定义yum超时时间：
vi /etc/yum.conf
timeout=120
修改源：（全部复制粘贴即可）
cd /etc/yum.repos.d/
mv rhel-debuginfo.repo rhel-debuginfo.repo.bak
wget http://www.yodav.com/project/centos/yum/rpmforge.repo
wget http://www.yodav.com/project/centos/yum/mirrors-rpmforge
wget http://www.yodav.com/project/centos/yum/dag.repo
wget http://www.yodav.com/project/centos/yum/rhel-debuginfo.repo
rpm --import http://mirror.leaseweb.com/centos/RPM-GPG-KEY-CentOS-5
cd /tmp
wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.1-1.el5.rf.x86_64.rpm
rpm -ivh rpmforge-release-0.5.1-1.el5.rf.x86_64.rpm
yum install yum-fastestmirror