yum 本地源 和No Packages xxx available的问题？

1.mount /dev/cdrom /mnt/

2.cp /etc/yum.repos.d/Centos-Base.repo /etc/yum.repos.d/Centos-Base.repo .bak

3.cd /etc/yum.repos.d

4.touch media.repo

5.vi media.repo
[yum]
name=CentOS7
baseurl=file:///mnt
gpgcheck=1
enable=1
gpgkey=file:///mnt/RPM-GPG-KEY-Centos-7

6.yum clean all

7.yum install screen -y

问题：No Packages xxx available
解决：yum的问题
安装上面的yum源解决.