刚申请了的阿里云服务器，在其Ubuntu上用apt-get install安装软件时，会遇到：
Failed to fetch http://aliyun.com/balabala 404 Not Found
这样的问题。后来搜了一下，原因是默认的aliyun下载源不可用。
解决方法是，换一个下载源，比如网易的。以下是我的做法：
1.把以前的下载源文件删了：rm /etc/apt/sources.list。当然，怕误删最好做个备份。
2.搜一个与Ubuntu版本匹配的网易源，对于Ubuntu 14.04可参考：
3.在Windows新建一个文件sources.list，该文件保存下列内容：
deb http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
4.用文件传输工具，如File Transfer将该sources.list上传到/etc/apt下
5.更新源：sudo apt-get update
注：核心思想就是，用新的源代替旧的源。如果你搜了个网易源，仍不管用，可能是该源很古老，现在已不可用，搜个其他的就好了。