Managing time
by Uday R. Sawant, William Leemans, Jonathan Hobson, Oliver Pelz
Linux: Powerful Server Administration
- Linux: Powerful Server Administration
- Table of Contents
- Linux: Powerful Server Administration
- Linux: Powerful Server Administration
- Credits
- Preface
- 1. Module 1
- 1. Managing Users and Groups
- 2. Networking
- Introduction
- Connecting to a network with a static IP
- Installing the DHCP server
- Installing the DNS server
- Hiding behind the proxy with squid
- Being on time with NTP
- Discussing load balancing with HAProxy
- Tuning the TCP stack
- Troubleshooting network connectivity
- Securing remote access with OpenVPN
- Securing a network with uncomplicated firewall
- Securing against brute force attacks
- Discussing Ubuntu security best practices
- 3. Working with Web Servers
- Introduction
- Installing and configuring the Apache web server
- Serving dynamic contents with PHP
- Hosting multiple websites with a virtual domain
- Securing web traffic with HTTPS
- Installing Nginx with PHP_FPM
- Setting Nginx as a reverse proxy
- Load balancing with Nginx
- Setting HTTPs on Nginx
- Benchmarking and performance tuning of Apache
- Securing the web server
- Troubleshooting the web server
- 4. Working with Mail Servers
- 5. Handling Databases
- Introduction
- Installing relational databases with MySQL
- Storing and retrieving data with MySQL
- Importing and exporting bulk data
- Adding users and assigning access rights
- Installing web access for MySQL
- Setting backups
- Optimizing MySQL performance – queries
- Optimizing MySQL performance – configuration
- Creating MySQL replicas for scaling and high availability
- Troubleshooting MySQL
- Installing MongoDB
- Storing and retrieving data with MongoDB
- 6. Network Storage
- 7. Cloud Computing
- 8. Working with Containers
- Introduction
- Installing LXD, the Linux container daemon
- Deploying your first container with LXD
- Managing LXD containers
- Managing LXD containers – advanced options
- Setting resource limits on LXD containers
- Networking with LXD
- Installing Docker
- Starting and managing Docker containers
- Creating images with a Dockerfile
- Understanding Docker volumes
- Deploying WordPress using a Docker network
- Monitoring Docker containers
- Securing Docker containers
- 9. Streaming with Ampache
- 10. Communication Server with XMPP
- 11. Git Hosting
- Introduction
- Installing Git
- Creating a local repository with Git CLI
- Storing file revisions with Git commit
- Synchronizing the repository with a remote server
- Receiving updates with Git pull
- Creating repository clones
- Installing GitLab, your own Git hosting
- Adding users to the GitLab server
- Creating a repository with GitLab
- Automating common tasks with Git hooks
- 12. Collaboration Tools
- 13. Performance Monitoring
- 14. Centralized Authentication Service
- 2. Module 2
- 1. Installing CentOS
- Introduction
- Downloading CentOS and confirming the checksum on Windows or OS X
- Creating USB installation media on Windows or OS X
- Performing an installation of CentOS using the graphical installer
- Running a netinstall over HTTP
- Installing CentOS 7 using a kickstart file
- Getting started and customising the boot loader
- Troubleshooting the system in rescue mode
- Updating the installation and enhancing the minimal install with additional administration and development tools
- 2. Configuring the System
- Introduction
- Navigating text files with less
- Introduction to Vim
- Speaking the right language
- Synchronizing the system clock with NTP and the chrony suite
- Setting your hostname and resolving the network
- Building a static network connection
- Becoming a superuser
- Customizing your system banners and messages
- Priming the kernel
- 3. Managing the System
- Introduction
- Knowing and managing your background services
- Troubleshooting background services
- Tracking system resources with journald
- Configuring journald to make it persistent
- Managing users and their groups
- Scheduling tasks with cron
- Synchronizing files and doing more with rsync
- Maintaining backups and taking snapshots
- Monitoring important server infrastructure
- Taking control with GIT and Subversion
- 4. Managing Packages with YUM
- 5. Administering the Filesystem
- 6. Providing Security
- 7. Building a Network
- 8. Working with FTP
- 9. Working with Domains
- 10. Working with Databases
- 11. Providing Mail Services
- 12. Providing Web Services
- 13. Operating System-Level Virtualization
- 14. Working with SELinux
- 15. Monitoring IT Infrastructure
- 1. Installing CentOS
- 3. Module 3
- 1. Working with KVM Guests
- 2. Deploying RHEL "En Masse"
- 3. Configuring Your Network
- 4. Configuring Your New System
- 5. Using SELinux
- 6. Orchestrating with Ansible
- 7. Puppet Configuration Management
- 8. Yum and Repositories
- 9. Securing RHEL 7
- 10. Monitoring and Performance Tuning
- Bibliography
- Index
RHEL 7 comes preinstalled with Chrony. While everybody knows Ntpd, Chrony is a newcomer to the game of timekeeping.
Chrony is a set of programs that maintains the time on your computer using different time sources, such as NTP servers, your system's clock, and even custom-made scripts/programs. It also calculates the rate at which the computer loses or gains time to compensate while no external reference is present—for example, if your NTP server(s) is(are) down.
Chrony is a good solution for systems which are intermittently disconnected and reconnected to a network.
Ntpd should be considered for systems that are normally kept on permanently.
When talking about managing time in RHEL, it can be done through:
- Chrony
- Ntpd
We'll take a look at each of the methods separately.
Ensure that chrony is installed and enabled, and perform the following steps:
- First, install
chronythrough the following command:~]# yum install -y chrony - Enable
chrony, as follows:~]# systemctl enable chrony ~]# systemctl start chrony
- Now, open
/etc/chrony.confwith your favorite editor and look for lines starting with theserverdirective using the following commands:server 0.rhel.pool.ntp.org iburst server 1.rhel.pool.ntp.org iburst server 2.rhel.pool.ntp.org iburst server 3.rhel.pool.ntp.org iburst
- Next, replace these lines with NTP servers that are near you and save the file:
server 0.pool.ntp.mydomain.lan iburst server 1.pool.ntp.mydomain.lan iburst
The
iburstoption causes NTP to send a burst of eight packets at the next poll instead of just one if the time master is unavailable, causing the NTP daemon to speed up time synchronization. - Finally, restart
chronyby executing the following command:~]# systemctl restart chrony
Ensure that ntpd is installed and enabled, and perform the following steps:
- First, install
ntpdby running the following:~]# yum install -y ntpd - Enable
ntpdthrough this command:~]# systemctl enable ntpd - Open
/etc/ntp.confwith your favorite editor and look for the lines starting with theserverdirective. Run the following:server 0.rhel.pool.ntp.org iburst server 1.rhel.pool.ntp.org iburst server 2.rhel.pool.ntp.org iburst server 3.rhel.pool.ntp.org iburst
- Replace these lines with the NTP servers near you and save the file:
server 0.pool.ntp.mydomain.lan iburst server 1.pool.ntp.mydomain.lan iburst
- Replace the contents of
/etc/ntp/step-tickerswith all your NTP servers, one per line:0.pool.ntp.mydomain.lan 1.pool.ntp.mydomain.lan
- Now, restart
ntpdby executing the following:~]# systemctl restart ntpd
While ntpd is the obvious choice for time synchronization, it doesn't fare well in environments where time masters are intermittently accessible (for whatever reason). In these environments, chronyd thrives. Also, ntpd can be quite complex to configure correctly, whereas chronyd is a little bit simpler.
The reason for modifying /etc/ntp/step-tickers when using the ntpd file is for the startup of the service. It uses ntpdate to synchronize time in one step before actually starting the NTP daemon itself, which is a lot slower in synchronizing time.
To figure out whether your system is synchronized, use the following command:
- For
chrony, use the following command:~]# chronyc sources - For
ntpd, run the following:~]# ntpq -p
Your output will be similar to:
remote refid st t when poll reach delay offset jitter ===================================================================== LOCAL(0) .LOCL. 5 l 60m 64 0 0.000 0.000 0.000 *master.exam 178.32.44.208 3 u 35 128 377 0.214 -0.651 14.285
The asterisk (*) in front of an entry means that your system is synchronized to this remote system's clock.
For more information on configuring chrony for RHEL 7, go to https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/ch-Configuring_NTP_Using_the_chrony_Suite.html.
For more information on configuring ntpd for RHEL 7, go to https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/ch-Configuring_NTP_Using_ntpd.html.
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