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：

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。

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，这正因为这样，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：

server1.cn.pool.ntp.org

server3.asia.pool.ntp.org

server2.asia.pool.ntp.org

2. 环境情况

准备两台电脑，分别为：

IP                 用途

10.0.0.202    ntpd服务器，用于与外部公共ntpd同步标准时间

10.0.0.203    ntpd客户端，用于与ntpd同步时间

3. 检查服务是否安装

使用rpm检查ntp包是否安装

[root@localhostkevin]# rpm -q ntp

ntp-4.2.6p5-19.el7.centos.3.x86_64

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

[root@localhostkevin]# yum -y install ntp

[root@localhostkevin]# systemctl enable ntpd

[root@localhostkevin]# systemctl start ntpd

4. 设置ntp服务器: 10.0.0.202

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

修改/etc/ntp.conf文件，红色字体是修改的内容

# For more information about thisfile, 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 withour time source, but do not

# permit the source to query ormodify the service on this system.

restrict default nomodify notrapnopeer noquery

# Permit all access over the loopbackinterface.  This could

# be tightened as well, but to do sowould effect some of

# the administrative functions.

restrict 127.0.0.1

restrict ::1

# Hosts on local network are lessrestricted.

#restrict 192.168.1.0 mask255.255.255.0 nomodify notrap

restrict 10.0.0.0 mask255.255.255.0 nomodify notrap

# Use public servers from thepool.ntp.org project.

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

#server0.centos.pool.ntp.org iburst

#server1.centos.pool.ntp.org iburst     ###注意此处四行要注释掉

#server2.centos.pool.ntp.org iburst

#server3.centos.pool.ntp.org iburst

server 2.cn.pool.ntp.org

server 1.asia.pool.ntp.org

server 2.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 239.255.254.254autokey # manycast client

restrict 2.cn.pool.ntp.orgnomodify notrap noquery

restrict 1.asia.pool.ntp.orgnomodify notrap noquery

restrict 2.asia.pool.ntp.orgnomodify notrap noquery

server 127.0.0.1 # localclock

fudge 127.0.0.1 stratum 10

# Enable public key cryptography.

#crypto

includefile /etc/ntp/crypto/pw

# Key file containing the keys andkey identifiers used when operating

# with symmetric key cryptography.

keys /etc/ntp/keys

# Specify the key identifiers whichare trusted.

#trustedkey 4 8 42

# Specify the key identifier to usewith the ntpdc utility.

#requestkey 8

# Specify the key identifier to usewith the ntpq utility.

#controlkey 8

# Enable writing of statistics records.

#statistics clockstats cryptostatsloopstats peerstats

# Disable the monitoring facility toprevent amplification attacks using ntpdc

# monlist command when defaultrestrict does not include the noquery flag. See

# CVE-2013-5211 for more details.

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

disable monitor

[root@lb01 ~]#

修改完成后重启ntpd服务systemctlrestart 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：

# ntpstat

unsynchronised

 time server re-starting

  polling server every 64 s

连接并同步后:

# ntpstat

synchronised to NTP server(202.112.10.36) at stratum 3

  time correct to within 275 ms

  polling server every 256 s

5. 设置ntp客户端: 10.0.0.203

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，红色字体为变化的内容。

# For more information about thisfile, 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 withour time source, but do not

# permit the source to query ormodify the service on this system.

restrict default nomodify notrapnopeer noquery

# Permit all access over the loopbackinterface.  This could

# be tightened as well, but to do sowould effect some of

# the administrative functions.

restrict 127.0.0.1

restrict ::1

# Hosts on local network are lessrestricted.

#restrict 192.168.1.0 mask255.255.255.0 nomodify notrap

# Use public servers from thepool.ntp.org project.

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

#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

server 10.0.0.202

restrict 10.0.0.202  nomodify notrap noquery

server 127.0.0.1

fudge 127.0.0.1 stratum 10

#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 239.255.254.254autokey # manycast client

# Enable public key cryptography.

#crypto

includefile /etc/ntp/crypto/pw

# Key file containing the keys andkey identifiers used when operating

# with symmetric key cryptography.

keys /etc/ntp/keys

# Specify the key identifiers whichare trusted.

#trustedkey 4 8 42

# Specify the key identifier to usewith the ntpdc utility.

#requestkey 8

# Specify the key identifier to usewith the ntpq utility.

#controlkey 8

# Enable writing of statisticsrecords.

#statistics clockstats cryptostatsloopstats peerstats

# Disable the monitoring facility toprevent amplification attacks using ntpdc

# monlist command when defaultrestrict does not include the noquery flag. See

# CVE-2013-5211 for more details.

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

disable monitor

[root@docker02 ~]#

重启ntpd服务

systemctl restart ntpd

启动后，查看同步情况，这个步骤一般根据服务器配置，同步时间不同，我的虚拟机同步为约两分钟

 ntpq -p

 [root@lb01 opt]# ntpstat

synchronised to NTP server(10.0.0.202) at stratum 4

  time correct to within 1243 ms

  polling server every 64 s

本文出自 “蓝叶子Sheep” 博客，请务必保留此出处http://dellinger.blog.51cto.com/12445009/1947889

CentOS7-ntp时间同步服务器