YumRepo Error: All mirror URLs are not using ftp, http[s] or file

有台机器使用Yum的时候，报错如下：

YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. $releasever is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors: /var/cache/yum/base/mirrorlist.txt
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again

猜测：

1.DNS问题

2. $releasever 取不到

1.改了DNS后测试，仍然有问题

2.查看/etc/yum.repos.d/CentOS-Base.repo

[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os

由于yum读取mirrorlist的时候，$releasever取不到，导致url访问失败。

那么解决方法也简单，要么把$releasever弄好，

要么直接修改CentOS-Base.repo文件，替换$releasever为6。（因为我的是centos6）。

怎么修复$releasever我还不清楚，懂了之后补上。