配置RHEL和centOS的阿里云镜像源

备份

sudo cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak

编辑 YUM 仓库配置文件

sudo vi /etc/yum.repos.d/CentOS-Base.repo

配置内容：

[base]
name=CentOS-7 - Base - Aliyun
baseurl=http://mirrors.aliyun.com/centos/7/os/$basearch/gpgcheck=1gpgkey=http://mirrors.aliyun.com/centos/7/os/$basearch/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-7 - Updates - Aliyun
baseurl=http://mirrors.aliyun.com/centos/7/updates/$basearch/gpgcheck=1gpgkey=http://mirrors.aliyun.com/centos/7/os/$basearch/RPM-GPG-KEY-CentOS-7

编辑 EPEL 仓库配置文件 epel.repo
sudo vi /etc/yum.repos.d/epel.repo

编辑内容：

[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch-Aliyunbaseurl=http://mirrors.aliyun.com/epel/7/$basearch
failovermethod=priority
enabled=1
gpgcheck=0
gpgkey=http://mirrors.aliyun.com/epel/RPM-GPG-KEY-EPEL-7

清理缓存

sudo yum clean all

生成缓存（重新生成缓存，以验证配置是否正确）

sudo yum makecache

尝试安装软件：

sudo yum install tftp-server tftp