安裝外網的yum步驟
cd /etc/yum.repos.d/
touch my.repo
vi /etc/yum.repos.d/my.repo
[base]
name=centos6.3
baseurl=http://mirrors.163.com/centos/$releasever/os/$basearch
gpgecheck=0
enable=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
保存退出
然後yum clean all（清空yum）,yum makecache
但是在yum makecache時出現如下錯誤：
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os error was
14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'"
於是就各種找答案，各種嘗試，最後發現是DNS的問題，
vi /etc/resolv.conf文件添加如下
nameserver 8.8.8.8(你自己的DNS服務器)
然後重新yum makecache即可成功
