安装NTP包
检查是否安装了ntp相关包。如果没有安装ntp相关包，使用rpm或yum安装，安装也非常简单方便。
如何检查是否安装：service ntp status 查看是否有此服务
yum install -y ntp
2.进入ntp配置目录：
vim /etc/ntp.conf
http://blog.csdn.net/scorpio3k/article/details/49645219
常见错误解决：
http://www.blogjava.net/spray/archive/2008/07/10/213964.html
NTP 4.2.7p441-1 "No association ID's returned"
问题解决：https://bbs.archlinux.org/viewtopic.php?id=181370
开机自启：
systemctl enable ntpd.service
如果不能自启：
systemctl disable chronyd.service
一．首先检查机器是否装有NTP：
systemctl start ntpd
如果没有 就GG了。。。或者联网 yum install –y ntp
二．NTP详细配置步骤（默认机器上装有ntp，红字部分为修改部分）
首先进入ntp配置文件：
vim /etc/ntp.conf
# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).
driftfile /var/lib/ntp/drift
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default nomodify notrap nopeer noquery
#禁止从网上同步
restrict -4 default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
# Permit all access over the loopback interface. This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
restrict ::1
# Hosts on local network are less restricted.
#这个地方是传说中的服务端配置，意思大概就是允许某一个网段的同步请求
#如果你要把机器配成服务端可以改下头这句话
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
#此处为设置NTP上层服务器，即从此机器同步
#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 192.168.9.100 perfer
#此处为运行上层server的所有权限 这句话和上头的server必须有
restrict 192.168.9.100
#设置本地时钟以及层数 当同步不好用的时候就将本机作为服务器
server 127.127.1.0
fudge 127.127.1.0 startum 1
#broadcast 192.168.1.255 autokey # broadcast server
#broadcastclient # broadcast client
#broadcast 224.0.1.1 autokey # multicast server
#multicastclient 224.0.1.1 # multicast client
#manycastserver 239.255.254.254 # manycast server
#manycastclient 239.255.254.254 autokey # manycast client
# Enable public key cryptography.
#crypto
到此为止 配置文件结束 保存退出即可
三.配置自动同步开机自启
命令行输入
crontab –e
然后在配置文件输入：
*/5 * * * * /usr/sbin/ntpdate -u pool.ntp.org ;/sbin/hwclock --systohc >> /root/sync_date.log 2>&1
意思是每5分钟执行一次同步语句
随后：
systemctl restart ntpd
systemctl restart crond
systemctl enable ntpd
systemctl enable crond
http://francs3.blog.163.com/blog/static/40576727201211231254105/
查看方式 ：可使用ntpq -p 和 watch ntpq -p查看ntp服务器状态以及联通情况