Build a Public NTP Server

Install NTPd and Configure NTP server for time adjustment.

[root@dlp ~]# yum -y install ntp
[root@dlp ~]# vi /etc/ntp.conf
# line 18: add the network range you allow to receive requests publicly
restrict 0.0.0.0 mask 128.0.0.0 nomodify notrap

# change servers for synchronization

#server 0.centos.pool.ntp.org iburst
#server 1.centos.pool.ntp.org iburst
#server 2.centos.pool.ntp.org iburst
#server 3.centos.pool.ntp.org iburst
server bsti1.time.gov.bd iburst
server bsti2.time.gov.bd iburst
[root@dlp ~]# systemctl start ntpd
[root@dlp ~]# systemctl enable ntpd

If Firewalld is running, allow NTP service. NTP uses 123/UDP.

[root@dlp ~]# firewall-cmd --add-service=ntp --permanent
success
[root@dlp ~]# firewall-cmd --reload
success

Verify it works normally like follows.

[root@dlp ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
+ntp1.jst.mfeed. 172.29.1.100 2 u 29 64 1 18.826 -0.126 0.000
+ntp2.jst.mfeed. 172.29.2.50 2 u 28 64 1 21.592 0.018 0.000
*ntp3.jst.mfeed. 133.243.236.18 2 u 28 64 1 22.666 -1.033 0.000

Next verify query an NTP server under Windows:

C:\> w32tm /stripchart /computer:foo
Tracking foo [1.2.3.4].
The current time is 19/07/2012 17:39:48 (local time).
17:39:48 d:-00.0000840s o:-25.3004160s [@ | ]
17:39:50 d:-00.0000680s o:-25.3070080s [@ | ]
17:39:52 d:-00.0000690s o:-25.2987405s [@ | ]
17:39:54 d:-00.0000680s o:-25.3071530s [@ | ]
17:39:56 d:-00.0000690s o:-25.2993985s [@ | ]
17:39:58 d:-00.0000680s o:-25.3070270s [@ |

Wireshark shows NTP packets being generated by this. Some versions of w32tm have a /query option

C:\>w32tm /stripchart /computer:us.pool.ntp.org /dataonly /samples:5
Tracking us.pool.ntp.org [208.87.221.228:123].
Collecting 5 samples.
The current time is 19/07/2012 18:36:32.
18:36:32, +03.3575094s
18:36:34, +03.3573679s
18:36:37, +03.3572986s
18:36:39, +03.3570290s
18:36:41, +03.3565357s

You can parallel check the NTP system using windows GUI tool such as Bytefusion’s NTPQuery. or Galleon

Src: 

https://www.server-world.info/en/note?os=CentOS_7&p=ntp&f=1

https://superuser.com/questions/451018/how-can-i-query-an-ntp-server-under-windows/451066

 

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