在ubuntu下执行sudo apt-get update时，经常会遇到报错：
1 Err http://security.ubuntu.com precise-security InRelease
2
3 Err http://security.ubuntu.com precise-security Release.gpg
4 Temporary failure resolving 'security.ubuntu.com'
5 Err http://cn.archive.ubuntu.com precise InRelease
6
7 Err http://cn.archive.ubuntu.com precise-updates InRelease
8
9 Err http://cn.archive.ubuntu.com precise-backports InRelease
10
11 Err http://cn.archive.ubuntu.com precise Release.gpg
12 Temporary failure resolving 'cn.archive.ubuntu.com'
13 Err http://cn.archive.ubuntu.com precise-updates Release.gpg
14 Temporary failure resolving 'cn.archive.ubuntu.com'
15 Err http://cn.archive.ubuntu.com precise-backports Release.gpg
16 Temporary failure resolving 'cn.archive.ubuntu.com'
17 Reading package lists... Done
18 W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise/InRelease
19
20 W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-updates/InRelease
21
22 W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/precise-backports/InRelease
23
24 W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/InRelease
25
26 W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/Release.gpg Temporary failure resolving 'security.ubuntu.com'
27
28 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
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：
1 sudo vi /etc/resolv.conf
在其中添加：
1 # Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
2 # DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
3 nameserver 127.0.1.1
4 #这里用的是阿里云的DNS服务器
5 nameserver 223.5.5.5
6 nameserver 223.6.6.6
