CentOS8下执行yum install xxx 时出现以上错误，解决措施如下：

1、将旧镜像库文件备份
    cd /etc/yum.repos.d
    mkdir ~/repo_old
    mv ./* ~/repo_old/
    cp ~/repo_old/CentOS-Linux-BaseOS.repo .
    cp ~/repo_old/CentOS-Linux-AppStream.repo .

2、修改镜像地址为vault
BaseOS.repo
    vim CentOS-Linux-BaseOS.repo
    #修改
    baseurl=https://vault.centos.org/centos/$releasever/BaseOS/$basearch/os/

AppStream.repo
    vim CentOS-Linux-AppStream.repo  
    #修改
    baseurl=https://vault.centos.org/centos/$releasever/AppStream/$basearch/os/


3、 重建缓存
    yum clean all
    rpm --rebuilddb
    yum update
    yum list
    yum makecache