[root@localhost yum.repos.d]# yum clean all
Loaded plugins: fastestmirror, langpacks, product-id, search-disabled-repos,
              : subscription-manager
This system is not registered with an entitlement server. You can use subscription-manager to register.
There are no enabled repos.
 Run "yum repolist all" to see the repos you have.
 To enable Red Hat Subscription Management repositories:
     subscription-manager repos --enable <repo>
 To enable custom repositories:
     yum-config-manager --enable <repo>

遇到上面的问题，转载一篇解决方案：http://www.linuxidc.com/Linux/2013-10/90932.htm

https://bbs.vpser.net/thread-7553-1-1.html

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今日网上搜索了很多关于RHEL6.4更改为CentOS源的文章，都是按照一步一步去操作，但都是不行的。后来自己综合了各篇文章，修修改改，终于成功更改了yum源。

第一步：删除默认的yum源。命令如下
rpm-aq | grep yum|xargs rpm -e--nodeps

第二步：下载最新的yum安装包

http://mirrors.163.com/centos/7/os/x86_64/Packages/

wget http://ftp.nara.wide.ad.jp/pub/Linux/centos/6.4/os/x86_64/Packages/python-iniparse-0.3.1-2.1.el6.noarch.rpm
wget http://ftp.nara.wide.ad.jp/pub/Linux/centos/6.4/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
wget http://ftp.nara.wide.ad.jp/pub/Linux/centos/6.4/os/x86_64/Packages/yum-3.2.29-40.el6.centos.noarch.rpm
wget http://ftp.nara.wide.ad.jp/pub/Linux/centos/6.4/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-14.el6.noarch.rpm

第三步：安装yum相关软件。
rpm-ivh python-iniparse-0.3.1-2.1.el6.noarch.rpm
rpm-ivh yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
rpm-ivh yum-3.2.29-40.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.30-14.el6.noarch.rpm

第四步：更改yum源，我们使用网易的镜像源，可以去这里http://mirrors.163.com/.help/centos.html 下载最新的CentOS-Base.repo文件，然后cp到/etc/yum.repos.d/目录下，修改把文件里面的$releasever全部替换为版本号，即6 最后保存。最后把这个文件命名为rhel-source.repo，替换掉原来该目录的同名文件。

第五步：清理yum缓存
yum clean all
yum makecache
最后显示如下，更换yum源成功。

Loaded plugins: fastestmirror, product-id, subscription-manager
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
Determining fastest mirrors
base | 3.7 kB 00:00 
base/group_gz | 212 kB 00:00 
base/filelists_db | 5.9 MB 00:12 
base/primary_db | 4.4 MB 00:09 
base/other_db | 2.7 MB 00:05 
extras | 3.4 kB 00:00 
extras/filelists_db | 10 kB 00:00 
extras/prestodelta | 905 B 00:00 
extras/primary_db | 18 kB 00:00 
extras/other_db | 5.7 kB 00:00 
updates | 3.4 kB 00:00 
updates/filelists_db | 4.0 MB 00:08 
updates/prestodelta | 978 kB 00:02 
updates/primary_db | 4.4 MB 00:09 
updates/other_db | 661 kB 00:01