CentOs yum configuration

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，两者只能选一个。
用下面的内容覆盖CentOS-Base.repo就行了。

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

#第一部分结束

#============分割线============ 注意，上下两部分只选一个

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

#第二部分结束

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 时总是出现如下错误

此时聪明的我马上想到了是不是服务器解析不了域名了，导致repomd.xml下载不下来呢？

然后我就ping了一下mirrors.ustc.edu.cn ， 果然出现错误：ping: unknown host mirrors.ustc.edu.cn。显然是DNS配置问题了，DNS域名服务器IP地址配的不对。

把/etc/hosts里面 加了一行

这样就可以解析mirrors.ustc.edu.cn了。yum成功！

小记：但是这不是解决这种问题最终办法。需要配置正确的DNS。方法是：打开/etc/resolv.conf 文件，加入一行 nameserver ip。此处把ip换成IDC运营商给的DNS的地址就可以了。