yum が失敗し、"[Errno 14] curl#6 - "Could not resolve host: cdn.redhat.com" エラーが出力される

Issue

• yum が Red Hat CDN からリポジトリーを取得できず、以下のエラーを出力します。

  # yum repolist
  Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
  https://cdn.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: cdn.redhat.com; Unknown error"
  Trying other mirror.
  https://cdn.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: cdn.redhat.com; Unknown error"
  Trying other mirror.
  repo id                                                                                            repo name                                                                                                 status
  rhel-7-server-rpms/7Server/x86_64                                                                  Red Hat Enterprise Linux 7 Server (RPMs)                                                                  0
  repolist: 0