NTP是网络时间协议(Network Time Protocol)，它是用来同步网络中各个计算机的时间的协议
NTP 工作原理
ntpd 进程通过定期与NTP时钟源服务器发送消息来获取时间信息。在进程初始启动时候，不论是第一次boot还是随后启动，nptd 会给服务器发送消息以获取时间本设置到本地系统。为了防止网络风暴，进程启动后会在定义好的间隔64秒之上再加一个随机延迟值，这个随机值的范围是0~16秒；因此进程启动后需要数分钟才会开始同步时间。
下载对应的rpm包：(能上网的机器)
ps:既然能看到这篇文章就说明你的机器可以上网
yum -y install ntp --downloadonly --downloaddir /ntp/
下载对应包到/ntp/目录下
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部署
首先将预备好的rpm包放到要部署ntp服务的机器上
rpm -ivh autogen-libopts-5.18-5.el7.x86_64.rpm
rpm -ivh ntpdate-4.2.6p5-29.el7.centos.x86_64.rpm
rpm -ivh ntp-4.2.6p5-29.el7.centos.x86_64.rpm
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修改server段配置文件
vim /etc/ntp.conf
# 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 192.168.0.0 mask 255.255.0.0 nomodify notrap ##允许内网其他机器同步时间
# 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
#外部服务不可用，使用本地时间作为服务
server 127.127.1.0 #本地时间就写这个ip
fudge 127.127.1.0 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
启动
systemctl restart ntpd
client段同上部署ntpd服务
修改配置文件
vim /etc/ntp.conf
# 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
##将原来的server注释掉，添加自己的ntp-server-ip
#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 192.168.100.105 iburst
...
启动ntp服务：
systemctl restart ntpd
systemctl enable ntpd
完成
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手动对时(需提前关闭ntpd-service)
ntpdate "ntp-server:ip"
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