现象如下：
VirtualBox:~$ sudo apt-get update
Err http://mirrors.aliyun.com trusty InRelease
Err http://mirrors.aliyun.com trusty-updates InRelease
Err http://mirrors.aliyun.com trusty-backports InRelease
Err http://mirrors.aliyun.com trusty-security InRelease
Err http://mirrors.aliyun.com trusty Release.gpg
Could not resolve 'mirrors.aliyun.com'
Err http://mirrors.aliyun.com trusty-updates Release.gpg
Could not resolve 'mirrors.aliyun.com'
Err http://mirrors.aliyun.com trusty-backports Release.gpg
Could not resolve 'mirrors.aliyun.com'
Err http://mirrors.aliyun.com trusty-security Release.gpg
Could not resolve 'mirrors.aliyun.com'
Reading package lists... Done
W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty/InRelease
换了阿里云和163云不能解决问题。
sudo apt-get clean
sudo apt-get update
也不能解决
查找资料，http://blog.csdn.net/ronghua_liu/article/details/8609450，需要更新DNS
sudo vi /etc/resolvconf/resolv.conf.d/base
在里面插入：
nameserver 8.8.8.8
nameserver 8.8.4.4
sudo resolvconf -u
cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8
nameserver 8.8.4.4
可以看到我们的设置已经加上了，然后再ping一个域名，当时就可以解析了，无需重启。
也可以直接修改/etc/resolv.conf，sudo /etc/init.d/networking restart
但重启失效
