[root@pachong yum-3.2.28]# yum search www
Loaded plugins: fastestmirror
Determining fastest mirrors
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. /
removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/6/base/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: base

今天发现yum报错如上所示，后来找啊找，找到了这个帖子 http://www.howtoforge.com/forums/showthread.php?t=54388
里面有关键的一个回帖，最后一栏
Can you check if your Internet connection is working and that you have valid nameservers in /etc/resolv.conf?

然后我打开 /etc/resolv.conf一看，果然这个里面指定的DNS的IP  ping不过去
最后的原因是DNS服务器的IP换了，果然是网络问题