实验状况：虚拟机中的一台Centos7系统想要连接内部网络NTP服务器（IP：192.168.100.100），但其本身不能连接互联网。
一、更改yum源从本地硬盘
打开VMware中“虚拟机（M）”选择“设置”，再选择“CD/DVD（IDE）”，选择“使用ISO映像文件（M）”，选择合适的系统版本。
二、挂载光盘
光盘放好后挂载光盘
先新建挂载点：# mkdir -p /mnt/cdrom，然后再重新进行挂载。
mount /dev/cdrom /mnt/cdrom
或者 mount /dev/sr0 /mnt/cdrom
三、修改yum源（临时更改）
[root@localhost ~]# cd /etc/yum.repos.d/
[root@svr_share_02 yum.repos.d]# vi CentOS-Base.repo
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=file:///mnt/cdrom #设置路径
gpgcheck=0 #表示对从这个源下载的rpm包不进行校验
enable=1 #表示启用这个源
最后 wq! 保存退出。
四、安装NTP
1、查看/etc/yum.repo.d列表里面已经安装和可以安装的程序包
[root@localhost ~]# yum list
若发现出现如下报错。基本的原因就是自己的网络设置问题，解决好就正常了。
若发现自己的网络问题已正常，使用yum list 还是报如下错误：
可以先查看自己之前挂载的情况：
发现自己的挂载没有了，需要重新再挂载。
这时候再使用[root@localhost ~]# yum list 查看，就会发现已正常。
2、安装NTP
[root@localhost ~]# yum install ntp
四、修改配置文件
[root@localhost ~]# 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 192.168.100.100 增加的NTP服务器地址
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
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
五、设置NTP服务
[root@localhost ~]# systemctl restart ntpd.service #重启NTP服务
[root@localhost ~]# systemctl status ntpd.service #查看NTP服务状态
● ntpd.service - Network Time Service
Loaded: loaded (/usr/lib/systemd/system/ntpd.service; disabled; vendor preset: disabled)
Active: active (running) since 一 2018-07-30 13:15:15 CST; 52s ago
Process: 34667 ExecStart=/usr/sbin/ntpd -u ntp:ntp $OPTIONS (code=exited, status=0/SUCCESS)
Main PID: 34668 (ntpd)
CGroup: /system.slice/ntpd.service
├─34668 /usr/sbin/ntpd -u ntp:ntp -g
└─34669 /usr/sbin/ntpd -u ntp:ntp -g
可以看到当前的状态为 running
六、查看是否成功
[root@localhost ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
192.168.100.100 .GPS. 1 u 1 64 7 7.840 240671. 3.119
七、设置开机自启动
[root@localhost ~]# systemctl enable ntpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/ntpd.service to /usr/lib/systemd/system/ntpd.service.