[root@localhost ~]# yum -y install telnet 
Loaded plugins: fastestmirror
Setting up Install Process
Determining fastest mirrors
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/6/base/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: base

网上很多资料都是说dns配置或网络问题导致的，经过排查发现公共源镜像关停了相关服务
经过多次测试找到如下解决办法

1、使用腾讯云的yum源

https://mirrors.cloud.tencent.com/help/centos.html

2、备份系统旧配置文件

mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

3、下载源配置 CentOS6

wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.cloud.tencent.com/repo/centos6_base.repo

4、把http替换https

sed -i 's#http#https#g' /etc/yum.repos.d/CentOS-Base.repo

5、清除缓存

yum clean all
yum makecache