为什么要这么做？
在我们的服务器集群中，有三种问题是最难排查的：
- 因为selinux造成的问题
- 因为防火墙造成的
- 因为时间不同造成的
所以在整个集群中时间的同步尤其重要，我们不能让数据库等服务器暴露在公网，同时我们也需要校对时间，所以我们需要配置一个时间同步服务
本机环境：
[root@m02 ~]# cat /etc/redhat-release CentOS Linux release 7.4.1708 (Core) [root@m02 ~]# uname -r 3.10.0-693.el7.x86_64
安装ntp服务：
[root@m02 ~]# yum install -y ntp Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: mirrors.aliyun.com * epel: mirrors.aliyun.com * extras: mirrors.aliyun.com * updates: mirrors.aliyun.com Package ntp-4.2.6p5-25.el7.centos.2.x86_64 already installed and latest version Nothing to do [root@m02 ~]# rpm -qa ntp ntp-4.2.6p5-25.el7.centos.2.x86_64
配置ntp：/etc/ntp.conf
两个关键点：
第一个位置：用来设置网段和子网掩码以及权限
第二个位置：ntp服务需要同步的上级服务
启动ntp服务：
[root@m02 ~]# systemctl start ntpd
[root@m02 ~]# systemctl status ntpd
● ntpd.service - Network Time Service
Loaded: loaded (/usr/lib/systemd/system/ntpd.service; disabled; vendor preset: disabled)
Active: active (running) since Thu 2017-11-16 09:17:03 CST; 6h ago
Main PID: 1575 (ntpd)
CGroup: /system.slice/ntpd.service
└─1575 /usr/sbin/ntpd -u ntp:ntp -g
Nov 16 11:30:11 m02 ntpd[1575]: new interface(s) found: waking up resolver
Nov 16 11:34:30 m02 ntpd[1575]: Deleting interface #10 eth0, fe80::8a2a:ff08:cb95:36c5...secs
Nov 16 11:34:34 m02 ntpd[1575]: Listen normally on 11 eth0 fe80::8a2a:ff08:cb95:36c5 UDP 123
Nov 16 11:34:34 m02 ntpd[1575]: new interface(s) found: waking up resolver
Nov 16 12:02:35 m02 ntpd[1575]: Deleting interface #11 eth0, fe80::8a2a:ff08:cb95:36c5...secs
Nov 16 12:02:38 m02 ntpd[1575]: Listen normally on 12 eth0 fe80::8a2a:ff08:cb95:36c5 UDP 123
Nov 16 12:02:38 m02 ntpd[1575]: new interface(s) found: waking up resolver
Nov 16 15:11:31 m02 ntpd[1575]: Deleting interface #12 eth0, fe80::8a2a:ff08:cb95:36c5...secs
Nov 16 15:11:34 m02 ntpd[1575]: Listen normally on 13 eth0 fe80::8a2a:ff08:cb95:36c5 UDP 123
Nov 16 15:11:34 m02 ntpd[1575]: new interface(s) found: waking up resolver
Hint: Some lines were ellipsized, use -l to show in full.
查看同步链接（监视ntp）
[root@m02 ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
182.92.12.11 .XFAC. 16 u 5h 1024 0 0.000 0.000 0.000
173.255.215.209 .XFAC. 16 u - 1024 0 0.000 0.000 0.000
202.108.6.95 .XFAC. 16 u 5h 1024 0 0.000 0.000 0.000
61.216.153.105 .XFAC. 16 u - 1024 0 0.000 0.000 0.000
212.47.249.141 .XFAC. 16 u 6h 1024 0 0.000 0.000 0.000
ntp的工作状态：
[root@m02 ~]# ntpstat synchronised to unspecified at stratum 3 time correct to within 292 ms polling server every 256 s
客户端测试：
[root@m01 ~]# ntpdate 172.16.1.62 16 Nov 09:23:36 ntpdate[1270]: step time server 172.16.1.62 offset -2.630123 sec [root@m01 ~]# ntpdate -d 172.16.1.62 16 Nov 09:24:04 ntpdate[1271]: ntpdate 4.2.6p5@1.2349-o Mon Feb 6 07:22:46 UTC 2017 (1) Looking for host 172.16.1.62 and service ntp host found : 172.16.1.62 transmit(172.16.1.62)