Why do we need NTP? What does it look like, and how should it be set up? Some people say that the NTP protocol keeps all your host clocks in sync. Let's take a closer look!
The Network Time Protocol (NTP) is designed to synchronize system clocks across different hosts on a network. All managed devices can align their time with a designated time server known as an NTP server. In turn, the NTP server synchronizes itself with either a public NTP server or any server of your choice. This ensures that all NTP-managed devices maintain precise time down to the millisecond.
In enterprise environments, if a company doesn’t want to open its firewall for NTP traffic, setting up an internal NTP server becomes essential. Employees can then use internal servers instead of public ones. In this guide, we’ll show you how to configure a CentOS system as an NTP server. Before diving into the details, let’s first understand what NTP is.
**Why do we need NTP?**
Due to manufacturing differences, not all clocks run at exactly the same speed. Some may gain time, while others lose it. Over time, this leads to a phenomenon called "clock drift" or "time drift." To reduce the impact of this, systems using NTP should regularly connect to a specified NTP server to keep their clocks accurate.
Time synchronization between devices is crucial for operations such as scheduled backups, intrusion detection logs, distributed task scheduling, and transaction tracking. It might even be required by regulatory compliance standards.
**NTP Hierarchy**
NTP operates in a hierarchical structure, where each level is referred to as a stratum. The stratum level indicates how many NTP hops a machine is away from the authoritative time source.
Stratum 0 consists of highly accurate reference clocks, such as atomic clocks. These cannot be accessed directly over the network. Stratum 1 servers are directly connected to Stratum 0 devices. Stratum 2 servers synchronize with Stratum 1, and so on. NTP supports up to 15 strata, and Stratum 16 is considered out of sync and unusable.
**Preparing the CentOS Server**
Now, let’s walk through setting up an NTP server on CentOS.
First, ensure the server's time zone is correctly configured. In CentOS 7, you can use the `timedatectl` command to view and change the time zone (e.g., "Australia/Adelaide").
```bash
# timedatectl list-timezones | grep Australia
# timedatectl set-timezone Australia/Adelaide
# timedatectl
```
Next, install the necessary software using YUM:
```bash
# yum install ntp
```
Then, configure the NTP server by editing the `/etc/ntp.conf` file:
```bash
# vim /etc/ntp.conf
```
Add the following lines to define global NTP servers:
```
server 0.oceania.pool.ntp.org
server 1.oceania.pool.ntp.org
server 2.oceania.pool.ntp.org
server 3.oceania.pool.ntp.org
```
By default, NTP logs are stored in `/var/log/messages`. If you prefer a custom log file, specify it in the configuration:
```
logfile /var/log/ntpd.log
```
Make sure to adjust the ownership and SELinux context:
```bash
# chown ntp:ntp /var/log/ntpd.log
# chcon -t ntpd_log_t /var/log/ntpd.log
```
Start the NTP service and enable it to start at boot:
```bash
# systemctl restart ntp
# systemctl enable ntp
```
To verify that the local clock is synchronized, use the `ntpq` command:
```bash
# ntpq -p
```
This will display information about the NTP peers and their status.
**Controlling Access to the NTP Server**
By default, the NTP server allows queries from all hosts. To restrict access, you can add rules to the firewall. For example:
```bash
# iptables -A INPUT -s 192.168.1.0/24 -p udp --dport 123 -j ACCEPT
# iptables -A INPUT -p udp --dport 123 -j DROP
```
This rule allows NTP traffic only from the 192.168.1.0/24 network and blocks all other traffic. Adjust the rules according to your network needs.
**Configuring an NTP Client**
1. **Linux**
On Linux clients, the `ntpdate` package is used to synchronize time with an NTP server. Install it using YUM or APT, then run the command with the server IP address:
```bash
# ntpdate "server-IP-address"
```
This works similarly on RHEL and Debian-based systems.
2. **Windows**
On Windows, go to Date and Time settings and select “Internet Time†to configure synchronization.
3. **Cisco Devices**
For Cisco equipment, use the following command in global configuration mode:
```bash
# ntp server "server-IP-address"
```
With this setup, your network can maintain accurate time across all devices, ensuring smooth operations and compliance.
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