在ubuntu下執行sudo apt-get update時，經常會遇到報錯：

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.

一般都會推薦使用國內的映象源，比如163或者阿里雲的映象伺服器
將下列文字新增到/etc/apt/sources.list檔案裡

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  

但是執行sudo apt-get update仍然報錯，問題在於DNS沒有配置好。
解決方法：

sudo vi /etc/resolv.conf

新增

# 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 127.0.1.1
#這裡用的是阿里雲的DNS伺服器
nameserver 223.5.5.5  
nameserver 223.6.6.6

之後執行sudo apt-get update就正常了。