CentOS7中使用NTP進行時間同步

CentOS7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，分別為直接同步處理和平滑同步：
直接同步處理
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點啟動並執行任務，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配置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曆兩次，它每次同步時間的位移量不會太陡，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設定檔中的建議寫法：
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：

IP	用途
192.168.11.212	ntpd伺服器，用於與外部公用ntpd同步標準時間
172.16.248.129	ntpd用戶端，用於與ntpd同步時間
172.16.248.130	ntpd用戶端，用於與ntpd同步時間
172,16,248.131	ntpd用戶端，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 ntpntp-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

配置前先使用命令：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 127.0.0.1restrict ::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 (http://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# 允許上層時間伺服器主動修改本機時間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

修改完成後重啟ntpd服務systemctl restart ntpd

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

使用ntpstat 命令查看時間同步狀態，這個一般需要5-10分鐘後才能成功串連和同步。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

串連並同步後:

# 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 127.0.0.1restrict ::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 (http://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

重啟ntpd服務

#systemctl restart ntpd

啟動後，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。