首先确保能ping通公网（已联网）
一.查询已安装的yum
1.#rpm -qa | grep yum
二.使用root权限卸载已安装的yum
1.#rpm -qa | grep yum | xargs rpm -e --nodeps
三.下载并安装阿里的yum源rpm包
1.# 创建安放rpm包的文件夹(创建目录位置随意)
cd /etc/yum.repos.d/
mkdir yum-rpm && cd yum-rpm
2.# 下载rpm包
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-3.4.3-161.el7.centos.noarch.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-langpacks-0.4.2-7.el7.noarch.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-rhn-plugin-2.0.1-10.el7.noarch.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-utils-1.1.31-50.el7.noarch.rpm
#wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/python-iniparse-0.4-9.el7.noarch.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/python-urlgrabber-3.10-9.el7.noarch.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/rpm-4.11.3-35.el7.x86_64.rpm
#wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-50.el7.noarch.rpm
以上命令，最好一条条复制下载，如无法下载安装，
根据rpm包主要关键字到阿里云CentOS-yum源库地址库中搜索
获取最新rpm包的地址后，使用wget进行下载安装即可。
四、安装rpm包
1.#cd /etc/yum.repos.d/yum-rpm
2.#rpm -ivh *.rpm --force --nodeps
加上--force参数强制安装，解决安装过程中的的依赖问题
五、下载阿里镜像到/etc/yum.repos.d/目录下
#wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
六、修改Centos-7.repo文件将所有$releasever替换为7
#vim /etc/yum.repos.d/CentOS-Base.repo
:%s/$releasever/7/g
七、运行以下命令
#yum clean all
#yum makecache
#yum update