实际业务需求中由于两台server的时间不一致。导致漏数据。损失巨大，被user批的不行不行的。还好IT有一台 server 专门用来校时。
校时服务器是和下面四个server校时的
server 0.rhel.pool.ntp.org
#server 1.rhel.pool.ntp.org
#server 2.rhel.pool.ntp.org
#server 3.rhel.pool.ntp.org 。
Network Time Protocol(NTP，网络时间协议)用于同步它所有客户端时钟的服务。NTP服务器将本地系统的时钟与一个公共的NTP服务器同步然后作为时间主机提供服务，使本地网络的所有客户端能同步时钟。
ntp时间服务器是一个简单的常用的服务器，工作中我们只要做到会用就行，能搭建起来就可以了，不用太去深入研究ntp时间服务器的原理。
server端的配置
# 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 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 -6 ::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.rhel.pool.ntp.org iburst
#server 1.rhel.pool.ntp.org iburst
#server 2.rhel.pool.ntp.org iburst
#server 3.rhel.pool.ntp.org iburst
#server 10.50.10.254 //此台server是和time.windows.com 每隔一小时对一次
server 10.56.5.240
#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
客户端需要做的事情很简单。
第一步，客户端安装NTP服务：
yum install -y ntp
第二步，同步时间：
1、crontab -l 查看校时脚本是否和设定server一致。
2、若不一致 使用 caontab -e修改目标server，否则进行 3
3、ntpdate 服务器IP或者域名
