1. NTP时钟同步方式说明NTP在linux下有两种时钟同步方式，分别为直接同步和平滑同步： 直接同步 使用ntpdate命令进行同步，直接进行时间变更。如果服务器上存在一个12点运行的任务，当前服务器时间是13点，但标准时间时11点，使用此命令可能会造成任务重复执行。因此使用ntpdate同步可能会引发风险，因此该命令也多用于配置时钟同步服务时第一次同步时间时使用。 平滑同步 使用ntpd进行时钟同步，可以保证一个时间不经历两次，它每次同步时间的偏移量不会太陡，是慢慢来的，这正因为这样，ntpd平滑同步可能耗费的时间比较长。
标准时钟同步服务 http://www.pool.ntp.org/zone/cn网站包含全球的标准时间同步服务，也包括对中国时间的同步，对应的URL为cn.pool.ntp.org，在其中也描述了ntp配置文件中的建议写法： server 1.cn.pool.ntp.org server 3.asia.pool.ntp.org server 2.asia.pool.ntp.org
2. 环境情况准备四台电脑，分别为：
3. 检查服务是否安装使用rpm检查ntp包是否安装
[root@localhost kevin]# rpm -q ntpntp-4.2.6p5-19.el7.CentOS.3.x86_64如果已经安装则略过此步，否则使用yum进行安装，并设置系统开机自动启动并启动服务
[root@localhost kevin]# yum -y install ntp[root@localhost kevin]# systemctl enable ntpd[root@localhost kevin]# systemctl start ntpd4. 设置ntp服务器: 192.168.11.212
配置前先使用命令：ntpdate -u cn.pool.ntp.org，同步服务器
修改/etc/ntp.conf文件，红色字体是修改的内容
# 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# 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 127.0.0.1restrict ::1# Hosts on local network are less restricted.#restrict 192.168.1.0 mask 255.255.255.0 nomodify notraprestrict 172.16.248.0 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).#server 0.centos.pool.ntp.org iburstserver 2.cn.pool.ntp.orgserver 1.asia.pool.ntp.orgserver 2.asia.pool.ntp.org#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# 允许上层时间服务器主动修改本机时间restrict 2.cn.pool.ntp.org nomodify notrap noqueryrestrict 1.asia.pool.ntp.org nomodify notrap noqueryrestrict 2.asia.pool.ntp.org nomodify notrap noqueryserver 127.0.0.1 # local clockfudge 127.0.0.1 stratum 10# Enable public key cryptography.#cryptoincludefile /etc/ntp/crypto/pw# Key file containing the keys and key identifiers used when operating# with symmetric key cryptography. keys /etc/ntp/keys# Specify the key identifiers which are trusted.#trustedkey 4 8 42# Specify the key identifier to use with the ntpdc utility.#requestkey 8# Specify the key identifier to use with the ntpq utility.#controlkey 8# Enable writing of statistics records.#statistics clockstats cryptostats loopstats peerstats# Disable the monitoring facility to prevent amplification attacks using ntpdc# monlist command when default restrict does not include the noquery flag. See# CVE-2013-5211 for more details.# Note: Monitoring will not be disabled with the limited restriction flag.disable monitor修改完成后重启ntpd服务systemctl restart ntpd
使用ntpq -p 查看网络中的NTP服务器，同时显示客户端和每个服务器的关系
使用ntpstat 命令查看时间同步状态，这个一般需要5-10分钟后才能成功连接和同步。所以，服务器启动后需要稍等下： 刚启动的时候，一般是：
# ntpstat unsynchronised time server re-starting polling server every 64 s连接并同步后:
# ntpstat synchronised to NTP server (202.112.10.36) at stratum 3 time correct to within 275 ms polling server every 256 s5. 设置ntp客户端: 172.16.248.129|130|131
安装ntp服务并设置为自动启动，和前面的设置方式相同。然后编辑/etc/ntp.conf文件，红色字体为变化的内容。
# 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# 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 127.0.0.1restrict ::1# Hosts on local network are less restricted.#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).#server 0.centos.pool.ntp.org iburstserver 172.16.248.1restrict 172.16.248.1 nomodify notrap noqueryserver 127.0.0.1fudge 127.0.0.1 stratum 10#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.#cryptoincludefile /etc/ntp/crypto/pw # Key file containing the keys and key identifiers used when operating# with symmetric key cryptography. keys /etc/ntp/keys # Specify the key identifiers which are trusted.#trustedkey 4 8 42# Specify the key identifier to use with the ntpdc utility.#requestkey 8# Specify the key identifier to use with the ntpq utility.#controlkey 8# Enable writing of statistics records.#statistics clockstats cryptostats loopstats peerstats# Disable the monitoring facility to prevent amplification attacks using ntpdc# monlist command when default restrict does not include the noquery flag. See# CVE-2013-5211 for more details.# Note: Monitoring will not be disabled with the limited restriction flag.disable monitor重启ntpd服务
#systemctl restart ntpd启动后，查看同步情况
# ntpq -p# ntpstat 因为是内网，一般ntpstat很快就可以同步上。
CentOS NTP服务器安装与配置 http://www.68idc.cn/Linux/2014-01/95258.htm
Linux实战部署系列之NTP服务器 http://www.68idc.cn/Linux/2013-11/92275.htm
多种操作系统NTP客户端配置 http://www.68idc.cn/Linux/2013-10/91928.htm
搭建企业级NTP时间服务器 http://www.68idc.cn/Linux/2014-02/97205.htm
Linux下快速搭建ntp时间同步服务器 http://www.68idc.cn/Linux/2014-07/104371.htm
CentOS 6.3 下实现NTP时间服务器 http://www.68idc.cn/Linux/2014-08/105414.htm
有时我们选择改变，并非经过深思熟虑，而更像是听见了天地间冥冥中的呼唤，
