CentOS 7 中使用NTP進行時間同步

1. NTP時鐘同步方式說明
NTP在linux下有兩種時鐘同步方式，分別為直接同步和平滑同步：
直接同步
使用ntpdate命令進行同步，直接進行時間變更。如果伺服器上存在一個12點執行的任務，當前伺服器時間是13點，但標準時間時11點，使用此命令可能會造成任務重複執行。因此使用ntpdate同步可能會引發風險，因此該命令也多用於設定時鐘同步服務時第一次同步時間時使用。
平滑同步
使用ntpd進行時鐘同步，可以保證一個時間不經歷兩次，它每次同步時間的偏移量不會太陡，是慢慢來的，這正因為這樣，ntpd平滑同步可能耗費的時間比較長。

標準時鐘同步服務
http://www.pool.ntp.org/zone/cn網站包含全球的標準時間同步服務，也包括對中國時間的同步，對應的URL為cn.pool.ntp.org，在其中也描述了ntp組態檔中的建議寫法：
server 1.cn.pool.ntp.org
server 3.asia.pool.ntp.org
server 2.asia.pool.ntp.org

2. 環境情況
準備四台電腦，分別為：

3. 檢查服務是否安裝

• 使用rpm檢查ntp包是否安裝

[root@localhost kevin]# rpm -q ntp
ntp-4.2.6p5-19.el7.CentOS.3.x86_64

• 如果已經安裝則略過此步，否則使用yum進行安裝，並設定系統開機自動啟動並啟動服務

[root@localhost kevin]# yum -y install ntp
[root@localhost kevin]# systemctl enable ntpd
[root@localhost kevin]# systemctl start ntpd

4. 設定ntp伺服器: 192.168.11.212

設定前先使用命令：ntpdate -u cn.pool.ntp.org，同步伺服器

• 修改/etc/ntp.conf檔案，紅色字型是修改的內容

# 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 127.0.0.1
restrict ::1

# 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

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
#server 0.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 239.255.254.254 autokey # manycast client
# 允許上層時間伺服器主動修改本機時間
restrict 2.cn.pool.ntp.org nomodify notrap noquery
restrict 1.asia.pool.ntp.org nomodify notrap noquery
restrict 2.asia.pool.ntp.org nomodify notrap noquery

server 127.0.0.1 # local clock
fudge 127.0.0.1 stratum 10

# 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 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

修改完成後重新啟動ntpd服務systemctl restart ntpd

使用ntpq -p 檢視網路中的NTP伺服器，同時顯示用戶端和每個伺服器的關係

使用ntpstat 命令檢視時間同步狀態，這個一般需要5-10分鐘後才能成功連線和同步。所以，伺服器啟動後需要稍等下：
剛啟動的時候，一般是：

# ntpstat 
unsynchronised
  time server re-starting
   polling server every 64 s

連線並同步後:

# ntpstat 
synchronised to NTP server (202.112.10.36) at stratum 3
   time correct to within 275 ms
   polling server every 256 s

5. 設定ntp用戶端: 172.16.248.129|130|131

安裝ntp服務並設定為自動啟動，和前面的設定方式相同。然後編輯/etc/ntp.conf檔案，紅色字型為變化的內容。

# 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 127.0.0.1
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 0.centos.pool.ntp.org iburst

server 172.16.248.1

restrict 172.16.248.1 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 239.255.254.254 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 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

重新啟動ntpd服務

#systemctl restart ntpd

啟動後，檢視同步情況

# ntpq -p
# ntpstat 

因為是內網，一般ntpstat很快就可以同步上。

CentOS NTP伺服器安裝與設定 http://www.linuxidc.com/Linux/2014-01/95258.htm

Linux實戰部署系列之NTP伺服器 http://www.linuxidc.com/Linux/2013-11/92275.htm

多種作業系統NTP用戶端設定 http://www.linuxidc.com/Linux/2013-10/91928.htm

搭建企業級NTP時間伺服器 http://www.linuxidc.com/Linux/2014-02/97205.htm

Linux下快速搭建ntp時間同步伺服器 http://www.linuxidc.com/Linux/2014-07/104371.htm

CentOS 6.3 下實現NTP時間伺服器 http://www.linuxidc.com/Linux/2014-08/105414.htm

本文永久更新連結地址：http://www.linuxidc.com/Linux/2015-11/124911.htm