Red Hat System Administration I

Objectives

After completing this section, students should be able to maintain accurate time synchronization and time zone configuration to ensure correct timestamps in system logs.

Set local clocks and time zone

Correct synchronized system time is very important for log file analysis across multiple systems. The Network Time Protocol (NTP) is a standard way for machines to provide and obtain correct time information on the Internet. A machine may get accurate time information from public NTP services on the Internet such as the NTP Pool Project. A high-quality hardware clock to serve accurate time to local clients is another option.

The timedatectl command shows an overview of the current time-related system settings, including current time, time zone, and NTP synchronization settings of the system.

[student@serverX ~]$ timedatectl
      Local time: Thu 2014-02-13 02:16:15 EST
  Universal time: Thu 2014-02-13 07:16:15 UTC
        RTC time: Thu 2014-02-13 07:16:15
        Timezone: America/New_York (EST, -0500)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: no
 Last DST change: DST ended at
                  Sun 2013-11-03 01:59:59 EDT
                  Sun 2013-11-03 01:00:00 EST
 Next DST change: DST begins (the clock jumps one hour forward) at
                  Sun 2014-03-09 01:59:59 EST
                  Sun 2014-03-09 03:00:00 EDT

A database with known time zones is available and can be listed with:

[student@serverX ~]$ timedatectl list-timezones
Africa/Abidjan
Africa/Accra
Africa/Addis_Ababa
Africa/Algiers
Africa/Asmara
Africa/Bamako
...

Time zone names are based on the public "tz" (or "zoneinfo") time zone database maintained by IANA. Time zones are named based on continent or ocean, then typically but not always the largest city within the time zone region. For example, most of the US Mountain time zone is "America/Denver."

Selecting the correct name can be non-intuitive in cases where localities inside the time zone have different daylight saving time rules. For example, in the USA, much of the state of Arizona (US Mountain time) does not have a daylight saving time adjustment at all and is in the time zone "America/Phoenix."

The command tzselect is useful for identifying correct zoneinfo time zone names. It interactively prompts the user with questions about the system's location, and outputs the name of the correct time zone. It does not make any changes to the time zone setting of the system.

The system setting for the current time zone can be adjusted as user root:

[root@serverX ~]# timedatectl set-timezone America/Phoenix
[root@serverX ~]# timedatectl
      Local time: Thu 2014-02-13 00:23:54 MST
  Universal time: Thu 2014-02-13 07:23:54 UTC
        RTC time: Thu 2014-02-13 07:23:53
        Timezone: America/Phoenix (MST, -0700)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

To change the current time and date settings with the timedatectl command, the set-time option is available. The time is specified in the "YYYY-MM-DD hh:mm:ss" format, where either date or time can be omitted. To change the time to 09:00:00, run:

[root@serverX ~]$ timedatectl set-time 9:00:00
[root@serverX ~]$ timedatectl
      Local time: Thu 2014-02-13 09:00:27 MST
  Universal time: Thu 2014-02-13 16:00:27 UTC
        RTC time: Thu 2014-02-13 16:00:28
        Timezone: America/Phoenix (MST, -0700)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

The set-ntp option enables or disables NTP synchronization for automatic time adjustment. The option requires either a true or false argument to turn it on or off. To turn on NTP synchronization, run:

[student@desktopX ~]$ timedatectl set-ntp true

Configuring and monitoring chronyd

The chronyd service keeps the usually-inaccurate local hardware clock (RTC) on track by synchronizing it to the configured NTP servers, or if no network connectivity is available, to the calculated RTC clock drift which is recorded in the driftfile specified in the /etc/chrony.conf configuration file.

By default, chronyd uses servers from the NTP Pool Project for the time synchronization and does not need additional configuration. It may be useful to change the NTP servers when the machine in question is on an isolated network.

The quality of an NTP time source is determined by the stratum value reported by the time source. The stratum determines the number of hops the machine is away from a high-performance reference clock. The reference clock is a stratum 0 time source. An NTP server directly attached to it is a stratum 1, while a machine synchronizing time from the NTP server is a stratum 2 time source.

There are two categories of time sources that can be configured in the /etc/chrony.conf configuration file, server and peer. The server is one stratum above the local NTP server, and the peer is at the same stratum level. More than one server and more than one peer can be specified, one per line.

The first argument of the server line is the IP address or DNS name of the NTP server. Following the server IP address or name, a series of options for the server can be listed. It is recommended to use the iburst option, because after the service starts, four measurements are taken in a short time period for a more accurate initial clock synchronization.

To reconfigure the chronyd server to synchronize with classroom.example.com instead of the default servers configured in the /etc/chrony.conf, remove the other server entries and replace them with the following configuration file entry:

# Use public servers from the pool.ntp.org project.
server classroom.example.com iburst

After pointing chronyd to the local time source, classroom.example.com, the service needs to be restarted:

[root@serverX ~]# systemctl restart chronyd

The chronyc command acts as a client to the chronyd service. After setting up NTP synchronization, it is useful to verify the NTP server was used to synchronize the system clock. This can be achieved with the chronyc sources command or, for more verbose output with additional explanations about the output, chronyc sources -v:

[root@serverX ~]$ chronyc sources -v
210 Number of sources = 1

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||                                                /   xxxx = adjusted offset,
||         Log2(Polling interval) -.             |    yyyy = measured offset,
||                                  \            |    zzzz = estimated error.
||                                   |           |                         
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* classroom.example.com         8   6    17    23   -497ns[-7000ns] +/-  956us

The * character in the S (Source state) field indicates that the classroom.example.com server has been used as a time source and is the NTP server the machine is currently synchronized to.