更新yum源
cd /etc/yum.repos.d
备份一下原来的yum源
mv CentOS-Base.repo｛，.bak｝
下载新的CentOS-Base.repo 到/etc/yum.repos.d/
如下的源用一个就好，三条命令选其一
//更新为阿里云的源
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
//更新为163的源
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.163.com/.help/CentOS7-Base-163.repo
//更新为搜狐的源
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.sohu.com/help/CentOS-Base-sohu.repo
运行yum makecache生成缓存
yum clean all
yum makecache
