踩坑说明：
yum install安装rpm包时经常报错：Failed connect to mirrors.cloud.aliyuncs.com:80; Connection refused
避坑指南：
究其原因，是由于非阿里云ECS用户无法解析主机“mirrors.cloud.aliyuncs.com”
只需将CentOS-Base里包含aliyuncs.com的行删除即可。
脱坑方案：
1. 备份CentOS-Base
[root@centos7 ~]# cp /etc/yum.repos.d/CentOS-Base.repo{,.bak}
2. 下载CentOS-Base
[root@centos7 ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
或
[root@centos7 ~]# curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
3. 编辑CentOS-Base
[root@centos7 ~]# sed -i -e '/mirrors.cloud.aliyuncs.com/d' -e '/mirrors.aliyuncs.com/d' /etc/yum.repos.d/CentOS-Base.repo