1. NTP時鍾同步方式說明
NTP在linux下有兩種時鍾同步方式，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。如果服務器上存在一個12點運行的任務，當前服務器時間是13點，但標准時間時11點，使用此命令可能會造成任務重復執行。因此使用ntpdate同步可能會引發風險，因此該命令也多用於配置時鍾同步服務時第一次同步時間時使用。
平滑同步
使用ntpd進行時鍾同步，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。
標准時鍾同步服務
http://www.pool.ntp.org/zone/cn網站包含全球的標准時間同步服務，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配置文件中的建議寫法：
server 1.cn.pool.ntp.org
server 3.asia.pool.ntp.org
server 2.asia.pool.ntp.org
2. 環境情況
准備四台電腦，分別為：
| IP | 用途 |
|---|---|
| 192.168.11.212 | ntpd服務器，用於與外部公共ntpd同步標准時間 |
| 172.16.248.129 | ntpd客戶端，用於與ntpd同步時間 |
| 172.16.248.130 | ntpd客戶端，用於與ntpd同步時間 |
| 172,16,248.131 | ntpd客戶端，用於與ntpd同步時間 |
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，並設置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 ntpd4. 設置ntp服務器: 192.168.11.212
配置前先使用命令：
ntpdate -u cn.pool.ntp.org，同步服務器
- 修改/etc/ntp.conf文件，紅色字體是修改的內容
# 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 # 允許上層時間服務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 修改完成后重啟ntpd服務
systemctl restart ntpd
使用ntpq -p 查看網絡中的NTP服務器，同時顯示客戶端和每個服務器的關系
使用ntpstat 命令查看時間同步狀態，這個一般需要5-10分鍾后才能成功連接和同步。所以，服務器啟動后需要稍等下：
剛啟動的時候，一般是：
# ntpstat unsynchronised time server re-starting polling server every 64 s連接並同步后:
# ntpstat synchronised to NTP server (202.112.10.36) at stratum 3 time correct to within 275 ms polling server every 256 s5. 設置ntp客戶端: 172.16.248.129|130|131
安裝ntp服務並設置為自動啟動，和前面的設置方式相同。然后編輯/etc/ntp.conf文件，紅色字體為變化的內容。
# 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 重啟ntpd服務
#systemctl restart ntpd啟動后，查看同步情況
# ntpq -p # ntpstat 因為是內網，一般ntpstat很快就可以同步上。