1. NTP配置文件 ntp.conf

配置文件路径：/etc/ntp.conf

配置格式：关键字(如server) 参数(如prefer)

以换行为结束，所以一个配置不能占多行。

ntp.conf包括两类配置命令集，一类叫配置命令(configuration commands)。另一类叫辅助命令(auxiliary commands)

1.1. 配置文件命令

1.2. driftfile 偏差文件路径

系统时间与BIOS时间的偏差记录; 将自己主机的bios芯片震荡频率与上层的Time server频率比较，将误差记录在这个文件里

driftfile /etc/ntp/drift 

1.3. restrict 控制相关权限

restrict address [mask mask] [flag][...]

address地址参数可以是IP地址或者能够解析的域名。mask是子网掩码。默认的是(address 0.0.0.0,mask 0.0.0.) Note that the text string default, with no mask option, may be used to indicate the default entry.

Some flags have the effect to deny service, some have the effect to enable service and some are conditioned by other flags. The flags. are not orthogonal, in that more restrictive flags will often make less restrictive ones redundant. The flags that deny service are classed in two categories, those that restrict time service and those that restrict informational queries and attempts to do run-time reconfiguration of the server. One or more of the following flags may be specified:

flake Discard received NTP packets with probability 0.1; that is, on average drop one packet in ten. This is for testing and amusement. The name comes from Bob Braden's flakeway, which once did a similar thing for early Internet testing.

Default restriction list entries with the flags ignore, ntpport, for each of the local host's interface addresses are inserted into the table at startup to prevent the server from attempting to synchronize to its own time. A default entry is also always present, though if it is otherwise unconfigured; no flags are associated with the default entry (i.e., everything besides your own NTP server is unrestricted).

语法为： restrict IP地址 mask 子网掩码 参数

其中IP地址也可以是default ，default 就是指所有的IP

参数有以下几个：

restrict    default nomodify notrap nopeer noquery
restrict -6 default nomodify notrap nopeer noquery
或者
restrict    default ignore
restrict -6 default ignore

默认权限配置是先禁止所有的权限，然后再开放指定的权限

restrict 127.0.0.1 
restrict ::1

一般会开放本地环回接口所有权限，例如可以通过本地时间作为时间源

1.4. server 时间源服务器

For type s and r addresses (only), this command mobilizes a persistent client mode association with the specified remote server or local reference clock. If the preempt flag is specified, a preemptable client mode association is mobilized instead.

语法为：server 时间源IP/域名 参数

server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int] This command can be used to configure reference clocks in special ways. The options are interpreted as follows:

1.5. fudge 服务器层次

这行是时间服务器的层次。stratum 0 表示1级primary reference，为顶级；如果要向别的NTP服务器更新时间，请不要把它设为0

server 127.127.1.0   
fudge 127.127.1.0 stratum 0  

fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1] This command can be used to configure reference clocks in special ways. It must immediately follow the server command which configures the driver. Note that the same capability is possible at run time using the ntpdc program. The options are interpreted as follows: time1 sec Specifies a constant to be added to the time offset produced by the driver, a fixed-point decimal number in seconds. This is used as a calibration constant to adjust the nominal time offset of a particular clock to agree with an external standard, such as a precision PPS signal. It also provides a way to correct a systematic error or bias due to serial port or operating system latencies, different cable lengths or receiver internal delay. The specified offset is in addition to the propagation delay provided by other means, such as internal DIPswitches. Where a calibration for an individual system and driver is available, an approximate correction is noted in the driver documentation pages. Note: in order to facilitate calibration when more than one radio clock or PPS signal is supported, a special calibration feature is available. It takes the form of an argument to the enable command described in the Miscellaneous Options page and operates as described in the Reference Clock Drivers page. time2 secs Specifies a fixed-point decimal number in seconds, which is interpreted in a driver-dependent way. See the descriptions of specific drivers in the reference clock drivers page. stratum int Specifies the stratum number assigned to the driver, an integer between 0 and 15. This number overrides the default stratum number ordinarily assigned by the driver itself, usually zero. refid string Specifies an ASCII string of from one to four characters which defines the reference identifier used by the driver. This string overrides the default identifier ordinarily assigned by the driver itself. flag1 flag2 flag3 flag4 These four flags are used for customizing the clock driver. The interpretation of these values, and whether they are used at all, is a function of the particular clock driver. However, by convention flag4 is used to enable recording monitoring data to the clockstats file configured with the filegen command. Further information on the filegen command can be found in the Monitoring Options page.

1.6. tinker

tinker [ allan allan | dispersion dispersion | freq freq | huffpuff huffpuff | panic panic | step step | stepout stepout ]

该命令改变当前系统给定的钟驯化算法(the clock discipline algorithm)，该默认值已经优化过了。很少需要改变该默认值；但是有些人非要改变该值得话，每个值得解释如下:

1.7. peer 时间互相同步

仅对于NTP服务器有效，这个命令指定了多台NTP服务器之间时间同步模式

peer 172.31.1.1 iburst minpoll 4 maxpoll 6  perfer   
peer 172.31.1.2 iburst minpoll 4 maxpoll 6

1.8. includefile 允许包含其他的配置文件

includefile includefile 允许包含其他的配置文件。

1.9. Links

• http://doc.ntp.org/4.2.6/comdex.html

• http://doc.ntp.org/4.2.6/miscopt.html