad numeric format rather than converting to the canonical host names.

A note about timedatectl command

If you are using systemd based system, run the following command to check the service status
# timedatectl status

systemd-timesyncd configuration

If NTP enabled is set to No. Try configuring by editing /etc/systemd/timesyncd.conf file as follows:
# vi /etc/systemd/timesyncd.conf
Append/edit [Time] as follows i.e. add time servers or change the provided ones, uncomment the relevant line and list their host name or IP separated by a space (default from my Debian 8.x server):


Save and close the file. Finally, start and enable it, run:
# timedatectl set-ntp true
# timedatectl status

[Oct 26, 2013] RHEL handling of DST change

Most server hardware clocks are use UTC. UTC stands for the Universal Time, Coordinated, also known as Greenwich Mean Time (GMT). Other time zones are determined by adding or subtracting from the UTC time. Server typically displays local time, which now is subject of DST correction twice a year.

Wikipedia defines DST as follows:

Daylight saving time (DST), also known as summer time in British English, is the convention of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour in late winter or early spring and are adjusted backward in autumn.

DST patch is only required in few countries such as USA.Please see this wikipedia article.

Linux will change to and from DST when the HWCLOCK setting in /etc/sysconfig/clock is set to -u, i.e. when the hardware clock is set to UTC (which is closely related to GMT), regardless of whether Linux was running at the time DST is entered or left.

When the HWCLOCK setting is set to `--localtime', Linux will not adjust the time, operating under the assumption that your system may be a dual boot system at that time and that the other OS takes care of the DST switch. If that was not the case, the DST change needs to be made manually.


EST is defined as being GMT -5 all year round. US/Eastern, on the other hand, means GMT-5 or GMT-4 depending on whether Daylight Savings Time (DST) is in effect or not.

The tzdata package contains data files with rules for various timezones around the world. When this package is updated, it will update multiple timezone changes for all previous timezone fixes.

[Sep 21, 2012 ] How to Configure NTP in a RHEL-CentOS Vmware Guest

Successful time keeping in a Virtual Machine can be a bit confusing. At times I have been told to use Vmware Tools to sync time between the Guest and the Host, and at time I have been advised to avoid this functionality and use NTP. The following information is direct from a VMware KB article (updated, 4/16/2010) so I am going to follow their lead on this and use NTP exclusively.

First off VMware advises using the NTP service to keep time in sync, but it suggests using an additional kernel parameter that you add to your grub.conf. See the KB Article for more info on how to do this.

Note that there are no additional params needed for 5.4

Also, inside the ntp.conf the following line should be added to the top of the file.

tinker panic 0 
This configuration directive instructs NTP not to give up if it sees a large jump in time. This is important for coping with large time drifts and also resuming virtual machines from their suspended state.

It is also important not to use the local clock as a time source, often referred to as the Undisciplined Local Clock. NTP has a tendency to fall back to this in preference to the remote servers when there is a large amount of time drift.

An example of such a configuration is below. You should comment out both lines.

fudge stratum 10 

Also, if you are using ntp, you want to make sure that you disable Vmware tools time sync. You can do so with the following command via the guest OS.

[ Sep 21, 2012] Configuring the NTP Daemon by Ed Halley

...Red Hat Linux comes with a sample /etc/ntp.conf, but if you just want your machine to synchronize with a public server, I recommend removing that file and starting with a far simpler script. Remove the existing file and create one that looks like the following.

# A very simple client-only ntp configuration.
server # local clock
fudge stratum 10 
driftfile /etc/ntp/drift
restrict default ignore
restrict mask
authenticate no

Choose your preferred NTP servers. Note that some servers are not available to the public, and some servers are too busy to handle new requests. Please do your homework in choosing appropriate servers that are geographically near you, and which will accept new public connections, and whether you must obtain their permission first.

You should select two or three servers for best results. For each server you find, discover its hostname and its fixed numerical address. Then add lines like the following to the /etc/ntp.conf file you created above.

server ntp5.someserver.etc
restrict nomodify  # ntp5.someserver.etc

(The over-simplified Red Hat Linux time and date settings application allow a single-server configuration, but usually mangles the lines in the file, unfortunately, so don't use that utility once you've edited your own /etc/ntp.conf manually.)

Next, a separate ticker file must be created that lists the servers. The following command will read the /etc/ntp.conf file you've made, and create the proper list of servers for the ticker file. This finds the hostnames listed on lines beginning with 'server', except for the initial local clock, and lists them in the ticker file.

$ su -
# awk '/^server/ {print $2}' /etc/ntp.conf | grep -v '' > /etc/ntp/step-tickers

Before starting the service, roughly synchronize your clock using one of the servers with ntpdate. Ensure your configuration is set to run the NTP daemon ntpd to track and alleviate clock drifting.

$ su -
# chkconfig --list ntpd
ntpd        0:off   1:off   2:off   3:off   4:off   5:off   6:off
# ntpdate -u ntp5.someserver.etc
 9 Dec 15:15:15 ntpdate[12345]: step time server offset 7.879896 sec
# chkconfig --level 345 ntpd on 

If your system clock is likely more than a few minutes off, it is best to shut down as many applications as possible before performing gross clock corrections. Some programs may have bugs which cause them to lose data or crash. A minimal environment such as running at runlevel 1 (single user mode) is the ideal situation for drastic clock changes.

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