一、原因

2020年11月30日CentOS 6 EOL。按照社区规则
CentOS 6的源地址 http://mirror.centos.org/centos-6/内容已移除
目前第三方的镜像站(阿里云和163)中均已移除CentOS 6的源。
阿里云的源 http://mirrors.cloud.aliyuncs.com和 http://mirrors.aliyun.com也无法同步到CentOS 6的源。

1.1、解决方法

# 查看系统版本
[root@aka]# cat /etc/redhat-release
CentOS Linux release 6.9.2003 (Core)

# 编辑 yum 文件
vim /etc/yum.repos.d/CentOS-Base.repo
# 将文件中的 $releasever 替换成你 centos 的具体版本。
# 更新
yum clean all
yum makecache

1.2、修改好的 yum 源

# 有需要直接复制黏贴执行即可
cat > /etc/yum.repos.d/CentOS-Base.repo << EOF
[base]
name=CentOS-6.9 - Base - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/6.9/os/$basearch/
        http://mirrors.aliyuncs.com/centos-vault/6.9/os/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos-vault/6.9/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6

#released updates 
[updates]
name=CentOS-6.9 - Updates - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/6.9/updates/$basearch/
        http://mirrors.aliyuncs.com/centos-vault/6.9/updates/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos-vault/6.9/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6

#additional packages that may be useful
[extras]
name=CentOS-6.9 - Extras - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/6.9/extras/$basearch/
        http://mirrors.aliyuncs.com/centos-vault/6.9/extras/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos-vault/6.9/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-6.9 - Plus - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/6.9/centosplus/$basearch/
        http://mirrors.aliyuncs.com/centos-vault/6.9/centosplus/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos-vault/6.9/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6

#contrib - packages by Centos Users
[contrib]
name=CentOS-6.9 - Contrib - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/6.9/contrib/$basearch/
        http://mirrors.aliyuncs.com/centos-vault/6.9/contrib/$basearch/
        http://mirrors.cloud.aliyuncs.com/centos-vault/6.9/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/RPM-GPG-KEY-CentOS-6
EOF

# 更新
yum clean all
yum makecache