执行yum 错误讯息Error: Cannot find a valid baseurl for repo: addon

执行YUM时出现以下错误讯息 :
[root@localhost ~]# yum -y install net-snmp
Loaded plugins: fastestmirror
Determining fastest mirrors
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=5&arch=x86_64&repo=addons error was
[Errno 4] IOError: <urlopen error (-3, 'Temporary failure in name resolution')>
Error: Cannot find a valid baseurl for repo: addons

当出现以上的错误后，请先试着Ping mirrorlist.centos.org，发现ping不通，出现 ping: unknown host mirrorlist.centos.org 的讯息。
请先确认DNS的设定上是否有问题

详细内容请参考http://blog.faq-book.com/?p=3092