[root@m01 tmp]# cat /etc/redhat-release

CentOS release 6.9 (Final)

[root@m01 tmp]# hostname -I

10.0.0.61（外网） 172.16.1.61（内网）

[root@m01 tmp]# rpm -qa ntp

ntp-4.2.6p5-.el6.centos..x86_64

sed -i '8c restrict default nomodify' /etc/ntp.conf替换配置文件的第8行内容，也可以使用vim
vim /etc/ntp.conf

# restrict default kod nomodify notrap nopeer noquery

restrict default nomodify#允许局域网时间同步

# Use public servers from the pool.ntp.org project.

# Please consider joining the pool (http://www.pool.ntp.org/join.html).

#server .centos.pool.ntp.org iburst

#server .centos.pool.ntp.org iburst

#server .centos.pool.ntp.org iburst

#server .centos.pool.ntp.org iburst

#将原有的不可用的源注释，添加新的时间同步源

server ntp1.aliyun.com

server time.nist.gov

[root@m01 tmp]# /etc/init.d/ntpd start

Starting ntpd:                                             [  OK  ]

[root@m01 ~]# ntpstat

synchronised to NTP server (182.92.12.11) at stratum

   time correct to within  ms

   polling server every  s

[root@nfs01 ~]# ntpdate 172.16.1.61

 Oct :: ntpdate[]: adjust time server 172.16.1.61 offset 0.000652 sec
#时间服务器启动后，需要过几分钟才能进行同步。
ntpdate[5687]: no server suitable for synchronization found

[root@nfs01 ~]# crontab -l

#time sync by yanxinjiang at -10-10

*/ * * * * /usr/sbin/ntpdate 172.16.1.61  >/dev/null >&