Ubuntu 11.04 (Natty) 已经停止支持,但可以使用old-releases源了,先备份源列表文件: 
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup 

sudo gedit /etc/apt/sources.list 
把里面的内容替换为以下内容: 
deb http://old-releases.ubuntu.com/ubuntu/ natty main restricted universe multiverse 
deb http://old-releases.ubuntu.com/ubuntu/ natty-security main restricted universe multiverse 
deb http://old-releases.ubuntu.com/ubuntu/ natty-updates main restricted universe multiverse 
deb http://old-releases.ubuntu.com/ubuntu/ natty-proposed main restricted universe multiverse 
deb http://old-releases.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse 
deb-src http://old-releases.ubuntu.com/ubuntu/ natty main restricted universe multiverse 
deb-src http://old-releases.ubuntu.com/ubuntu/ natty-security main restricted universe multiverse 
deb-src http://old-releases.ubuntu.com/ubuntu/ natty-updates main restricted universe multiverse 
deb-src http://old-releases.ubuntu.com/ubuntu/ natty-proposed main restricted universe multiverse 
deb-src http://old-releases.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse 

重载源列表,使配置生效: 
sudo apt-get update 

安装SSH服务器和客户端: 
sudo apt-get install openssh-server openssh-client 

sudo apt-get update 不了