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'
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.
一般都会推荐使用国内的镜像源，比如163或者阿里云的镜像服务器
将下列文本添加到/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
# 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
之后执行sudo apt-get update就正常了。
参考：
ubuntu apt-get update 失败解决。
Ubuntu操作系统“Failed to fetch”错误的解决方法
docker ubuntu容器更换阿里源
