通过网络ntp自动同步
如果只需要同步一次，那么执行如下命令即可
rdate -s time.nist.gov
其中time.nist.gov 是一个时间服务器，当然你也可以使用另外的时间服务器。
如果是多台服务器的时间需要保持完全同步，那么最好还是给这些服务器配置统一的自动同步任务，以免因为各种原因，造成它们的时间有所偏差。方法如下
使用 crontab 让linux自动同步时间
vi /etc/crontab
加上一句：
00 0 1 * * root rdate -s time.nist.gov
time.nist.gov 是一个时间服务器.
rdate同步配置
centos最小化安装默认不安装，先确认已经安装过，否则先安装。
其中rdate本身是用来获取远程时间服务器上时间用的，带上 -s 参数，就可以将获取到的时间应用到本地系统。
NAME
rdate - get the time via the network
SYNOPSIS
rdate [-p] [-s] [-u] [-l] [-t sec] [host...]
DESCRIPTION
rdate connects to an RFC www.111cn.net 868 time server over a TCP/IP network, printing the returned time
and/or setting the system clock.
OPTIONS
-p Print the time returned by the remote machine.
-s Set the system time to the returned time.
-u Use UDP instead of TCP as the transport.
-l Use syslog to output errors (cron.warning) and output (cron.info).
-t Set timeout in seconds for every attempt to retreive date.
rdate -s time.nist.gov
ntpdate 本身就是用来同步时间的工具。
示例 ：ntpdate ntp.sjtu.edu.cn
NAME
ntpdate - set the date and time via NTP
Disclaimer: The functionality of this program is now available in the ntpd program. See the -q com-
mand line option in the ntpd - Network Time Protocol (NTP) daemon page. After a suitable period of
mourning, the ntpdate program is to be retired from this distribution
SYNOPSIS
ntpdate [ -46bBdqsuv ] [ -a key ] [ -e authdelay ] [ -k keyfile ] [ -o version ] [ -p samples ] [ -t
timeout ] [ -U user_name ] server [ ... ]
DESCRIPTION
[略，具体参看linux man手册]
Linux集群时间同步
打算将11台Linux集群时间同步一下，操作系统都为Ubuntu 11.04 x86 64位
选定其中一台133.133.10.2作为时间服务器（ntp server），其他节点与该节点进行时间同步。
1、在133.133.10.2（monitor）上安装ntp server。
root@monitor:~# apt-get install ntp
2、修改/etc/ntp.conf
# /etc/ntp.conf, configuration for ntpd; see ntp.conf(5) for help
driftfile /var/lib/ntp/ntp.drift
# Enable this if you want statistics to be logged.
#statsdir /var/log/ntpstats/
statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
filegen clockstats file clockstats type day enable
# 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.
#server 0.ubuntu.pool.ntp.org
#server 1.ubuntu.pool.ntp.org
#server 2.ubuntu.pool.ntp.org
#server 3.ubuntu.pool.ntp.org
# Use Ubuntu's ntp server as a fallback.
#server ntp.ubuntu.com
server 127.127.1.0 minpoll 4 maxpoll 5
fudge 127.127.1.0 stratum 2
# Access control configuration; see /usr/share/doc/ntp-doc/html/accopt.html for
# details. The web page <http://support.ntp.org/bin/view/Support/AccessRestrictions>
# might also be helpful.
#
# Note that "restrict" applies to both servers and clients, so a configuration
# that might be intended to block requests from certain clients could also end
# up blocking replies from your own upstream servers.
# By default, exchange time with everybody, but don't allow configuration.
#restrict -4 default kod notrap nomodify nopeer noquery
#restrict -6 default kod notrap nomodify nopeer noquery
restrict -4 default kod nomodify noper
restrict -6 default kod nomodify noper
# Local users may interrogate the ntp server more closely.
restrict 127.0.0.1
restrict ::1
# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
#restrict 192.168.123.0 mask 255.255.255.0 notrust
# If you want to provide time to your local subnet, change the next line.
# (Again, the address is an example only.)
#broadcast 192.168.123.255
# If you want to listen to time broadcasts on your local subnet, de-comment the
# next lines. Please do this only if you trust everybody on the network!
#disable auth
#broadcastclient
3、运行ntpdate命令
首先重启一下ntp服务
root@monitor:/etc#/etc/init.d/ntp restart
再运行
root@monitor:/etc# ntpdate -q 127.0.0.1
得到
server 127.0.0.1, stratum 3, offset -0.000003, delay 0.02565
23 Sep 17:12:00 ntpdate[11502]: adjust time server 127.0.0.1 offset -0.000003 sec
4、将其他节点133.133.10.*的时间与133.133.10.2同步
在其他每一个节点运行命令
root@slave1:~# ntpdate 133.133.10.2
这时候发现节点间的时间同步了，但ntpdate只在开机运行，我们若要设置为1小时同步一次，那么进行第5步
5、将其他节点133.133.10.*的时间定期与133.133.10.2同步
安装完以后使用crontab来自动更新时间：
#crontab -e
添加如下行：
* */1 * * * /usr/sbin/ntpdate 133.133.10.2
这行的意思是每个小时与与133.133.10.2时间同步一次。
6、通过date命令发现所有节点时间都一致了