通过ntp服务实现时间同步。
1：首先搭建本地yum源见网址（http://www.cnblogs.com/zj21/p/6223462.html）
2：先安装ntp服务
[root@jw37 ~]# yum install ntp -y
3：修改ntp的配置文件
[root@jw37 ~]# vi /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
# 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 192.168.100.140 iburst #目标网络地址
#server 1.centos.pool.ntp.org iburst #注释掉默认的ntp服务器
#server 2.centos.pool.ntp.org iburst
#server 3.centos.pool.ntp.org iburst
#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
保存退出。
4：启动服务并设置开启自启
[root@jw37 ~]# systemctl start ntpd.service #启动服务
[root@jw37 ~]# systemctl enable ntpd.service #设置为开机启动
[root@jw37 ~]# systemctl status ntpd.service #查看运行状态
● ntpd.service - Network Time Service
Loaded: loaded (/usr/lib/systemd/system/ntpd.service; enabled; vendor preset: disabled)
Active: active (running) since 二 2016-11-08 02:59:39 CST; 5min ago #红色部分为当前运行状态为running
Main PID: 3185 (ntpd)
CGroup: /system.slice/ntpd.service
└─3185 /usr/sbin/ntpd -u ntp:ntp -g
11月 08 02:59:39 jw37 ntpd[3185]: Listen and drop on 0 v4wildcard 0.0.0.0 UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listen and drop on 1 v6wildcard :: UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listen normally on 2 lo 127.0.0.1 UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listen normally on 3 eno16777736 192.168.100.137 UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listen normally on 4 lo ::1 UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listen normally on 5 eno16777736 fe80::20c:29ff:fe87:3eb2 UDP 123
11月 08 02:59:39 jw37 ntpd[3185]: Listening on routing socket on fd #22 for interface updates
11月 08 02:59:39 jw37 ntpd[3185]: 0.0.0.0 c016 06 restart
11月 08 02:59:39 jw37 ntpd[3185]: 0.0.0.0 c012 02 freq_set kernel 0.000 PPM
11月 08 02:59:39 jw37 ntpd[3185]: 0.0.0.0 c011 01 freq_not_set