Yumrepo Error:all Mirror URLs is not using FTP, Http[s] or file

When a machine uses Yum, the error is as follows:

Yumrepo Error:all Mirror URLs is 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. Verify its path and try again

Guess:

1.DNS problem

2. $releasever do not have access to

1. After the DNS test is changed, there are still problems

2. View/etc/yum.repos.d/centos-base.repo

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

Because Yum reads mirrorlist, $releasever cannot be fetched, resulting in URL access failure.

Then the solution is simple, either to fix the $releasever,

Either modify the Centos-base.repo file directly, and replace the $releasever with 6. (because mine is CENTOS6).

How to repair $releasever I am not clear, understand and fill up.

Yumrepo Error:all Mirror URLs is not using FTP, Http[s] or file