原因：centos6的默认源在2012年-12月左右被官方搞掉了

下列是错误详情
Bash

[root@c8-20 ~]# yum makecache
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
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

或者
Bash

[root@li496-237 ~]# yum -y install unzip zip
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/
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
http://mirrors.linode.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
To address this issue please refer to the below knowledge base article 

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please open a ticket with Red Hat Support.

Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify 

第一种方法：一键修复(一条一条执行即可):

一般执行了前3条命令就好了
Bash

sed -i "s|enabled=1|enabled=0|g" /etc/yum/pluginconf.d/fastestmirror.conf
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Aliyun.repo 
yum clean all
yum makecache

第二种方法：手动修复教程:

首先把fastestmirrors关了
Markup

#编辑

vi /etc/yum/pluginconf.d/fastestmirror.conf

•  

#修改

enable=0

#或者执行以下命令

sed -i "s|enabled=1|enabled=0|g" /etc/yum/pluginconf.d/fastestmirror.conf

先把之前的repo挪到备份，然后下面两个二选一
Markup

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

替换为官方Vault源
Markup

curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Official.repo

替换为阿里云Vault镜像
Markup

curl -o /etc/yum.repos.d/CentOS-Base.repo https://www.xmpan.com/Centos-6-Vault-Aliyun.repo