执行yum操作，报错：
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. $releasever is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors: /var/cache/yum/base/mirrorlist.txt
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again
最后问题解决：开启桥接模式下的混杂模式，设置全部允许就搞定了。
Vbox虚拟机装CentOS，在桥接模式下无法使用yum的问题解决
原文：http://www.cnblogs.com/baiban/p/4431317.html