在ubuntu下执行sudo apt-get update 异常Err http://security.ubuntu.com precise-security InReleaseErr

Err http://security.ubuntu.com precise-security InReleaseErr http://security.ubuntu.com precise-security Release.gpg  Temporary failure resolving 'security.ubuntu.com'Err http://cn.archive.ubuntu.com precise InReleaseErr http://cn.archive.ubuntu.com precise-updates InReleaseErr http://cn.archive.ubuntu.com precise-backports InReleaseErr 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... DoneW: 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.

一般都会推荐使用国内的镜像源，比如163或者阿里云的镜像服务器 
将下列文本添加到/etc/apt/sources.list文件里

deb http://mirrors.aliyun.com/ubuntu/ raring main restricted universe multiverse  deb http://mirrors.aliyun.com/ubuntu/ raring-security main restricted universe multiverse  deb http://mirrors.aliyun.com/ubuntu/ raring-updates main restricted universe multiverse  deb http://mirrors.aliyun.com/ubuntu/ raring-proposed main restricted universe multiverse  deb http://mirrors.aliyun.com/ubuntu/ raring-backports main restricted universe multiverse  deb-src http://mirrors.aliyun.com/ubuntu/ raring main restricted universe multiverse  deb-src http://mirrors.aliyun.com/ubuntu/ raring-security main restricted universe multiverse  deb-src http://mirrors.aliyun.com/ubuntu/ raring-updates main restricted universe multiverse  deb-src http://mirrors.aliyun.com/ubuntu/ raring-proposed main restricted universe multiverse  deb-src http://mirrors.aliyun.com/ubuntu/ raring-backports main restricted universe multiverse  

但是执行sudo apt-get update仍然报错，问题在于DNS没有配置好。 
解决方法：

sudo vi /etc/resolv.conf

添加

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTENnameserver 127.0.1.1#这里用的是阿里云的DNS服务器nameserver 223.5.5.5  nameserver 223.6.6.6

之后执行sudo apt-get update就正常了。

参考： 
ubuntu apt-get update 失败解决。 
Ubuntu操作系统“Failed to fetch”错误的解决方法