ubuntu apt-get update 失败解决。

当运行apt-get update后出现如下错误时：
E: Some index files failed to download, they have been ignored, or old ones used instead.

可以将目录下/var/lib/apt/lists/partial/所有的文件清掉，再次运行apt-get update即可！自带源在大陆不好。

出现以下错误：

/etc/apt$ sudo apt-get update
Err http://security.ubuntu.com precise-security InRelease
  
Err http://security.ubuntu.com precise-security Release.gpg
  Temporary failure resolving 'security.ubuntu.com'
Err http://cn.archive.ubuntu.com precise InRelease
  
Err http://cn.archive.ubuntu.com precise-updates InRelease
  
Err http://cn.archive.ubuntu.com precise-backports InRelease
  
Err http://cn.archive.ubuntu.com precise Release.gpg
  Temporary failure resolving 'cn.archive.ubuntu.com'
Err http://cn.archive.ubuntu.com precise-updates Release.gpg
  Temporary failure resolving 'cn.archive.ubuntu.com'
Err http://cn.archive.ubuntu.com precise-backports Release.gpg
  Temporary failure resolving 'cn.archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise/InRelease  

W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-updates/InRelease  

W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-backports/InRelease  

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/InRelease  

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/Release.gpg  Temporary failure resolving 'security.ubuntu.com'

W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise/Release.gpg  Temporary failure resolving 'cn.archive.ubuntu.com'

W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-updates/Release.gpg  Temporary failure resolving 'cn.archive.ubuntu.com'

W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-backports/Release.gpg  Temporary failure resolving 'cn.archive.ubuntu.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

修改dns：

配置文件地址 /etc/resolv.conf

使用编辑器打开

改为如下内容：
search localdomain
nameserver 202.96.128.86 希望修改成的DNS
nameserver 202.96.128.166 备用DNS

重启网络：sudo /etc/init.d/networking restart。即可