解决：Failed to download metadata for repo ‘base‘

1.先查看本机的系统信息

[root@h0436 h0436 zlong]# cat /etc/redhat-release

2.进入yum.repos.d

[root@h0436 zlong]# cd /etc/yum.repos.d

3.查看镜像文件（看看你需要修改的哪个文件）

[root@h0436 yum.repos.d]# ls
CentOS-Base.repo         CentOS-Base.repo.bak  epel.repo
CentOS-Base.repo.backup  docker-ce.repo

4.修改文件（我的是CentOS-Base.repo.backup文件）

[root@h0436 yum.repos.d]# vim CentOS-Base.repo.backup 

5.如果你找不到在哪个文件夹，你就每个文件夹都vim 进去看看，找到CentOS-Base.repo就行

6.复制下面这一段配置就行，按键盘上“i”编辑，按ESC退出。如果要保存退出就按:wq，如果不保存退出就按:q （图中标红的地方都是改过之后的地方）

[base]
name=CentOS-8 - Base - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/8/os/$basearch/
        http://mirrors.aliyuncs.com/centos/8/os/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos/8/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-8

#released updates 
[updates]
name=CentOS-8 - Updates - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/8/updates/$basearch/
        http://mirrors.aliyuncs.com/centos/8/updates/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos/8/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-8

#additional packages that may be useful
[extras]
name=CentOS-8 - Extras - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/8/extras/$basearch/
        http://mirrors.aliyuncs.com/centos/8/extras/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos/8/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-8

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-8 - Plus - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/8/centosplus/$basearch/
        http://mirrors.aliyuncs.com/centos/8/centosplus/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos/8/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-8

#contrib - packages by Centos Users
[contrib]
name=CentOS-8 - Contrib - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/8/contrib/$basearch/
        http://mirrors.aliyuncs.com/centos/8/contrib/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos/8/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-8

7.清除缓存

[root@h0436 yum.repos.d]# yum clean all

8.加载缓存

[root@h0436 yum.repos.d]# yum makecache