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RedHat  Enterprise  Linux  7  has new service named  "chronyd".  RedHat recommends using this service for the NTP Server/Client.  Chrondy can be configured easily to run as a client or server. But if you will prefer to run  NTPD  classic service that you should disable "chronyd" service at boot. Because  "chronyd" service will deny "ntpd" service while the server  is booting.

If you prefer to use "chrony" service instead of  "ntpd" ,  you should define your NTP configuration on "/etc/chrony.conf " file. Then enable service.   On the other hand, to enable classic  "ntpd" service on your server you should follow these steps.

Step 1: Check  and  Disable  chrony

# systemctl status chronyd
● chronyd.service - NTP client/server
Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
Active: active (running) since Wed 2018-07-04 10:47:28 +03; 2h 33min ago
Process: 24275 ExecStartPost=/usr/libexec/chrony-helper update-daemon (code=exited, status=0/SUCCESS)
Process: 24271 ExecStart=/usr/sbin/chronyd $OPTIONS (code=exited, status=0/SUCCESS)
Main PID: 24273 (chronyd)
CGroup: /system.slice/chronyd.service
└─24273 /usr/sbin/chronyd


#systemctl disable  chronyd

Step 2:Configure  Classic  NTPD service

#vi   /etc/ntp.conf
driftfile /var/lib/ntp/drift
restrict default nomodify notrap nopeer noquery
restrict 127.0.0.1
restrict ::1
server ntpserver01 iburst
server ntpserver02 iburst
includefile /etc/ntp/crypto/pw
keys /etc/ntp/keys
disable monitor

Step 3:Start and Enable  Service

#ntpdate  ntpserver01
#systemctl enable ntpserver01
#systemctl start  ntpd