1. NTP時鐘同步方式說明
NTP在linux下有兩種時鐘同步方式，分別為直接同步和平滑同步：
直接同步
使用ntpdate命令進行同步，直接進行時間變更。如果服務器上存在一個12點運行的任務，當前服務器時間是13點，但標准時間時11點，使用此命令可能會造成任務重復執行。因此使用ntpdate同步可能會引發風險，因此該命令也多用於配置時鐘同步服務時第一次同步時間時使用。
平滑同步
使用ntpd進行時鐘同步，可以保證一個時間不經歷兩次，它每次同步時間的偏移量不會太陡，是慢慢來的，這正因為這樣，ntpd平滑同步可能耗費的時間比較長。
標准時鐘同步服務
http://www.pool.ntp.org/zone/cn網站包含全球的標准時間同步服務，也包括對中國時間的同步，對應的URL為cn.pool.ntp.org，在其中也描述了ntp配置文件中的建議寫法：
server 1.cn.pool.ntp.org
server 3.asia.pool.ntp.org
server 2.asia.pool.ntp.org
2. 環境情況
准備四台電腦，分別為：
[root@localhost kevin]# rpm -q ntp
ntp-4.2.6p5-19.el7.CentOS.3.x86_64[root@localhost kevin]# yum -y install ntp
[root@localhost kevin]# systemctl enable ntpd
[root@localhost kevin]# systemctl start ntpd配置前先使用命令：
ntpdate -u cn.pool.ntp.org，同步服務器
# 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 127.0.0.1
restrict ::1
# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
restrict 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 iburst
server 2.cn.pool.ntp.org
server 1.asia.pool.ntp.org
server 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
# 允許上層時間服務器主動修改本機時間
restrict 2.cn.pool.ntp.org nomodify notrap noquery
restrict 1.asia.pool.ntp.org nomodify notrap noquery
restrict 2.asia.pool.ntp.org nomodify notrap noquery
server 127.0.0.1 # local clock
fudge 127.0.0.1 stratum 10
# Enable public key cryptography.
#crypto
includefile /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
修改完成後重啟ntpd服務
systemctl restart ntpd
使用ntpq -p 查看網絡中的NTP服務器，同時顯示客戶端和每個服務器的關系
使用ntpstat 命令查看時間同步狀態，這個一般需要5-10分鐘後才能成功連接和同步。所以，服務器啟動後需要稍等下：
剛啟動的時候，一般是：
# ntpstat
unsynchronised
time server re-starting
polling server every 64 s連接並同步後:
# ntpstat
synchronised to NTP server (202.112.10.36) at stratum 3
time correct to within 275 ms
polling server every 256 s安裝ntp服務並設置為自動啟動，和前面的設置方式相同。然後編輯/etc/ntp.conf文件，紅色字體為變化的內容。
# 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 127.0.0.1
restrict ::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 iburst
server 172.16.248.1
restrict 172.16.248.1 nomodify notrap noquery
server 127.0.0.1
fudge 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.
#crypto
includefile /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
重啟ntpd服務
#systemctl restart ntpd啟動後，查看同步情況
# ntpq -p
# ntpstat 因為是內網，一般ntpstat很快就可以同步上。
CentOS NTP服務器安裝與配置 http://www.linuxidc.com/Linux/2014-01/95258.htm
Linux實戰部署系列之NTP服務器 http://www.linuxidc.com/Linux/2013-11/92275.htm
多種操作系統NTP客戶端配置 http://www.linuxidc.com/Linux/2013-10/91928.htm
搭建企業級NTP時間服務器 http://www.linuxidc.com/Linux/2014-02/97205.htm
Linux下快速搭建ntp時間同步服務器 http://www.linuxidc.com/Linux/2014-07/104371.htm
CentOS 6.3 下實現NTP時間服務器 http://www.linuxidc.com/Linux/2014-08/105414.htm
