centosplusリポジトリからpostfixをインストールする「ホストを確認できませんでした：mirrors.linode.com;不明なエラー」

一人をフォローしています。地図時間MariaDBをサポートするPostfixをインストールします。デフォルト/etc/yum.repos.d/CentOS-Base.repoのリポジトリはMariaDBでPostfixをサポートしていないため、exclude=postfix行を使用してenabled=1ファイルを変更しました。

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

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

#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://mirrors.linode.com/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://mirrors.linode.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1

実行しましたが、yum --enablerepo=centosplus install postfixエラーが発生します。

Loaded plugins: fastestmirror
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
http://mirrors.linode.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.linode.com; Unknown error"
Trying other mirror.
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=centosplus&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
    http://mirrors.linode.com/centos/7/centosplus/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.linode.com; Unknown error"
    Trying other mirror.


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

この問題を解決するために何ができるか知っていますか？