?错误：

yum install maven
Repository epel is listed more than once in the configuration
CentOS Linux 8 - AppStream ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?17 MB/s | ?18 MB ? ? 00:01 ? ?
CentOS Linux 8 - BaseOS ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?7.3 kB/s | 2.3 kB ? ? 00:00 ? ?
Errors during downloading metadata for repository 'baseos':
? - Status code: 404 for https://mirrors.aliyun.com/centos/8/BaseOS/x86_64/os/repodata/repomd.xml (IP: 39.96.118.192)
Error: Failed to download metadata for repo 'baseos': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

因为测试需要，安装了CentOS 6.4,进行更新时yum提示不可用。

按照阿里云开源镜像(https://developer.aliyun.com/mirror/centos)的说明：

1	`wget -O?/etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-6.repo`

1	`配置完成后，运行yum makecache时报错，提示repomd.xml找不到，错误码404.拷贝对应的地址到浏览器确实404打不开。`

除了readme，什么也没有了,readme文件内容如下：

This directory (and version of CentOS) is deprecated.  For normal users,
you should use /6/ and not /6.4/ in your path. Please see this FAQ
concerning the CentOS release scheme:

https://wiki.centos.org/FAQ/General

If you know what you are doing, and absolutely want to remain at the 6.4
level, go to http://vault.centos.org/ for packages. 

Please keep in mind that 6.0, 6.1, 6.2, 6.3 and 6.4 no longer gets any updates, nor
any security fix's.

后来查阅资料(https://wiki.centos.org/FAQ/General)，是停止维护了

CentoS 6?停止维护更新日期2020年11月30日

CentOS 7?停止维护更新日期2024年6月30日

CentOS 8?停止维护更新日期2021年12月31日

?原因：

mirrors.aliyun.comhttps://mirrors.aliyun.com/centos/8/BaseOS/x86_64/os/repodata/repomd.xml

?https://mirrors.aliyun.com/centos/8-stream/BaseOS/x86_64/os/repodata/repomd.xmlhttps://mirrors.aliyun.com/centos/8-stream/BaseOS/x86_64/os/repodata/repomd.xml

解决办法

还有一个阿里云源(centos-vault)可以使用，整理内容如下，直接复制可用。

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
#
 
[base]
name=CentOS-6.4 - Base
baseurl=http://mirrors.aliyun.com/centos-vault/6.4/os/x86_64/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
 
[updates]
name=CentOS-6.4 - Updates
baseurl=http://mirrors.aliyun.com/centos-vault/6.4/updates/x86_64/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
 
[extras]
name=CentOS-6.4 - Extras
baseurl=http://mirrors.aliyun.com/centos-vault/6.4/extras/x86_64/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
 
[centosplus]
name=CentOS-6.4 - Plus
baseurl=http://mirrors.aliyun.com/centos-vault/6.4/centosplus/x86_64/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
 
[contrib]
name=CentOS-6.4 - Contrib
baseurl=http://mirrors.aliyun.com/centos-vault/6.4/contrib/x86_64/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6

?OK，问题解决~