简单记录使用ntp在实践中同步centos7。
安装结构时，如果虚拟机之间的实践差异太大，则会发生异常。 也可以使用命令直接修改时间，但考虑到后期维护，选择使用ntp同步时间。
此配置使用五个虚拟机进行演示。
IP用途172.17.0.82ntp客户端：与内部ntp服务器(85 )同步时间172.17.0.83ntp客户端：与内部ntp服务器) 85同步时间172.17.0.84ntp客户端：内部
首先检查是否安装了ntp。rpm -q ntp
如果未安装，则使用命令：yum -y install ntp安装ntp。
如果已安装，请继续执行下一步。
安装完成后，按如下方式设置并启动ntp，然后启动ntp :
系统启用ntpd
系统启动ntpd
首先设置ntp服务器(172.17.0.85 )
编辑配置文件(vim /etc/ntp.conf )，注释默认ntp服务地址，并使用国内地址(https://www.pool.ntp.org/zone/cn和http://www
# formoreinformationaboutthisfile，see the man pages
#NTP.conf(5，ntp_acc(5) 5，ntp_auth(5) 5，ntp_clock(5) 5，ntp_misc(5) 5，ntp_mon(5) 5
差速文件/var/lib/NTP /差速
# permittimesynchronizationwithourtimesource，but do not
# permitthesourcetoqueryormodifytheserviceonthissystem。
restrictdefaultnomodifynotrapnopeernoquery
# permitallaccessovertheloopbackinterface.this could
# be tightened as well，but to do so would effect some of
# the管理员tive functions。
restrict 127.0.0.1
restrict :1
# hostsonlocalnetworkarelessrestricted。
# restrict 192.168.1.0 mask 255.255.255.0否修改说明
restrict 172.17.0.0 mask 255.255.255.0否修改说明
# usepublicserversfromthepool.NTP.org project。
# pleaseconsiderjoiningthepool (http://www.pool.NTP.org/join.html )。
# server0. centos.pool.NTP.org I burst
# server1. centos.pool.NTP.org I burst
# server2. centos.pool.NTP.org I burst
# server3. centos.pool.NTP.org I burst
服务器0.cn.pool.NTP.org
服务器1.cn.pool.NTP.org
服务器2.cn.pool.NTP.org
服务器3.cn.pool.NTP.org
restrict0. cn.pool.NTP.orgnomodifynotrapnoquery
restrict1. cn.pool.NTP.orgnomodifynotrapnoquery
restrict2. cn.pool.NTP.orgnomodifynotrapnoquery
restrict3. cn.pool.NTP.orgnomodifynotrapnoquery
服务器127.0.0.1 #本地时钟
fudge 127.0.0.1 stratum 10
# broadcast 192.168.1.255 autokey # broadcast ser
ver#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
修改完成后保存退出，并重启ntp（systemctl restart ntpd）。
设置ntp客户端（以172.17.0.82为例，其余相同，略）
编辑配置文件（vim /etc/ntp.conf），注释掉默认ntp服务，使用我们自己配置的ntp服务器（即 172.17.0.85），具体修改部分突出显示，如下：
【！！！】这里需要注意的是，在使用ntp服务器时，如果有防火墙，则需要开启ntp服务器的udp协议123端口，否则其他虚拟机不能成功同步时间。
# 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 1.centos.pool.ntp.org iburst
#server 2.centos.pool.ntp.org iburst
#server 3.centos.pool.ntp.org iburst
server 172.17.0.85
restrict 172.17.0.85 nomodify notrap noquery
server 127.0.0.1 # local clock
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
修改完成后保存退出并重启ntp（systemctl restart ntpd）。
重启后可以使用ntpq -p 查看网络中的NTP服务器，如下分别是85和82（其余客户端与82类似）：
还可以使用ntpstat 命令查看时间同步状态，但是需要等待几分钟才回看到同步信息，同样，下面分别是85和82：
可以看到82在开始时候并没有同步时间，而是等待一段时间后才开始同步。
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