测试环境：
NTPserver 192.168.1.252
NTPclient 192.168.1.251
准备工作：
关闭selinux：
vi /etc/selinux/config
SELINUX=disabled
关闭iptables：
service iptables stop
chkconfig iptables off
一.安装NTP软件包：
yum -y install ntp /*yum安装NTP服务*/
chkconfig --add ntpd /*添加NTP*/
chkconfig ntpd on /*开机自启动NTP*/
二.修改NTP配置文件：
vi /etc/ntp.conf
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# 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
restrict default ignore 设置默认策略为拒绝所有访问方式的请求
# 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 允许局域网内机器同步时间
# 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
server 1.centos.pool.ntp.org 设置同步服务器
server 2.centos.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
restrict 0.centos.pool.ntp.org nomodify notrap noquery
restrict 1.centos.pool.ntp.org nomodify notrap noquery 允许与上层服务器同步时间
restrict 2.centos.pool.ntp.org nomodify notrap noquery
# Undisciplined Local Clock. This is a fake driver intended for backup
# and when no outside source of synchronized time is available.
server 127.127.1.0 # local clock
fudge 127.127.1.0 stratum 10 外界同步源无法联系时，使用本地时间为同步服务
# 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 loops
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