在Ubuntu 11.04中执行sudo apt-get update命令时，遇到以下错误：
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/universe/binary-amd64/Packages 404 Not Found [IP: 91.189.91.24 80]
到 http://security.ubuntu.com/ubuntu/dists 网站上一看，果然没有了n开头的，所以报了404错误，经查资料，得出了以下结论：
The repositories for older releases that are not supported (like 11.04, 11.10 and 13.04) get moved to an archive server. There are repositories available at http://old-releases.ubuntu.com
Ubuntu 11.04的生命周期已经结束，不再提供更新，如扔使用，需要到 http://old-releases.ubuntu.com 上下载所需包。
解决方法：
If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com to old-releases.ubuntu.com
执行以下命令：
sudo sed -i -re ‘s/([a-z]{2}.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g’ /etc/apt/sources.list