yumはCent-OSで動作しません

新しいCent-OS 7をインストールしますが、更新または新しいパッケージを取得したいときに
このエラーが発生します。

Loaded plugins: fastestmirror, langpacks
 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:
 1. Contact the upstream for the repository and get them to fix the problem.

 2. Reconfigure the baseurl/etc. for the repository, to point to a working
    upstream. This is most often useful if you are using a newer
    distribution release than is supported by the repository (and the
    packages for the previous distribution release still work).

 3. Disable the repository, so yum won't use it by default. Yum will then
    just ignore the repository until you permanently enable it again or use
    --enablerepo for temporary usage:

        yum-config-manager --disable <repoid>

 4. Configure the failing repository to be skipped, if it is unavailable.
    Note that yum will try to contact the repo. when it runs most commands,
    so will have to try and fail each time (and thus. yum will be be much
    slower). If it is a very temporary problem though, this is often a nice
    compromise:

        yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: base/7/x86_64`

これはyum repolist all：

repo id                                                              repo name                                                              status
C7.0.1406-base/x86_64                                                CentOS-7.0.1406 - Base                                                 disabled
C7.0.1406-centosplus/x86_64                                          CentOS-7.0.1406 - CentOSPlus                                           disabled
C7.0.1406-extras/x86_64                                              CentOS-7.0.1406 - Extras                                               disabled
C7.0.1406-fasttrack/x86_64                                           CentOS-7.0.1406 - CentOSPlus                                           disabled
C7.0.1406-updates/x86_64                                             CentOS-7.0.1406 - Updates                                              disabled
base/7/x86_64                                                        CentOS-7 - Base                                                        enabled: 0
base-debuginfo/x86_64                                                CentOS-7 - Debuginfo                                                   disabled
base-source/7                                                        CentOS-7 - Base Sources                                                disabled
centosplus/7/x86_64                                                  CentOS-7 - Plus                                                        disabled
centosplus-source/7                                                  CentOS-7 - Plus Sources                                                disabled
cr/7/x86_64                                                          CentOS-7 - cr                                                          disabled
extras/7/x86_64                                                      CentOS-7 - Extras                                                      enabled: 0
extras-source/7                                                      CentOS-7 - Extras Sources                                              disabled
fasttrack/7/x86_64                                                   CentOS-7 - fasttrack                                                   disabled
updates/7/x86_64                                                     CentOS-7 - Updates                                                     enabled: 0
updates-source/7                                                     CentOS-7 - Updates Sources                                             disabled
repolist: 0

および内容/etc/yum.repos.d/CentOS-Base.repo：

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates 
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

のコメントを外した後baseurl、CentOS-Base.repo私は
再びこのエラーを受け取りましたが、最後に異なっています：

failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirror.centos.org/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirror.centos.org; Unknown error"

失敗する理由はyum、リポジトリURLを知る必要があるためです。構成ファイルには、次のものがあります。

#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/

#記号は、それがコメント化されていること、したがって、yumその行を無視し、URLがあるか分からないでしょう。ハッシュ記号を削除して、yum通常どおり使用します。

注意：あなたがからそれを削除する必要があります任意の baseurl他のレポも同様に重要であるとして、あなたがコメントしていること。