在ubuntu下執行sudo apt-get update時，經常會遇到報錯：
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.一般都會推薦使用國內的鏡像源，比如163或者阿里雲的鏡像服務器
將下列文本添加到/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#這里用的是阿里雲的DNS服務器nameserver 223.5.5.5 nameserver 223.6.6.6之后執行sudo apt-get update就正常了。
參考：
ubuntu apt-get update 失敗解決。
Ubuntu操作系統“Failed to fetch”錯誤的解決方法