安装NTP服务可以说很容易，几个要点搞定就成。下面是我装NTP服务器的记录。
一、首先当然是惯例更新
sudo apt update
sudo apt upgrade
axing@84:~$sudo apt install ntp
Reading state information... Done
The following packages were automatically installed and are no longer required:
libcurses-perl libcurses-ui-perl libpkcs11-helper1 libterm-readkey-perl
Use 'sudo apt autoremove' to remove them.
Suggested packages:
ntp-doc
The following NEW packages will be installed:
ntp
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 640 kB of archives.
After this operation, 1,952 kB of additional disk space will be used.
Get:1 http://gb.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 ntp amd64 1:4.2.8p10+dfsg-5ubuntu7.1 [640 kB]
Fetched 640 kB in 0s (13.1 MB/s)
Selecting previously unselected package ntp.
然后修改下配置文件。
这里我们建议去找最近的ntp公共服务器，访问站点 https://www.ntppool.org/
查找你的区域，比如我需要找 Europe/United Kingdom，可以发现有这么几个，记下来要在配置文件里改的。
server 0.uk.pool.ntp.org
server 1.uk.pool.ntp.org
server 2.uk.pool.ntp.org
server 3.uk.pool.ntp.org
这里是中国常用的NTP服务器，以供参考：
210.72.145.44 (国家授时中心服务器IP地址)
202.112.10.36 # 1.cn.pool.ntp.org
59.124.196.83 # 0.asia.pool.ntp.org
s2m.time.edu.cn 北京大学
s2c.time.edu.cn 北京邮电大学
二、修改配置文件 /etc/ntp.conf
# Specify one or more NTP servers.
# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for
# more information.
#这里常用的选项是prefer - 优先主机， iburst -当服务器不可用时将发包检测
server 0.uk.pool.ntp.org iburst
server 1.uk.pool.ntp.org iburst
server 2.uk.pool.ntp.org iburst
server 3.uk.pool.ntp.org iburst
#pool 0.ubuntu.pool.ntp.org iburst
#pool 1.ubuntu.pool.ntp.org iburst
#pool 2.ubuntu.pool.ntp.org iburst
#pool 3.ubuntu.pool.ntp.org iburst
# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
#访问网段限制
restrict xx.xxx.xx.0 mask 255.255.255.0 notrust
三、重启服务，查看服务状态
axing@84:~$ sudo service ntp restart
axing@84:~$ sudo service ntp status
● ntp.service - Network Time Service
Loaded: loaded (/lib/systemd/system/ntp.service; enabled; vendor preset: enabled)
Active: active (running) since Wed 2019-01-16 16:02:17 GMT; 7s ago
Docs: man:ntpd(8)
Process: 24693 ExecStart=/usr/lib/ntp/ntp-systemd-wrapper (code=exited, status=0/SUCCESS)
Main PID: 24711 (ntpd)
Tasks: 2 (limit: 1110)
CGroup: /system.slice/ntp.service
└─24711 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 113:118
Jan 16 16:02:17 84 ntpd[24711]: Listen normally on 2 lo 127.0.0.1:123
Jan 16 16:02:17 84 ntpd[24711]: Listen normally on 3 ens3 xx.xxx.xxx.xx:123
Jan 16 16:02:17 84 ntpd[24711]: Listen normally on 4 lo [::1]:123
Jan 16 16:02:17 84 ntpd[24711]: Listen normally on 5 ens3 [fe80::5054:ff:febd:720d%2]:123
Jan 16 16:02:17 84 ntpd[24711]: Listening on routing socket on fd #22 for interface updates
Jan 16 16:02:18 84 ntpd[24711]: Soliciting pool server 91.189.89.199
Jan 16 16:02:19 84 ntpd[24711]: Soliciting pool server 91.189.94.4
Jan 16 16:02:20 84 ntpd[24711]: Soliciting pool server 91.189.89.198
Jan 16 16:02:21 84 ntpd[24711]: Soliciting pool server 91.189.91.157
Jan 16 16:02:22 84 ntpd[24711]: Soliciting pool server 2001:67c:1560:8003::c8
四、开放防火墙端口，我们看到ntp使用udp 123端口
axing@84:~$ sudo ufw allow 123/udp
Rule added
Rule added (v6)
现在，服务器端就配置完了。 查看与上层ntp服务器的状态
root@84:~# watch ntpq -p
Every 2.0s: ntpq -p C84: Wed Jan 16 16:24:16 2019
remote refid st t when poll reach delay offset jitter
==============================================================================
ntp.ubuntu.com .POOL. 16 p - 64 0 0.000 0.000 0.000
+lond-web-1.spee 87.242.168.84 2 u 60 64 377 1.501 -0.110 0.215
+bode.spiderspac 85.199.214.99 2 u 65 64 377 1.342 -0.112 0.096
*87.242.168.84 ( .UPPS. 1 u 58 64 377 11.249 -0.027 0.079
+ntp1.wirehive.n 195.66.241.2 2 u - 64 377 5.466 -0.073 0.245
-alphyn.canonica 132.246.11.231 2 u 10 64 377 75.283 -3.859 0.057
-pugot.canonical 17.253.52.253 2 u 11 64 377 1.762 0.029 0.150
-chilipepper.can 145.238.203.14 2 u 6 64 377 1.293 0.061 0.057
-golem.canonical 17.253.52.125 2 u 16 64 377 1.288 0.029 0.551
其中这些状态的意思是：
remote: 连接的远程NTP服务器；
refid: 提供时间同步的服务器IP；
st: 远程服务器的层级别（stratum）。由于NTP是层型结构，有顶端的服务器、多层的Relay服务器、再到客户端。所以服务器级别从高到低可以设定为1-16。为了减缓负荷和网络堵塞，原则上应该避免直接连接到级别为1的服务器；
when: 几秒钟前曾经做过时间同步更新的动作；
poll: 本地主机和远程服务器多少时间进行一次同步(单位：s)；
reach: 已经向上层NTP服务器要求更新的次数；
delay: 网络传输过程当中延迟的时间（单位：10^(-6)s，微秒）；
offset: 时间补偿的结果（单位与：10^(-3)s，毫秒）；
jitter: Linux系统时间与BIOS硬件时间的差值（单位：10^(-6)s，微秒）。其绝对值越小，主机和对时服务器的时间就越接近；
*: 远端的服务器已经被确认为主NTP Server，系统时间将由这台机器所提供；
+: 作为辅助的NTP Server，与带有*号的服务器一起为我们提供同步服务. 当```*```号服务器不可用时，它就可以接管；
-: 远程服务器被clustering algorithm认为是不合格的NTP Server；
x: 远程服务器不可用；
五、linux客户端配置ntp
5.1先安装ntpdate，测试NTP工具
axing@THN:~$ sudo apt install ntpdate
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed
ntpdate
0 to upgrade, 1 to newly install, 0 to remove and 0 not to upgrade.
Need to get 51.8 kB of archives.
After this operation, 183 kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 ntpdate amd64 1:4.2.8p10+dfsg-5ubuntu7.1 [51.8 kB]
Fetched 51.8 kB in 0s (162 kB/s)
Selecting previously unselected package ntpdate.
(Reading database ... 92007 files and directories currently installed.)
Preparing to unpack .../ntpdate_1%3a4.2.8p10+dfsg-5ubuntu7.1_amd64.deb ...
Unpacking ntpdate (1:4.2.8p10+dfsg-5ubuntu7.1) ...
Setting up ntpdate (1:4.2.8p10+dfsg-5ubuntu7.1) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
测试NTP服务器是否好用（如有提示“ntpdate[XXX]: the ntp socket is in use, exiting”， 可加参数 -u )
axing@THN:~$ sudo ntpdate xx.xxx.xxx.xx
16 Jan 16:48:51 ntpdate[22307]: adjust time server xx.xxx.xxx.xx offset -0.001725 sec
5.2 很好，那么我们先停止ubuntu自带的时间同步服务
axing@THN:~$ sudo timedatectl set-ntp off
5.3 安装ntp服务并添加主NTP服务器，可以修改/etc/ntp.cfg，或者直接使用下面语句添加：
axing@THN:~$ sudo bash -c "echo server xx.xxx.xxx.xx prefer iburst >> /etc/ntp.conf"
#重启NTP服务
axing@THN:~$ sudo service ntp restart
看一下同步状态：
axing@THN:~$ watch ntpq -p
Every 2.0s: ntpq -p THN: Wed Jan 16 17:32:29 2019
remote refid st t when poll reach delay offset jitter
==============================================================================
ntp.ubuntu.com .POOL. 16 p - 64 0 0.000 0.000 0.000
*xx.xxx.xxx.xx xx.xxx.xx.4 3 u 57 64 77 0.825 -0.951 0.706
+ns1.luns.net.uk 158.43.192.66 2 u 58 64 77 8.456 -0.887 0.751
-37.220.20.12 (h 82.69.97.89 2 u 53 64 77 3.795 -3.091 0.580
+time.rdg.uk.as4 87.242.168.84 2 u 56 64 77 2.171 -1.905 0.719
-armcd.co.uk 130.88.212.143 3 u 55 64 77 1.756 -0.599 0.706
-pugot.canonical 140.203.204.77 2 u 60 64 77 1.737 -0.923 0.492
-alphyn.canonica 132.246.11.231 2 u 1 64 177 75.296 -4.481 0.427
-golem.canonical 17.253.52.125 2 u 65 64 77 1.313 -1.000 0.693
-chilipepper.can 193.79.237.14 2 u 3 64 177 1.477 -0.380 0.335
可以看到，已经和主服务器同步成功了。
