今天偶尔启动了以下长时间未用的云服务器，使用yum安装某些软件，发现报网络延迟的错误
Errors during downloading metadata for repository 'docker-ce-stable':
- Curl error (28): Timeout was reached for https://download.docker.com/linux/centos/8/x86_64/stable/repodata/repomd.xml [Connection timed out after 30000 milliseconds]
按照网上的解决方案进行解决，重新配置镜像源，出现以下的错误，这种情况应该是源失效了，解决思路参考这篇文章
CentOS-3 - Base - mirrors.aliyun.com 76 B/s | 2.3 kB 00:30
Errors during downloading metadata for repository 'base':
- Status code: 404 for http://mirrors.cloud.aliyuncs.com/centos/3/os/x86_64/repodata/repomd.xml (IP: 100.100.2.148)
- Status code: 404 for http://mirrors.aliyun.com/centos/3/os/x86_64/repodata/repomd.xml (IP: 121.89.58.189)
- Curl error (28): Timeout was reached for http://mirrors.aliyuncs.com/centos/3/os/x86_64/repodata/repomd.xml [Connection timed out after 30000 milliseconds]
如果上述文章中的链接不行(我是不太行)，建议:
centos6（centos6官方源已下线，建议切换centos-vault源）
wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-vault-6.10.repo
成功解决问题
阿里云镜像官方链接：https://developer.aliyun.com/mirror/
其他的操作系统相关镜像都可以在里面找到