yum解決 "Couldn't resolve host 'apt.sw.be'" 錯誤

1.yum無法安裝工具
    failure: repodata/repomd.xml from dag: [Errno 256] No more mirrors to try.
http://apt.sw.be/redhat/el7/en/x86_64/dag/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: apt.sw.be; Unknown error"
說明yum 源太老---解決辦法：
最好的辦法,換成國內的源
-bash-4.2$ ls
bak               CentOS-Debuginfo.repo  CentOS-Sources.repo  zabbix.repo
CentOS-Base.repo  CentOS-fasttrack.repo  CentOS-Vault.repo
CentOS-CR.repo    CentOS-Media.repo      dag.repo
首先備份
-bash-4.2$ mv * bak
下載阿里的源
-bash-4.2$ sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
-bash-4.2$ ls
bak  CentOS-Base.repo
清除緩存 並更新
-bash-4.2$ sudo yum makecache
現在就可以裝工具了
-bash-4.2$ sudo yum install tree -y