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@ -1,147 +0,0 @@
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Translating by qhwdw
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Keep Accurate Time on Linux with NTP
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======
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How to keep the correct time and keep your computers synchronized without abusing time servers, using NTP and systemd.
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### What Time is It?
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Linux is funky when it comes to telling the time. You might think that the `time` tells the time, but it doesn't because it is a timer that measures how long a process runs. To get the time, you run the `date` command, and to view more than one date, you use `cal`. Timestamps on files are also a source of confusion as they are typically displayed in two different ways, depending on your distro defaults. This example is from Ubuntu 16.04 LTS:
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```
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$ ls -l
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drwxrwxr-x 5 carla carla 4096 Mar 27 2017 stuff
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drwxrwxr-x 2 carla carla 4096 Dec 8 11:32 things
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-rw-rw-r-- 1 carla carla 626052 Nov 21 12:07 fatpdf.pdf
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-rw-rw-r-- 1 carla carla 2781 Apr 18 2017 oddlots.txt
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```
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Some display the year, some display the time, which makes ordering your files rather a mess. The GNU default is files dated within the last six months display the time instead of the year. I suppose there is a reason for this. If your Linux does this, try `ls -l --time-style=long-iso` to display the timestamps all the same way, sorted alphabetically. See [How to Change the Linux Date and Time: Simple Commands][1] to learn all manner of fascinating ways to manage the time on Linux.
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### Check Current Settings
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NTP, the network time protocol, is the old-fashioned way of keeping correct time on computers. `ntpd`, the NTP daemon, periodically queries a public time server and adjusts your system time as needed. It's a simple lightweight protocol that is easy to set up for basic use. Systemd has barged into NTP territory with the `systemd-timesyncd.service`, which acts as a client to `ntpd`.
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Before messing with NTP, let's take a minute to check that current time settings are correct.
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There are (at least) two timekeepers on your system: system time, which is managed by the Linux kernel, and the hardware clock on your motherboard, which is also called the real-time clock (RTC). When you enter your system BIOS, you see the hardware clock time and you can change its settings. When you install a new Linux, and in some graphical time managers, you are asked if you want your RTC set to the UTC (Coordinated Universal Time) zone. It should be set to UTC, because all time zone and daylight savings time calculations are based on UTC. Use the `hwclock` command to check:
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```
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$ sudo hwclock --debug
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hwclock from util-linux 2.27.1
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Using the /dev interface to the clock.
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Hardware clock is on UTC time
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Assuming hardware clock is kept in UTC time.
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Waiting for clock tick...
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...got clock tick
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Time read from Hardware Clock: 2018/01/22 22:14:31
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Hw clock time : 2018/01/22 22:14:31 = 1516659271 seconds since 1969
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Time since last adjustment is 1516659271 seconds
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Calculated Hardware Clock drift is 0.000000 seconds
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Mon 22 Jan 2018 02:14:30 PM PST .202760 seconds
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```
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"Hardware clock is kept in UTC time" confirms that your RTC is on UTC, even though it translates the time to your local time. If it were set to local time it would report "Hardware clock is kept in local time."
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You should have a `/etc/adjtime` file. If you don't, sync your RTC to system time:
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```
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$ sudo hwclock -w
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```
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This should generate the file, and the contents should look like this example:
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```
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$ cat /etc/adjtime
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0.000000 1516661953 0.000000
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1516661953
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UTC
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```
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The new-fangled systemd way is to run `timedatectl`, which does not need root permissions:
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```
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$ timedatectl
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Local time: Mon 2018-01-22 14:17:51 PST
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Universal time: Mon 2018-01-22 22:17:51 UTC
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RTC time: Mon 2018-01-22 22:17:51
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Time zone: America/Los_Angeles (PST, -0800)
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Network time on: yes
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NTP synchronized: yes
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RTC in local TZ: no
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```
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"RTC in local TZ: no" confirms that it is on UTC time. What if it is on local time? There are, as always, multiple ways to change it. The easy way is with a nice graphical configuration tool, like YaST in openSUSE. You can use `timedatectl`:
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```
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$ timedatectl set-local-rtc 0
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```
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Or edit `/etc/adjtime`, replacing UTC with LOCAL.
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### systemd-timesyncd Client
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Now I'm tired, and we've just gotten to the good part. Who knew timekeeping was so complex? We haven't even scratched the surface; read `man 8 hwclock` to get an idea of how time is kept on computers.
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Systemd provides the `systemd-timesyncd.service` client, which queries remote time servers and adjusts your system time. Configure your servers in `/etc/systemd/timesyncd.conf`. Most Linux distributions provide a default configuration that points to time servers that they maintain, like Fedora:
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```
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[Time]
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#NTP=
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#FallbackNTP=0.fedora.pool.ntp.org 1.fedora.pool.ntp.org
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```
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You may enter any other servers you desire, such as your own local NTP server, on the `NTP=` line in a space-delimited list. (Remember to uncomment this line.) Anything you put on the `NTP=` line overrides the fallback.
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What if you are not using systemd? Then you need only NTP.
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### Setting up NTP Server and Client
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It is a good practice to set up your own LAN NTP server, so that you are not pummeling public NTP servers from all of your computers. On most Linuxes NTP comes in the `ntp` package, and most of them provide `/etc/ntp.conf` to configure the service. Consult [NTP Pool Time Servers][2] to find the NTP server pool that is appropriate for your region. Then enter 4-5 servers in your `/etc/ntp.conf` file, with each server on its own line:
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```
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driftfile /var/ntp.drift
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logfile /var/log/ntp.log
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server 0.europe.pool.ntp.org
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server 1.europe.pool.ntp.org
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server 2.europe.pool.ntp.org
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server 3.europe.pool.ntp.org
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```
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The `driftfile` tells `ntpd` where to store the information it needs to quickly synchronize your system clock with the time servers at startup, and your logs should have their own home instead of getting dumped into the syslog. Use your Linux distribution defaults for these files if it provides them.
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Now start the daemon; on most Linuxes this is `sudo systemctl start ntpd`. Let it run for a few minutes, then check its status:
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```
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$ ntpq -p
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remote refid st t when poll reach delay offset jitter
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==============================================================
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+dev.smatwebdesi 192.168.194.89 3 u 25 64 37 92.456 -6.395 18.530
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*chl.la 127.67.113.92 2 u 23 64 37 75.175 8.820 8.230
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+four0.fairy.mat 35.73.197.144 2 u 22 64 37 116.272 -10.033 40.151
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-195.21.152.161 195.66.241.2 2 u 27 64 37 107.559 1.822 27.346
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```
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I have no idea what any of that means, other than your daemon is talking to the remote time servers, and that is what you want. To permanently enable it, run `sudo systemctl enable ntpd`. If your Linux doesn't use systemd then it is your homework to figure out how to run `ntpd`.
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Now you can set up `systemd-timesyncd` on your other LAN hosts to use your local NTP server, or install NTP on them and enter your local server in their `/etc/ntp.conf` files.
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NTP servers take a beating, and demand continually increases. You can help by running your own public NTP server. Come back next week to learn how.
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Learn more about Linux through the free ["Introduction to Linux" ][3]course from The Linux Foundation and edX.
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--------------------------------------------------------------------------------
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via: https://www.linux.com/learn/intro-to-linux/2018/1/keep-accurate-time-linux-ntp
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作者：[CARLA SCHRODER][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.linux.com/users/cschroder
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[1]:https://www.linux.com/learn/how-change-linux-date-and-time-simple-commands
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[2]:http://support.ntp.org/bin/view/Servers/NTPPoolServers
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[3]:https://training.linuxfoundation.org/linux-courses/system-administration-training/introduction-to-linux
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146
translated/tech/20180125 Keep Accurate Time on Linux with NTP.md
Normal file
146
translated/tech/20180125 Keep Accurate Time on Linux with NTP.md
Normal file
@ -0,0 +1,146 @@
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在 Linux 上使用 NTP 保持精确的时间
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======
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如何保持正确的时间，如何使用 NTP 和 systemd 让你的计算机在不滥用时间服务器的前提下保持同步。
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### 它的时间是多少？
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当让 Linux 来告诉你时间的时候，它是很奇怪的。你可能认为是使用 `time` 命令来告诉你时间，其实并不是，因为 `time` 只是一个测量一个进程运行了多少时间的计时器。为得到时间，你需要运行的是 `date` 命令，你想查看更多的日期，你可以运行 `cal` 命令。文件上的时间戳也是一个容易混淆的地方，因为根据你的发行版默认情况不同，它一般有两种不同的显示方法。下面是来自 Ubuntu 16.04 LTS 的示例：
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```
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$ ls -l
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drwxrwxr-x 5 carla carla 4096 Mar 27 2017 stuff
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drwxrwxr-x 2 carla carla 4096 Dec 8 11:32 things
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-rw-rw-r-- 1 carla carla 626052 Nov 21 12:07 fatpdf.pdf
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-rw-rw-r-- 1 carla carla 2781 Apr 18 2017 oddlots.txt
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```
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有些显示年，有些显示时间，这样的方式让你的文件更混乱。GNU 默认的情况是，如果你的文件在六个月以内，则显示时间而不是年。我想这样做可能是有原因的。如果你的 Linux 是这样的，尝试用 `ls -l --time-style=long-iso` 命令，让时间戳用同一种方式去显示，按字母顺序排序。查阅 [如何更改 Linux 的日期和时间：简单的命令][1] 去学习 Linux 上管理时间的各种方法。
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### 检查当前设置
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NTP —— 网络时间协议，它是老式的保持计算机正确时间的方法。`ntpd` 是 NTP 守护程序，它通过周期性地查询公共时间服务器来按需调整你的计算机时间。它是一个简单的、轻量级的协议，使用它的基本功能时设置非常容易。Systemd 通过使用 `systemd-timesyncd.service` 已经越俎代庖 “干了 NTP 的活”，它可以用作 `ntpd` 的客户端。
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在我们开始与 NTP “打交道” 之前，先花一些时间来了检查一下当前的时间设置是否正确。
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你的系统上（至少）有两个时钟：系统时间 —— 它由 Linux 内核管理，第二个是你的主板上的硬件时钟，它也称为实时时钟（RTC）。当你进入系统的 BIOS 时，你可以看到你的硬件时钟的时间，你也可以去改变它的设置。当你安装一个新的 Linux 时，在一些图形化的时间管理器中，你会被询问是否设置你的 RTC 为 UTC（协调世界时间）时区，因为所有的时区和夏令时都是基于 UTC 的。你可以使用 `hwclock` 命令去检查：
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```
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$ sudo hwclock --debug
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hwclock from util-linux 2.27.1
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Using the /dev interface to the clock.
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Hardware clock is on UTC time
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Assuming hardware clock is kept in UTC time.
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Waiting for clock tick...
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...got clock tick
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Time read from Hardware Clock: 2018/01/22 22:14:31
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Hw clock time : 2018/01/22 22:14:31 = 1516659271 seconds since 1969
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Time since last adjustment is 1516659271 seconds
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Calculated Hardware Clock drift is 0.000000 seconds
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Mon 22 Jan 2018 02:14:30 PM PST .202760 seconds
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```
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"硬件时钟用 UTC 时间维护" 确认了你的计算机的 RTC 是使用 UTC 时间，虽然你的本地时间是通过 UTC 转换来的。如果设置本地时间，它将报告 “硬件时钟用本地时间维护”。
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如果你不同步你的 RTC 到系统时间，你应该有一个 `/etc/adjtime` 文件。
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```
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$ sudo hwclock -w
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```
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这个命令将生成这个文件，它将包含如下示例中的内容：
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```
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$ cat /etc/adjtime
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0.000000 1516661953 0.000000
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1516661953
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UTC
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```
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新发明的 systemd 方式是去运行 `timedatectl` 命令，运行它不需要 root 权限：
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```
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$ timedatectl
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Local time: Mon 2018-01-22 14:17:51 PST
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Universal time: Mon 2018-01-22 22:17:51 UTC
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RTC time: Mon 2018-01-22 22:17:51
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Time zone: America/Los_Angeles (PST, -0800)
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Network time on: yes
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NTP synchronized: yes
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RTC in local TZ: no
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```
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"RTC in local TZ: no" 确认了它没有使用 UTC 时间。如果要改变它的本地时间，怎么办？这里有许多种方法可以做到。最简单的方法是使用一个图形配置工具，比如像 openSUSE 中的 YaST。你可使用 `timedatectl`：
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```
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$ timedatectl set-local-rtc 0
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```
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或者编辑 `/etc/adjtime`，将 UTC 替换为 LOCAL。
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### systemd-timesyncd 客户端
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现在，我已经累了，但是我们刚到非常精彩的部分。谁能想到计时如此复杂？我们甚至还没有了解到它的皮毛；阅读 `man 8 hwclock` 去了解你的计算机如何保持时间的详细内容。
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Systemd 提供了 `systemd-timesyncd.service` 客户端，它查询远程时间服务器并调整你的本地系统时间。在 `/etc/systemd/timesyncd.conf` 中配置你的服务器。大多数 Linux 发行版都提供一个默认配置，它指向他们维护的时间服务器上，比如，以下是 Fedora 的：
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```
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[Time]
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#NTP=
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#FallbackNTP=0.fedora.pool.ntp.org 1.fedora.pool.ntp.org
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```
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你可以输入你希望的其它时间服务器，比如你自己的本地 NTP 服务器，在 `NTP=` 行上输入一个以空格分隔的服务器列表。（别忘了取消这一行的注释）`NTP=` 行上的任何内容都将覆盖掉 fallback 行上的配置项。
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如果你不想使用 systemd 呢？那么，你将需要一个 NTP。
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### 配置 NTP 服务器和客户端
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配置你自己的局域网 NTP 服务器是一个非常好的实践，这样你的网内计算机就不需要不停查询公共 NTP 服务器。在大多数 Linux 的 `ntp` 包中都带了 NTP，它们大多都提供 `/etc/ntp.conf` 文件去配置服务器。查阅 [NTP 时间服务器池][2] 去找到你所在的区域的合适的 NTP 服务器池。然后在你的 `/etc/ntp.conf` 中输入 4- 5 个服务器，每个服务器用单独的一行：
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```
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driftfile /var/ntp.drift
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logfile /var/log/ntp.log
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server 0.europe.pool.ntp.org
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server 1.europe.pool.ntp.org
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server 2.europe.pool.ntp.org
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server 3.europe.pool.ntp.org
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```
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`driftfile` 告诉 `ntpd` 在这里保存的信息是用于在启动时，使用时间服务器去快速同步你的系统时钟的。而日志将保存在他们自己指定的目录中，而不是转储到 syslog 中。如果你的 Linux 发行版默认提供了这些文件，请使用它们。
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现在去启动守护程序；在大多数主流的 Linux 中它的命令是 `sudo systemctl start ntpd`。让它运行几分钟之后，我们再次去检查它的状态：
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```
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$ ntpq -p
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remote refid st t when poll reach delay offset jitter
|
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==============================================================
|
||||
+dev.smatwebdesi 192.168.194.89 3 u 25 64 37 92.456 -6.395 18.530
|
||||
*chl.la 127.67.113.92 2 u 23 64 37 75.175 8.820 8.230
|
||||
+four0.fairy.mat 35.73.197.144 2 u 22 64 37 116.272 -10.033 40.151
|
||||
-195.21.152.161 195.66.241.2 2 u 27 64 37 107.559 1.822 27.346
|
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|
||||
```
|
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我不知道这些内容是什么意思，但重要的是，你的守护程序已经与时间服务器开始对话了，而这正是我们所需要的。你可以去运行 `sudo systemctl enable ntpd` 命令，永久启用它。如果你的 Linux 没有使用 systemd，那么，给你留下的家庭作业就是找出如何去运行 `ntpd`。
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|
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现在，你可以在你的局域网中的其它计算机上设置 `systemd-timesyncd`，这样它们就可以使用你的本地 NTP 服务器了，或者，在它们上面安装 NTP，然后在它们的 `/etc/ntp.conf` 上输入你的本地 NTP 服务器。
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||||
NTP 服务器持续地接受客户端查询，并且这种需求在不断增加。你可以通过运行你自己的公共 NTP 服务器来提供帮助。下周我们将学习如何运行你自己的公共服务器。
|
||||
|
||||
通过来自 Linux 基金会和 edX 的免费课程 ["Linux 入门" ][3] 来学习更多 Linux 的知识。
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://www.linux.com/learn/intro-to-linux/2018/1/keep-accurate-time-linux-ntp
|
||||
|
||||
作者：[CARLA SCHRODER][a]
|
||||
译者：[qhwdw](https://github.com/qhwdw)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:https://www.linux.com/users/cschroder
|
||||
[1]:https://www.linux.com/learn/how-change-linux-date-and-time-simple-commands
|
||||
[2]:http://support.ntp.org/bin/view/Servers/NTPPoolServers
|
||||
[3]:https://training.linuxfoundation.org/linux-courses/system-administration-training/introduction-to-linux
|
||||
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Block a user