更换镜像源
1.备份：
sudo mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2.创建一个文件:
cd /etc/yum.repos.d/
touch CentOS-Base.repo
3.往CentOS-Base.repo添加内容
vi CentOS-Base.repo
添加以下内容：
[base]
name=CentOS-\$releasever - Base - mirrors.aliyun.com
baseurl=http://mirrors.aliyun.com/centos/\$releasever/os/\$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
[updates]
name=CentOS-\$releasever - Updates - mirrors.aliyun.com
baseurl=http://mirrors.aliyun.com/centos/\$releasever/updates/\$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
[extras]
name=CentOS-\$releasever - Extras - mirrors.aliyun.com
baseurl=http://mirrors.aliyun.com/centos/\$releasever/extras/\$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
[centosplus]
name=CentOS-\$releasever - Plus - mirrors.aliyun.com
baseurl=http://mirrors.aliyun.com/centos/\$releasever/centosplus/\$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7 清理并重建缓存
yum clean all
yum makecache
遇到的问题
在更换完镜像源后使用yum makecache命令提示错误
解决办法：
解析DNS
vi /etc/sysconfig/network-scripts/ifcfg-ens33
新增DNS
DNS1=8.8.8.8
DNS2=8.8.4.4
重启network
service network restart
更新yum
yum update -y