修改Centos7默认yum源为阿里云源

1.备份本地yum源：

mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

2.下载新的CentOS-Base.repo 到/etc/yum.repos.d/

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

3.生存缓存：

yum clean all
yum makecache

4.查看yum源：

yum repolist