CentOS 7 中使用NTP进行时间同步

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:
直接同步
使用ntpdate命令进行同步,直接进行变更。如果上存在一个12点运行的任务,当前是13点,但标准时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使用。
平滑同步
使用ntpd进行时钟同步,可以保证一个不经历两次,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
https://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:
server 1.cn.pool.ntp.org
server 3.asia.pool.ntp.org
server 2.asia.pool.ntp.org

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

  • 使用rpm检查ntp包是否安装
[root@localhost kevin]# rpm -q ntpntp-4.2.6p5-19.el7..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@localhost kevin]# yum -y install ntp[root@localhost kevin]# systemctl enable ntpd[root@localhost kevin]# systemctl start ntpd

4. 设置ntp服务器: 192.168.11.212

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

  • 修改/etc/ntp.conf文件,红色字体是修改的内容
# 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 127.0.0.1restrict ::1# Hosts on local network are less restricted.#restrict 192.168.1.0 mask 255.255.255.0 nomodify notraprestrict 172.16.248.0 mask 255.255.255.0 nomodify notrap# Use public servers from the pool.ntp.org project.# Please consider joining the pool (https://www.pool.ntp.org/join.html).#server 0.centos.pool.ntp.org iburstserver 2.cn.pool.ntp.orgserver 1.asia.pool.ntp.orgserver 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 239.255.254.254 autokey # manycast client# 允许上层时间主动修改本机时间restrict 2.cn.pool.ntp.org nomodify notrap noqueryrestrict 1.asia.pool.ntp.org nomodify notrap noqueryrestrict 2.asia.pool.ntp.org nomodify notrap noqueryserver 127.0.0.1 # local clockfudge 127.0.0.1 stratum 10# Enable public key cryptography.#cryptoincludefile /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# CVE-2013-5211 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: 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,红色字体为变化的内容。

# 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 127.0.0.1restrict ::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 (https://www.pool.ntp.org/join.html).#server 0.centos.pool.ntp.org iburstserver 172.16.248.1restrict 172.16.248.1 nomodify notrap noqueryserver 127.0.0.1fudge 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 239.255.254.254 autokey # manycast client# Enable public key cryptography.#cryptoincludefile /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# CVE-2013-5211 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。

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转载自:https://www.linuxidc.com/Linux/2015-11/124911.htm

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