Time synchronization is crucial for the proper functioning of servers and applications running on the CentOS 7 operating system. Precise time synchronization ensures that log files, transactions, and other time-sensitive tasks are consistent and reliable. This article provides a detailed overview of how to resolve time and time synchronization issues on CentOS 7.

Basic Concepts

NTP (Network Time Protocol) is a protocol used to synchronize the clocks of computers over networks with variable latency. CentOS 7 typically utilizes the chronyd or ntpd daemons for time synchronization using NTP.

chronyd vs. ntpd: chronyd is the default time synchronization daemon in CentOS 7, designed for systems that frequently change networks or are in sleep mode. ntpd is the traditional time synchronization daemon that can be used on stable servers with continuous network connectivity.

Installation and Configuration of NTP

1. Installation of chronyd

   • To install chronyd, use the command:
   • After installation, enable and start chronyd:

2. Configuration of chronyd

   • The main configuration file for chronyd is /etc/chrony.conf. Here, you can set NTP servers for synchronization.
   • Example configuration for using public NTP servers:

     sudo systemctl restart chronyd
     ​

   • After making changes in the configuration, restart chronyd:

     sudo systemctl restart chronyd
     ​

3. Verification of functionality

   • To verify that your system is properly synchronized, use the command:

     chronyc sources
     ​

   • This command will display a list of NTP servers your system is connected to and the synchronization status.

Troubleshooting Time Issues

1. Checking daemon status

   • Ensure that the chronyd or ntpd service is running:

     sudo systemctl status chronyd
     ​

   • If the service is not active, start it.

2. Firewall configuration

   • Check if the firewall is blocking incoming or outgoing communication on port 123/UDP, which is used for NTP.

3. Manual time setting

   • If you need to quickly set the time, you can use the timedatectl command:

     sudo timedatectl set-time 'YYYY-MM-DD HH:MM:SS'
     ​

   • This is not recommended for long-term use but can help in diagnosing issues.

Proper configuration and maintenance of time synchronization on the CentOS 7 system are crucial for ensuring the reliability and security of your system. By following the steps outlined above and regularly checking the configuration and status of services, you can ensure that your systems have accurate time, which is fundamental for the proper functioning of most networked applications.