Centos 7配置ntp时间同步

1.NTP时钟同步方式说明
    
NTP在linux下有两种时钟同步方式，分别为直接同步和平滑同步：
1)直接同步
     使用ntpdate命令进行同步，直接进行时间变更。如果服务器上存在一个12点运行的任务，当前服务器时间是13点，但标准时间时11点，使用此命令可能会造成任务重复执行。因此使用ntpdate同步可能会引发风险，因此该命令也多用于配置时钟同步服务时第一次同步时间时使用。
2)平滑同步
    使用ntpd进行时钟同步，可以保证一个时间不经历两次，它每次同步时间的偏移量不会太陡，是慢慢来的，正是因为这样，ntpd平滑同步可能耗费的时间比较长。

标准时钟同步服务
http://www.pool.ntp.org/zone/cn网站包含全球的标准时间同步服务，也包括对中国时间的同步，对应的URL为cn.pool.ntp.org，在其中也描述了ntp配置文件中的建议写法：

2.
环境情况
准备四台电脑，分别为：

IP	用途
192.168.11.212	ntpd服务器，用于与外部公共ntpd同步标准时间
172.16.248.129	ntpd客户端，用于与ntpd同步时间
172.16.248.130	ntpd客户端，用于与ntpd同步时间
172.16.248.131	ntpd客户端，用于与ntpd同步时间

3. 检查服务是否安装

1）使用rpm检查ntp包是否安装

2）如果已经安装则略过此步，否则使用yum进行安装，并设置系统开机自动启动并启动服务(每个节点都要安装)

[root@hadoop1 ~]# yum -y install ntp

[root@ hadoop1 ~]# systemctl enable ntpd(设置开机自启动)

[root@ hadoop1 ~]# systemctl start ntpd(启动ntp服务)

3)查看是否设置为开机自启动

4. 设置ntp服务器: 192.168.11.212

配置前先使用命令：ntpdate -u 0.asia.pool.ntp.org，同步服务器

1）修改/etc/ntp.conf文件

# For more information about this file, see the man pages

# ntp.conf(), ntp_acc(), ntp_auth(), ntp_clock(), ntp_misc(), ntp_mon().

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 ::

# Hosts on local network are less restricted.

#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

restrict 172.16.248.0 mask 255.255.255.0 nomodify notrap

只允许192.168.2.0网段的客户机进行时间同步

# Use public servers from the pool.ntp.org project.

# Please consider joining the pool (http://www.pool.ntp.org/join.html).

#server .centos.pool.ntp.org iburst

server .cn.pool.ntp.org

server .asia.pool.ntp.org

server .asia.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

# 允许上层时间服务器主动修改本机时间

restrict .cn.pool.ntp.org nomodify notrap noquery

restrict .asia.pool.ntp.org nomodify notrap noquery

restrict .asia.pool.ntp.org nomodify notrap noquery

server    127.127.1.0     # local clock

fudge     127.127.1.0 stratum 

# 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   

# Specify the key identifier to use with the ntpdc utility.

#requestkey 

# Specify the key identifier to use with the ntpq utility.

#controlkey 

# 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

# CVE-- for more details.

# Note: Monitoring will not be disabled with the limited restriction flag.

disable monitor

修改完成后重启ntpd服务systemctl restart ntpd

使用ntpq -p 查看网络中的NTP服务器，同时显示客户端和每个服务器的关系


使用ntpstat 命令查看时间同步状态，这个一般需要5-10分钟后才能成功连接

和同步。所以，服务器启动后需要稍等下：
刚启动的时候，一般是：


连接并同步后:

 
5. 设置ntp客户端: 172.16.248.129|130|131

安装ntp服务并设置为自动启动，和前面的设置方式相同。然后编辑/etc/ntp.conf文件。

# For more information about this file, see the man pages

# ntp.conf(), ntp_acc(), ntp_auth(), ntp_clock(), ntp_misc(), ntp_mon().

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 ::

# 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 .centos.pool.ntp.org iburst

server 172.16.248.1

restrict 172.16.248.1 nomodify notrap noquery

server    127.127.1.0     # local clock

fudge     127.127.1.0 stratum 

#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   

# Specify the key identifier to use with the ntpdc utility.

#requestkey 

# Specify the key identifier to use with the ntpq utility.

#controlkey 

# 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

# CVE-- for more details.

# Note: Monitoring will not be disabled with the limited restriction flag.

disable monitor

重启ntpd服务

#systemctl restart ntpd

启动后，查看同步情况

# ntpq -p

# ntpstat

因为是内网，一般ntpstat很快就可以同步上。