ubuntu 执行apt-get update报错Failed to fetch

在ubuntu下执行sudo apt-get update时，经常会遇到报错：

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'

这是因为镜像源除出了问题，一般都会推荐使用国内的镜像源，比如163或者阿里云或者清华大学的镜像服务器 （强烈建议使用清华镜像）

清华镜像源官网：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/

将下列文本添加到/etc/apt/sources.list文件里

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-security main restricted universe multiverse

# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse

或者进入repogen.simplylinux.ch 选择合适的Ubuntu版本后，在ubuntu Branches下全选

拉至网页最下，点击生产，复制list下内容，添加到/etc/apt/sources.list中即可

具体参考https://jingyan.baidu.com/article/afd8f4de66efcf34e286e993.html?st=2&os=0&bd_page_type=1&net_type=2

修改镜像源之后需要重新更新：

sudo apt-get update；sudo apt-get upgrade

如果执行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

在其中添加：

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 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