CentOS7 -[에러코드] Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

# 경로 이동
cd /etc/yum.repos.d

# 백업 폴더 생성
mkdir old_repo

# 기존 파일 이동
mv CentOS-* old_repo

# 폴더 이동
mv old_repo /opt/apps/

# 새 레포지토리 파일 생성
# original이 기존, new가 새로운 URL입니다.
# mirrorlist 부분은 모두 주석하였습니다.
vi CentOS-Base.repo

[base]
name=CentOS-$releasever - Base
# original
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
# new
baseurl=http://centos.mirror.cdnetworks.com/7/os/x86_64
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates
# original
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
# new
baseurl=http://centos.mirror.cdnetworks.com/7/updates/x86_64
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
# original
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
# new
baseurl=http://centos.mirror.cdnetworks.com/7/extras/x86_64
gpgcheck=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
# original
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
# new
baseurl=http://centos.mirror.cdnetworks.com/7/centosplus/x86_64
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

# yum cache 삭제
yum clean all

# 테스트
yum install -y libtool