Why do we need NTP? What does it look like, and how should it be set up? Some people say that the NTP protocol ensures that all host clocks are synchronized. Let's take a closer look!
The Network Time Protocol (NTP) is designed to synchronize the system clocks of different hosts across a network. All managed devices can sync their time with a designated time server known as an NTP server. In turn, this NTP server can synchronize its time with any public NTP server or a preferred internal one. When properly configured, NTP allows all devices to maintain accurate time down to the millisecond level.
In enterprise environments, if a company doesn't want to open firewall ports for NTP traffic, it’s recommended to set up an internal NTP server. This way, employees can use internal servers instead of public ones. In this tutorial, we'll guide you through configuring a CentOS system as an NTP server. Before diving into the details, let's first understand what NTP is and why it matters.
**Why is NTP important?**
Due to manufacturing differences, not all clocks run at the same speed. Some may gain or lose time over time, leading to what is called "clock drift." To minimize this effect, systems using NTP should regularly connect to a specified NTP server to keep their clocks in sync.
Time synchronization is crucial for various operations such as scheduled backups, intrusion detection logs, distributed task scheduling, and transaction tracking. It might even be a requirement for compliance with certain regulations.
**NTP Hierarchy**
NTP operates in a hierarchical structure, where each level is referred to as a stratum. The stratum number indicates how many NTP hops a device is away from an authoritative time source.
Stratum 0 includes highly accurate clocks like atomic clocks, which are not directly accessible over the network. Stratum 1 servers are connected directly to Stratum 0 devices, while Stratum 2 servers sync with Stratum 1, and so on. NTP supports up to 15 strata, with Stratum 16 considered out of sync and unreliable.
**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 or change the time zone:
```bash
# timedatectl list-timezones | grep Australia
# timedatectl set-timezone Australia/Adelaide
# timedatectl
```
Next, install the NTP package using `yum`:
```bash
# yum install ntp
```
Then, configure the NTP server by editing the `/etc/ntp.conf` file. Add public NTP servers for synchronization:
```bash
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, you can specify it:
```bash
logfile /var/log/ntpd.log
```
Make sure to adjust ownership and SELinux context if needed:
```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 on boot:
```bash
# systemctl restart ntp
# systemctl enable ntp
```
To verify that the NTP server is working correctly, use the `ntpq` command:
```bash
# ntpq -p
```
This will show the status of the local clock and its synchronization with remote servers.
**Controlling Access to the NTP Server**
By default, the NTP server accepts queries from all hosts. To restrict access, you can configure your firewall. For example, allow NTP traffic only from a specific subnet:
```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 connections from the `192.168.1.0/24` network and blocks all others. Adjust the rules according to your network requirements.
**Configuring an NTP Client**
For Linux clients, install the `ntpdate` package and use it to synchronize time with the NTP server:
```bash
# ntpdate "server-IP-address"
```
On Windows, go to Date and Time settings and select "Internet Time" to configure it. For Cisco devices, use the following command in global configuration mode:
```bash
# ntp server "server-IP-address"
```
By implementing NTP, organizations can ensure consistent and accurate time across their networks, which is essential for security, logging, and operational efficiency.
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