我今天下载了 Red Hat Enterprise Linux，当我尝试使用 yum 安装软件包时，我得到：
[root@localhost yum.repos.d]# yum install openssh
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
This system is not registered with an entitlement server. You can use subscription-manager to register.
Nothing to do
我该怎么办？