ログイン後、rootユーザにスイッチします
[centos@centos ~]$ su -
パスワード: rootユーザーのパスワード

NTPサーバーをインストールします
[root@centos ~]# yum -y install ntp
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
* base: ftp.iij.ad.jp
* updates: rsync.atworks.co.jp
* addons: www.ftp.ne.jp
* extras: www.ftp.ne.jp
Setting up Install Process
Parsing package install arguments
Resolving Dependencies
–> Running transaction check
—> Package ntp.i386 0:x.x.xxx-x.xxx.centos set to be updated
–> Finished Dependency Resolution

Dependencies Resolved

===================================================================================================================================
Package Arch Version Repository Size
===================================================================================================================================
Installing:
ntp i386 x.x.xxx-x.xxx.centos base 1.3 M

Transaction Summary
===================================================================================================================================
Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)

Total download size: 1.3 M
Downloading Packages:
ntp-x.x.xxx-x.xxx.centos.i386.rpm | 1.3 MB 00:00
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing : ntp [1/1]

Installed: ntp.i386 x:x.x.xxx-x.xxx.centos
Complete!

[root@centos ~]# vi /etc/ntp.conf
viエディタが起動するのでNTPサーバー設定ファイルを修正します
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery

# Permit all access over the loopback interface. This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
restrict -6 ::1

# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
ローカルネットワークの時間同期を許可します
※設定しているIPアドレスはyasuの環境になりますので、ご自身の環境にあわせて設定をしてください
restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
NTP/推奨公開サーバ – wiki@nothingを参考にして時間同期する為のNTPサーバーを変更します
#server 0.centos.pool.ntp.org　先頭に#をつけてコメント
#server 1.centos.pool.ntp.org　先頭に#をつけてコメント
#server 2.centos.pool.ntp.org　先頭に#をつけてコメント
日本標準時プロジェクトのNTPサーバー
server ntp.nict.jp
server ntp.nict.jp
server ntp.nict.jp
インターネットマルチフィード時刻情報サービスのNTPサーバー
server ntp1.jst.mfeed.ad.jp
server ntp2.jst.mfeed.ad.jp
server ntp3.jst.mfeed.ad.jp

#broadcast 192.168.1.255 key 42 # broadcast server
#broadcastclient # broadcast client
#broadcast 224.0.1.1 key 42 # multicast server
#multicastclient 224.0.1.1 # multicast client
#manycastserver 239.255.254.254 # manycast server
#manycastclient 239.255.254.254 key 42 # manycast client

# Undisciplined Local Clock. This is a fake driver intended for backup
# and when no outside source of synchronized time is available.
server 127.127.1.0 # local clock
fudge 127.127.1.0 stratum 10

# Drift file. Put this in a directory which the daemon can write to.
# No symbolic links allowed, either, since the daemon updates the file
# by creating a temporary in the same directory and then rename()’ing
# it to the file.
driftfile /var/lib/ntp/drift

# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography.
keys /etc/ntp/keys

# Specify the key identifiers which are trusted.
#trustedkey 4 8 42

# Specify the key identifier to use with the ntpdc utility.
#requestkey 8

# Specify the key identifier to use with the ntpq utility.
#controlkey 8

内容を保存しviエディタを終了します

[root@centos ~]# vi /etc/sysconfig/iptables
viエディタが起動するので設定ファイルを変更します。
# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT – [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp –icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp –dport 5353 -d 224.0.0.251 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp -m udp –dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 22 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 53 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m udp -p udp –dport 53 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 80 -j ACCEPT
下記2行を追加してTCPとUDPの123番ポート(ntp)を開放します
-A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 123 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m udp -p udp –dport 123 -j ACCEPT
-A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 443 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT –reject-with icmp-host-prohibited

内容を保存しviエディタを終了します

設定内容を反映します
[root@centos ~]# /etc/rc.d/init.d/iptables restart
ファイアウォールルールを適用中: [ OK ]
チェインポリシーを ACCEPT に設定中filter [ OK ]
iptables モジュールを取り外し中 [ OK ]
iptables ファイアウォールルールを適用中: [ OK ]
iptables モジュールを読み込み中ip_conntrack_netbios_ns [ OK ]

手動でNTPサーバーに接続して時刻を合わせます(NTPサーバー起動に大幅に時間がずれているとNTPサーバーが起動できない為)
[root@centos html]# ntpdate ntp.nict.jp
2 Apr 14:26:50 ntpdate[3542]: step time server 133.243.238.244 offset -94.600070 sec

NTPサーバーを起動します
[root@centos ~]# /etc/rc.d/init.d/ntpd start
ntpd を起動中: [ OK ]

NTPサーバーの自動起動設定を行います
[root@centos =]# chkconfig ntpd on
[root@centos =]# chkconfig –list ntpd
ntpd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

NTPサーバーの自動時間同期の確認を行います
[root@centos ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
+ntp-b3.nict.go. .NICT. 1 u 46 64 17 9.557 -0.351 0.152
*ntp-b2.nict.go. .NICT. 1 u 49 64 17 10.121 -0.547 0.707
+ntp-a3.nict.go. .NICT. 1 u 47 64 17 8.473 -0.385 1.096
ntp1.jst.mfeed. 210.173.160.56 2 u 40 64 17 5.442 0.648 0.428
-ntp2.jst.mfeed. 210.173.176.4 2 u 44 64 17 5.619 0.330 0.855
ntp3.jst.mfeed. 210.173.160.86 2 u 40 64 17 5.867 0.890 0.166
LOCAL(0) .LOCL. 10 l 43 64 17 0.000 0.000 0.004

ntpq出力結果の説明はこちら

/etc/default/ntpdateファイルを変更します
ubuntu@ubuntu:~$ sudo vi /etc/default/ntpdate
[sudo] password for ubuntu: ubuntuユーザのパスワードを応答します
viエディタが起動するのでNTPサーバー設定ファイルを修正します
# The settings in this file are used by the program ntpdate-debian, but not
# by the upstream program ntpdate.

# Set to “yes” to take the server list from /etc/ntp.conf, from package ntp,
# so you only have to keep it in one place.
NTPDATE_USE_NTP_CONF=yes

# List of NTP servers to use (Separate multiple servers with spaces.)
# Not used if NTPDATE_USE_NTP_CONF is yes.
NTPSERVERS=“sa-sa-ki.jp”

# Additional options to pass to ntpdate
NTPOPTIONS=”"

内容を保存しviエディタを終了します

設定内容を反映します
ubuntu@ubuntu:~$ sudo /etc/network/if-up.d/ntpdate