yum が "[Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403" エラーで失敗する理由

Issue

• yum update が失敗し、以下のエラーが表示されます。

  # yum update
  Loaded plugins: product-id, refresh-packagekit, rhnplugin, security, subscription-manager  
  Updating certificate-based repositories.  
  https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"  
  Trying other mirror.  
  Setting up Update Process  
  No Packages marked for Update  
  
  # yum install gcc  
  Loaded plugins: product-id, refresh-packagekit, rhnplugin, security, subscription-manager  
  Updating certificate-based repositories.  
  https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"  
  Trying other mirror.
  

• yum コマンドが [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403 Forbidden"" エラーで失敗します。

• RHN Classic にシステムが登録されており、その間はプロキシーが使用されています。パッケージをインストールすると、以下のエラーが発生します。

  # yum update
  Loaded plugins: product-id, rhnplugin, security
  This system is receiving updates from RHN Classic or RHN Satellite.
  Setting up Update Process
  https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"
  Trying other mirror.
  Error: Cannot retrieve repository metadata (repomd.xml) for repository: rhel-6-server-rpms. Please verify its path and try again