failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.

异常

yum makecache

failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.aliyun.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found

一般而言，出现此异常的原因是变量 $releasever 未找到；

这个变量对应 yum.conf 的 distroverpkg=centos-release 配置，centos-release 从包=centos-releasezhong获取，下面是安装步骤：

# 查看对应包
rpm -qi centos-release
>>> package centos-release is not installed

# 下载包，需要找到对应版本的包
wget http://mirrors.aliyun.com/centos/7/os/x86_64/Packages/centos-release-7-9.2009.0.el7.centos.x86_64.rpm

# 安装
rpm -Uvh --replacepkgs centos-release-7-9.2009.0.el7.centos.x86_64.rpm 

# 再次查看
rpm -qi centos-release
>>> 
Name        : centos-release
Version     : 7
Release     : 9.2009.0.el7.centos
Architecture: x86_64
Install Date: Tue 14 Feb 2023 11:32:53 AM CST
Group       : System Environment/Base
Size        : 44787
License     : GPLv2
Signature   : RSA/SHA256, Mon 26 Oct 2020 08:40:20 PM CST, Key ID 24c6a8a7f4a80eb5
Source RPM  : centos-release-7-9.2009.0.el7.centos.src.rpm
Build Date  : Fri 23 Oct 2020 10:53:59 PM CST
Build Host  : x86-01.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
Summary     : CentOS Linux release file
Description :
CentOS Linux release files

再次运行 yum clean all && yum makecache 即可