form:http://hi.baidu.com/firelet/blog
执行命令：
cd /etc/yum.repos.d/
mv CentOS-Base.repo CentOS-Base.repo.save
建立一个空文件，拷入最下面的文字，然后存成名为CentOS-Base.repo的文件，
将生成的CentOS-Base.repo文件考到/etc/yum.repos.d/下，然后执行：
64位:
rpm --import ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/os/x86_64/RPM-GPG-KEY-CentOS-5
32位:
rpm --import ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5
最后执行：
yum update
##########################################################################
[base]
name=CentOS-5 - Base
repo=os
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/os/$basearch/
gpgcheck=1
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
[update]
name=CentOS-5 - Updates
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/updates/$basearch/
gpgcheck=1
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
[addons]
name=CentOS-5 - Addons
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/addons/$basearch/
gpgcheck=1
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
[extras]
name=CentOS-5 - Extras
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/extras/$basearch/
gpgcheck=1
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
[centosplus]
name=CentOS-5 - Plus
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
[contrib]
name=CentOS-5 - Contrib
baseurl=ftp://ftp3.tsinghua.edu.cn/mirror/centos/5.1/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=ftp://ftp3.tsinghua.edu.cn/mirror/centos/RPM-GPG-KEY-CentOS-5
