Detailed explanation of 2 methods to synchronize network time in Linux/CentOS system

Due to hardware reasons, the machines may not keep up with the standard time to a certain extent, with the error ranging from a few seconds to a few minutes in a month. If the server time is inaccurate, it will cause a lot of trouble. For example, when making a payment, you are unable to place an order or the game cannot be logged in.

Method 1: Use ntpdate to update the time from the time server

If the system does not have the ntpdate command, you can install it online:

yum -y install ntp

After installation, you don’t need to do any configuration, just test it directly

[root@snsgou-pc src]# date
Wednesday, May 20, 2015 22:42:19 CST
[root@snsgou-pc src]# ntpdate time.nist.gov 
20 May 22:42:38 ntpdate[26759]: step time server 131.107.13.100 offset 2.117558 sec
[root@snsgou-pc src]# date
Wednesday, May 20, 2015 22:43:17 CST

The above situation indicates that the network time synchronization is successful.

We can use scheduled tasks to synchronize time regularly

Use the crontab -e command to enter the crontab editing state and append the following scheduled task text

*/10 * * * * ntpdate time.nist.gov #domain name or IP

Indicates synchronization every ten minutes. Recommended time servers:

time.nist.gov
time.nuri.net
0.asia.pool.ntp.org
1.asia.pool.ntp.org
2.asia.pool.ntp.org
3.asia.pool.ntp.org

Method 2: Use ntp to build your own time server

When we build our own time server, we don't need crontab to run it regularly.

1. Install the time server ntp

yum install ntp

2. Configure ntp

[root@localhost ~]# cat /etc/ntp.conf |awk '{if($0 !~ /^$/ && $0 !~ /^#/) {print $0}}'
restrict default ignore //By default, modification or query of ntp is not allowed, and special packets are not received restrict 127.0.0.1 //Give all permissions to the local machine restrict 192.168.1.0 mask 255.255.255.0 notrap nomodify //Give the LAN machine the permission to synchronize time server time.nist.gov prefer //Set the time server, add prefer to indicate priority server 0.asia.pool.ntp.org
server 1.asia.pool.ntp.org
server 2.asia.pool.ntp.org
server 127.127.1.0 # local clock
fudge 127.127.1.0 stratum 10
driftfile /var/lib/ntp/drift
keys /etc/ntp/keys

3. Start ntp

[root@localhost ~]# /etc/init.d/ntpd start

4. Check and test

[root@localhost ~]# netstat -upnl |grep ntpd //View the schedule [root@localhost ~]# ntpq -pn //View the synchronized server IP
 remote refid st t when poll reach delay offset jitter
==============================================================================
 50.77.217.185 .INIT. 16 u - 64 0 0.000 0.000 0.000
 202.90.158.4 .INIT. 16 u - 64 0 0.000 0.000 0.000
 202.71.100.89 .INIT. 16 u - 64 0 0.000 0.000 0.000
 202.134.1.10 .INIT. 16 u - 64 0 0.000 0.000 0.000
*127.127.1.0 .LOCL. 10 l 18 64 377 0.000 0.000 0.001
 
[root@localhost ~]# ntpstat //Synchronised to local net at stratum 11
 time correct to within 12 ms
 Polling server every 512 seconds

remote: the IP or host name of the NTP host. Pay attention to the symbol on the far left. If it is a "+", it means that the upper-level NTP is currently in effect. If it is a "*", it means that it is also online, but it is a secondary NTP host.

refid: the address of the upper-layer NTP host to which reference is applied

st: stratum

when: The time synchronization update operation was performed a few seconds ago

poll: The next update will be in a few seconds

reach: The number of times the upper-level NTP server has been requested to update

Delay: The delay time of the network transmission process

offset: the result of time compensation

jitter: The difference between Linux system time and BIOS hardware time

The above are the two methods of synchronizing network time in Linux system that this article will talk about. I hope it will be helpful to everyone.