Network Time Protocol daemon (简体中文)

From ArchWiki
(Redirected from NTP (简体中文))
Jump to: navigation, search
翻译状态: 本文是英文页面 Network_Time_Protocol_daemon翻译,最后翻译时间:2016-05-08,点击这里可以查看翻译后英文页面的改动。

Network Time Protocol (网络时间协议)是 GNU/Linux 系统通过互联网时间服务器同步系统软件时钟的最常见方法。设计时考虑到了各种网络延迟,通过公共网络同步时,误差可以降低到10毫秒以内;通过本地网络同步时,误差可以降低到 1 毫秒。

NTP 项目提供了一个名为简单 NTP 的参考实现。本文介绍如何设置和运行服务器和客户端 NTP 进程。

安装

安装 ntp 软件包。如果不做任何配置, ntpd 默认工作于客户端模式。如果使用 Arch Linux 默认的配置,请跳转到 #使用。作为服务器的配置,请参阅 #NTP 服务器模式

配置

主要的后台进程是 ntpd, 可以通过 /etc/ntp.conf 配置。详细信息可以参考手册 man ntp.conf 和相关的 man {ntpd|ntp_auth|ntp_mon|ntp_acc|ntp_clock|ntp_misc}.

连接到 NTP 服务器

NTP 服务器通过一个层级系统进行分类,不同的层级称为 strata: 独立的时间源为stratum 0; 直接连接到 stratum 0 的设备为 stratum 1;直接连接到 stratum 1 的源为 stratum 2,以此类推。

服务器的 stratum 并不能完全等同于它的精度和可靠度。通常的时间同步都使用 stratum 2 服务器。通过pool.ntp.org 服务器或这个链接 可以选择比较近的服务器池。

下面几行仅仅是例子:

/etc/ntp.conf
server 0.fr.pool.ntp.org iburst
server 1.fr.pool.ntp.org iburst
server 2.fr.pool.ntp.org iburst
server 3.fr.pool.ntp.org iburst

推荐使用iburst选项,如果第一次尝试无法建立连接,程序会发送一系列的包。burst 选项则总是发送一系列的包,即使第一次也是这样。如果没有明确的允许的话不要使用 burst 选项,有可能被封禁。

NTP 服务器模式

如果建立一个 NTP 服务器,你需要添加 local clock 作为一个服务器,这样,即便它失去网络连接,它也可以继续为网络提供服务;添加 local clock 作为一个 stratum 10 服务器 (使用 fudge 命令)这样它就只会在失去连接时使用本地时钟:

server 127.127.1.0
fudge  127.127.1.0 stratum 10

下一步,定义规则允许客户端连接你的服务(localhost 也被认为是一个客户端)。使用 restrict 命令;你应该在文件中已经有一行:

restrict default nomodify nopeer noquery

这限制了每个人做任何修改并阻止每个人请求你的时间服务器状态:nomodify 防止重新配置你的ntpd(使用ntpqntpdc ),noquery 防止从你的nptd(或是ntpqntpdc)获取状态数据。

你也能添加其它选项:

restrict default kod nomodify notrap nopeer noquery
注意: 这会允许其他人查询你的时间服务器。你需要添加 noserve 来停止提供时间。

"restrict"选项的完整文档可以从 man ntp_acc 中查找到。详见 https://support.ntp.org/bin/view/Support/AccessRestrictions

你需要在这一行之后告诉 ntpd 什么可以访问你的服务器;如果你不是在配置一台 NTP 服务器的话,下面一行就足够了。

restrict 127.0.0.1

如果你想要强制DNS解析到IPv6域名,在IP地址或域名前写上 -6-4 则强制使用IPv4域名),例如:

restrict -6 default kod nomodify notrap nopeer noquery
restrict -6 ::1    # ::1 is the IPv6 equivalent for 127.0.0.1

最后,指定drift文件(它能时刻监控你的时钟的时间漂移)和log文件的位置:

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log

一份基础的配置文件是这样的 (为了清晰起见,已删掉了所有的注释):

/etc/ntp.conf
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery

restrict 127.0.0.1
restrict -6 ::1  

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log
注意: 定义日志文件不是必须的,但是它对于反馈 ntpd 操作是有好处的。

使用

软件包默认包含客户端模式的配置,并且使用单独的用户和群组,启动时就会移除 root 权限。如果在终端中启动,请使用 -u 选项:

# ntpd -u ntp:ntp

systemd 服务默认使用 -u 选项和 -g 选项禁用一个阈值(panic-gate). 这样即使 ntp-server 的时间和系统时间的差异超过阈值,依然会同步时间。

Warning: 使用 panic-gate 的原因是某些后台任务或服务会引起时间跳跃. 如果系统的时间从来没有同步过,请考虑先禁用其他服务再进行同步。

两个服务都依赖系统网络状况,会在检测到网络连接时开始同步。

启动时启用 ntpd

启用 ntpd.service 服务.

Note: systemd 命令 timedatectl 仅可以控制 systemd-timesyncd, 用 root 执行 timedatectl set-ntp 1 会停止运行中的 ntpd.service.[1]

ntpq 可以查看同步的状态:

$ ntpq -p

delay, offset 和 jitter 不应该为零,ntpd 同步的服务器前有星号,ntpd 可能等待很多分钟后才会进行同步,请等 17 分钟 (1024 秒).

每次启动同步一次

另一种方式是 启用 ntpdate.service 服务,每次启动都同步一次(-q) 并且是 non-forking (-n), 进程不会在后台运行。如果是服务器或者很多天才会重启一次,不建议使用此方式。

如果需要把同步到的时间写入硬件时钟,请按照 这里 的说明修改服务并启动:

/etc/systemd/system/ntpdate.service.d/hwclock.conf
[Service]
ExecStart=/usr/bin/hwclock -w

技巧

Start ntpd on network connection

ntpd can be started by your network manager, so that the daemon only runs when the computer is online.

Netctl

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: add -u and optionally refer to #Usage, or use systemctl if possible (Discuss in Talk:Network Time Protocol daemon (简体中文)#)

Append the following lines to your netctl profile:

ExecUpPost='/usr/bin/ntpd || true'
ExecDownPre='killall ntpd || true'
NetworkManager

The ntpd daemon can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. The networkmanager-dispatcher-ntpd package installs one, pre-configured to start and stop the ntpd service[broken link: invalid section] with a connection.

Wicd

For Wicd, create a start script in the postconnect directory and a stop script in the predisconnect directory. Remember to make them executable:

/etc/wicd/scripts/postconnect/ntpd
#!/bin/bash
systemctl start ntpd &
/etc/wicd/scripts/predisconnect/ntpd
#!/bin/bash
systemctl stop ntpd &
Note: You are advised to customize the options for the ntpd command as explained in #Usage[broken link: invalid section].

See also Wicd#Scripts.

KDE

KDE can use NTP (ntp must be installed) by right clicking the clock and selecting Adjust date/time. However, this requires the ntp daemon to be disabled before configuring KDE to use NTP. [2]

Using ntpd with GPS

Most of the articles online about configuring ntpd to receive time from a GPS suggest to use the SHM (shared memory) method. However, at least since ntpd version 4.2.8 a much better method is available. It connects directly to gpsd, so gpsd needs to be installed.

Add these lines to your /etc/ntp.conf:

/etc/ntp.conf
#=========================================================
#  GPSD native ntpd driver
#=========================================================
# This driver exists from at least ntp version 4.2.8
# Details at
#   https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver46.html
server 127.127.46.0 
fudge 127.127.46.0 time1 0.0 time2 0.0 refid GPS

This will work as long as you have gpsd working. It connects to gpsd via the local socket and queries the "gpsd_json" object that is returned.

To test the setup, first ensure that gpsd is working by running:

 $ cgps -s 

Then wait a few minutes and run ntpq -p. This will show if ntpd is talking to gpsd:

$ ntpq -p
remote           refid            st t when poll reach   delay   offset  jitter
 ==================================================================================
*GPSD_JSON(0)    .GPS.            0 l   55   64  377    0.000    2.556  14.109
Tip: If the reach column is 0, it means ntpd has not been able to talk to gpsd. Wait a few minutes and try again. Sometimes it takes ntpd a while.

Running in a chroot

Note: ntpd should be started as non-root (default in the Arch Linux package) before attempting to jail it in a chroot, since chroots are relatively useless at securing processes running as root.

Create a new directory /etc/systemd/system/ntpd.service.d/ if it does not exist and a file named customexec.conf inside with the following content:

[Service]
ExecStart=
ExecStart=/usr/bin/ntpd -g -i /var/lib/ntp -u ntp:ntp -p /run/ntpd.pid

Then, edit /etc/ntp.conf to change the driftfile path such that it is relative to the chroot directory, rather than to the real system root. Change:

driftfile       /var/lib/ntp/ntp.drift

to

driftfile       /ntp.drift

Create a suitable chroot environment so that getaddrinfo() will work by creating pertinent directories and files (as root):

# mkdir /var/lib/ntp/etc /var/lib/ntp/lib /var/lib/ntp/proc
# touch /var/lib/ntp/etc/resolv.conf /var/lib/ntp/etc/services

and by bind-mounting the aformentioned files:

/etc/fstab
...
#ntpd chroot mounts
/etc/resolv.conf  /var/lib/ntp/etc/resolv.conf none bind 0 0
/etc/services	  /var/lib/ntp/etc/services none bind 0 0
/lib		  /var/lib/ntp/lib none bind 0 0
/proc		  /var/lib/ntp/proc none bind 0 0
# mount -a

Finally, restart ntpd daemon again. Once it restarted you can verify that the daemon process is chrooted by checking where /proc/{PID}/root symlinks to:

# ps -C ntpd | awk '{print $1}' | sed 1d | while read -r PID; do ls -l /proc/$PID/root; done

should now link to /var/lib/ntp instead of /.

It is relatively difficult to be sure that your driftfile configuration is actually working without waiting a while, as ntpd does not read or write it very often. If you get it wrong, it will log an error; if you get it right, it will update the timestamp. If you do not see any errors about it after a full day of running, and the timestamp is updated, you should be confident of success.

参见