cp /etc/yum.repos.d/CentOS-Base.repo CentOS-Base.repo_bak
vi /etc/yum.repo.d/CentOS-Base.repo
可以用中科大(ustc)yum源，也可以用上海交大(sjtu)yum源，兩者只能選一個。
用下面的內容覆蓋CentOS-Base.repo就行了。

#中科大(ustc)yum源，http://centos.ustc.edu.cn/centos/5/就是最新的，
#也可以設置成http://centos.ustc.edu.cn/centos/5.5/

[base]
name=CentOS-5 - Base
baseurl=http://centos.ustc.edu.cn/centos/5/os/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#released updates
[update]
name=CentOS-5 - Updates
baseurl=http://centos.ustc.edu.cn/centos/5/updates/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#packages used/produced in the build but not released
[addons]
name=CentOS-5 - Addons
baseurl=http://centos.ustc.edu.cn/centos/5/addons/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#additional packages that may be useful
[extras]
name=CentOS-5 - Extras
baseurl=http://centos.ustc.edu.cn/centos/5/extras/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5 - Plus
baseurl=http://centos.ustc.edu.cn/centos/5/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#contrib - packages by Centos Users
[contrib]
name=CentOS-5 - Contrib
baseurl=http://centos.ustc.edu.cn/centos/5/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#第一部分結束

#============分割線============ 注意，上下兩部分只選一個

# sjtu yum源，http://ftp.sjtu.edu.cn/centos/5/就是最新的，
#也可以設置成http://ftp.sjtu.edu.cn/centos/5.5/

[base]
name=CentOS-5 - Base
baseurl=http://ftp.sjtu.edu.cn/centos/5/os/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#released updates
[update]
name=CentOS-5 - Updates
baseurl=http://ftp.sjtu.edu.cn/centos/5/updates/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#packages used/produced in the build but not released
[addons]
name=CentOS-5 - Addons
baseurl=http://ftp.sjtu.edu.cn/centos/5/addons/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#additional packages that may be useful
[extras]
name=CentOS-5 - Extras
baseurl=http://ftp.sjtu.edu.cn/centos/5/extras/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5 - Plus
baseurl=http://ftp.sjtu.edu.cn/centos/5/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#contrib - packages by Centos Users
[contrib]
name=CentOS-5 - Contrib
baseurl=http://ftp.sjtu.edu.cn/centos/5/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5

#第二部分結束

第2步

中科大(ustc)yum源
32位: rpm --import http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5
64位: rpm --import http://centos.ustc.edu.cn/centos/5/os/x86_64/RPM-GPG-KEY-CentOS-5

上海交大(sjtu)yum源
32位: rpm --import http://ftp.sjtu.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5
64位: rpm --import http://ftp.sjtu.edu.cn/centos/5/os/x86_64/RPM-GPG-KEY-CentOS-5

第3步

yum update

After that if you have a proble like that.Do like this:

每次yum install 時總是出現如下錯誤

此時聰明的我馬上想到了是不是服務器解析不了域名了，導致repomd.xml下載不下來呢？

然后我就ping了一下mirrors.ustc.edu.cn ， 果然出現錯誤：ping: unknown host?mirrors.ustc.edu.cn。顯然是DNS配置問題了，DNS域名服務器IP地址配的不對。

把/etc/hosts里面 加了一行

這樣就可以解析mirrors.ustc.edu.cn了。yum成功！

小記：但是這不是解決這種問題最終辦法。需要配置正確的DNS。方法是：打開/etc/resolv.conf 文件，加入一行 nameserver?ip。此處把ip換成IDC運營商給的DNS的地址就可以了。