新安装了redhat6.5.安装后，登录系统，使用yum update 更新系统。提示：
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to
register.
无法更新。
redhat 默认自带的 yum 源需要注册，才能更新。我们想不花钱也可以更新，需要替换掉redhat的yum源。

1.检查是否安装yum包
查看RHEL是否安装了yum，若是安装了，那么又有哪些yum包：

[root@localhost ~]# rpm -qa |grep yum

yum-metadata-parser-1.0-8.fc6
yum-3.0.1-5.el5
yum-rhn-plugin-0.4.3-1.el5
yum-updatesd-3.0.1-5.el5
2 删除redhat自带的yum包
卸载上面显示的所有yum包：

[root@localhost ~]# rpm -qa|grep yum|xargs rpm -e --nodeps

（不检查依赖，直接删除rpm包）
再用

[root@localhost ~]# rpm -qa |grep yum [root@localhost ~]#

查看，无信息显示表示已经卸载完成。

3.下载新的yum包。使用Centos6.5的yum包

#wget http://centos.ustc.edu.cn/centos/6.5/os/x86_64/Packages/yum-3.2.29-40.el6.centos.noarch.rpm #wget http://centos.ustc.edu.cn/centos/6.5/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm #wget http://centos.ustc.edu.cn/centos/6.5/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-14.el6.noarch.rpm

安装yum软件包
注意:单个的安装包可能会依赖其它包（例如yum和yumfastestmirror
会相互依赖），所以我们可以把所有这些包
放在一起，用一行命令将它们同时安装即可：

#rpm -ivh *

4.更换yum源。

# cd /etc/yum.repos.d/ # vi CentOS‐Base.repo

编辑文件，添加以下内容：

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base - mirrors.ustc.edu.cn
baseurl=http://mirrors.ustc.edu.cn/centos/$releasever/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
gpgcheck=1
gpgkey=http://mirrors.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

#released updates
[updates]
name=CentOS-$releasever - Updates - mirrors.ustc.edu.cn
baseurl=http://mirrors.ustc.edu.cn/centos/$releasever/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=http://mirrors.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - mirrors.ustc.edu.cn
baseurl=http://mirrors.ustc.edu.cn/centos/$releasever/extras/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
gpgcheck=1
gpgkey=http://mirrors.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - mirrors.ustc.edu.cn
baseurl=http://mirrors.ustc.edu.cn/centos/$releasever/centosplus/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
gpgcheck=1
enabled=0
gpgkey=http://mirrors.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib - mirrors.ustc.edu.cn
baseurl=http://mirrors.ustc.edu.cn/centos/$releasever/contrib/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
gpgcheck=1
enabled=0
gpgkey=http://mirrors.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

5. 导入KEY 文件，注意大小写

#rpm --import http://centos.ustc.edu.cn/centos/RPM-GPG-KEY-CentOS-6

6.测试下

#yum list

测试的时候报错，异常为：

http://mirrors.ustc.edu.cn/centos/%24releasever/os/x86_64/repodata/repomd.xml   [Errno 14] HTTP Error 404 :

需要删除2个redhat 的rpm ,分别为 redhat-indexhtml-6-6.el6.noarch 和 redhat-release-server-6Server-6.5.0.1.el6.x86_64

#rpm -e --nodeps redhat-indexhtml-6-6.el6.noarch #rpm -e --nodeps redhat-release-server-6Server-6.5.0.1.el6.x86_64
#删除
8. 下载Centos的release和indexhtml包，并重装。

#wget http://centos.ustc.edu.cn/centos/6.5/os/x86_64/Packages/centos-release-6-5.el6.centos.11.1.x86_64.rpm #wget http://centos.ustc.edu.cn/centos/6.5/os/x86_64/Packages/centos-indexhtml-6-1.el6.centos.noarch.rpm

再安装centos-release和centos-indexhtml 这两个rpm 。

重新测试下

#yum list

清除原有缓存

#yum clean all

重建缓存，以提高搜索安装软件的速度

#yum makecache

这样就配置完毕。

参考资料：

https://lug.ustc.edu.cn/wiki/mirrors/help/centos

http://blog.csdn.net/wwwgeyang777/article/details/12945471