在ubuntu下執行sudo apt-get update時，經常會遇到報錯：
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'
這是因為鏡像源除出了問題，一般都會推薦使用國內的鏡像源，比如163或者阿里雲或者清華大學的鏡像服務器 （強烈建議使用清華鏡像）
清華鏡像源官網：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
將下列文本添加到/etc/apt/sources.list文件里
# 默認注釋了源碼鏡像以提高 apt update 速度，如有需要可自行取消注釋 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 # 預發布軟件源，不建議啟用 # 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
或者進入repogen.simplylinux.ch 選擇合適的Ubuntu版本后，在ubuntu Branches下全選
拉至網頁最下，點擊生產，復制list下內容，添加到/etc/apt/sources.list中即可
具體參考https://jingyan.baidu.com/article/afd8f4de66efcf34e286e993.html?st=2&os=0&bd_page_type=1&net_type=2
修改鏡像源之后需要重新更新：
sudo apt-get update；sudo apt-get upgrade
如果執行sudo apt-get update仍然報錯，問題在於DNS沒有配置好。
解決方法：
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 #這里用的是阿里雲的DNS服務器 5 nameserver 223.5.5.5 6 nameserver 223.6.6.6