RHEL的YUM源需要注册用户才能更新使用，由于CentOS和RHEL基本没有区别，并且CentOS已经被REHL收购。所以将RHEL的YUM源替换为CentOS即可。
问题如下：
[root@bogon ~]# yum repolist
Loaded plugins: product-id, refresh-packagekit, security, subscription-manager
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
repolist: 0
或者# yum install ntp
Loaded plugins: product-id, refresh-packagekit, security, subscription-manager
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
Setting up Install Process
Nothing to do
解决方法：
1.清除原有RHEL的YUM及相关软件包。
rpm -qa | grep yum | xargs rpm -e --nodeps
rpm -qa |grep python-urlgrabber|xargs rpm -e --nodeps
2.下载centos6的相关软件包。
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-3.2.29-73.el6.centos.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-37.el6.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/python-iniparse-0.3.1-2.1.el6.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/python-urlgrabber-3.9.1-11.el6.noarch.rpm
*注释:这些软件包的具体的版本，请到http://mirrors.163.com/centos/6/os/x86_64/Packages/ 目录下，选择下载上述软件包。网上很多文章没法直接使用就是因为软件包的版本问题。

3.安装软件包。
rpm -ivh python-iniparse-0.3.1-2.1.el6.noarch.rpm
rpm -ivh python-urlgrabber-3.9.1-11.el6.noarch.rpm
rpm -ivh yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
rpm -ivh yum-plugin-fastestmirror-1.1.30-37.el6.noarch.rpm yum-3.2.29-73.el6.centos.noarch.rpm
*注释yum-plugin-fastestmirror和yum-3.2.29要一起安装。

如果按照遇到问题：发现错误，python-urlgrabber版本必须大于等于3.9.1-10
?# rpm -ivh? yum-3.2.29-73.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.30-37.el6.noarch.rpm
warning: yum-3.2.29-73.el6.centos.noarch.rpm: Header V3 RSA/SHA1 Signature, key ID c105b9de: NOKEY
error: Failed dependencies:
?? ?python-urlgrabber >= 3.9.1-10 is needed by yum-3.2.29-73.el6.centos.noarch
查看系统的python信息
# rpm -qa|grep python

发现python-urlgrabber-3.9.1-9.el6.noarch 版本低于3.9.1-10?
卸载：python-urlgrabber-3.9.1-9.el6.noarch
?rpm -e python-urlgrabber-3.9.1-9.el6.noarch
安装：rpm -ivh python-urlgrabber-3.9.1-11.el6.noarch.rpm

安装yum
# rpm -ivh yum-*
更改yum源
cd /etc/yum.repos.d/
下载配置文件
wget http://mirrors.163.com/.help/CentOS6-Base-163.repo
将此配置文件替换/etc/yum.repos.d同名文件
sed -i "s;\$releasever;6;g" CentOS6-Base-163.repo
mv rhel-source.repo rhel-source.repo.bak

或者编辑配置文件

#vim CentOS-Base.repo

把$releasever替换成现有系统的版本号(6)
vim替换命令
：%s/$releasever/6/g
退出保存

清理yum缓存
#yum clean all
将服务器软件包信息缓存至本地，提高搜索安装效率
#yum makecache

测试
#yum seach vim

执行yum repolist查看，如果显示出repo仓库列表，并显示软件包数量则OK。（或者使用yum makecache）

6.使用yum安装软件包测试。
yum -y install lrzsz

至此，如果不出现意外，lrzsz软件包安装成功。yum也就可以正常使用了。

一.安装NTP软件包：

yum -y install ntp /*yum安装NTP服务*/

chkconfig --add ntpd /*添加NTP*/

chkconfig ntpd on /*开机自启动NTP*/

二.修改NTP配置文件：

vi /etc/ntp.conf
***************************************************************

# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).

driftfile /var/lib/ntp/drift
restrict default ignore 设置默认策略为拒绝所有访问方式的请求
# 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 允许局域网内机器同步时间

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.CentOS.pool.ntp.org
server 1.centos.pool.ntp.org 设置同步服务器
server 2.centos.pool.ntp.org

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

restrict 0.centos.pool.ntp.org nomodify notrap noquery
restrict 1.centos.pool.ntp.org nomodify notrap noquery 允许与上层服务器同步时间
restrict 2.centos.pool.ntp.org nomodify notrap noquery

# 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 外界同步源无法联系时，使用本地时间为同步服务

# Enable public key cryptography.
#crypto

includefile /etc/ntp/crypto/pw

# 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

# Enable writing of statistics records.
#statistics clockstats cryptostats loops

***************************************************************

时间同步
命令：
# ntpdate ip (同步时间服务器ip)
#hwclock -w //将时间写入硬件
例子
ntpdate 210.167.182.10
同步时间服务器：
server
ntp-sop.inria.frserver 210.72.145.44(中国国家授时中心服务器IP地址)
server ntp.sjtu.edu.cn(上海交通大学网络中心NTP服务器地址)

执行：
# ntpdate 210.167.182.10 (同步时间服务器ip)
#hwclock -w //将时间写入硬件

借鉴地址：忘了

RHEL6解决无法使用YUM源问题