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/
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basearch/ gpgcheck=1 gpgkey=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/
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basearch/ gpgcheck=1 gpgkey=http://mirrors.aliyun.com/centos/7/os/
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 -
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basearch - Aliyun baseurl=http://mirrors.aliyun.com/epel/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