启动SSH-server出现错误。Package openssh-server is not available, but is referred to by another package.

       sudo apt-get install openssh-server

往刚装好的ubuntu上一敲，出现如下错误：

steve@basil:~$ sudo apt-get install openssh-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openssh-server is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package openssh-server has no installation candidate

http://icesquare.com/wordpress/package-ssh-is-not-available-but-is-referred-to-by-another-package-this-may-mean-that-the-package-is-missing-has-been-obsoleted-or-is-only-available-from-another-source/

在source.list中添加最新的源
：

       sudo apt-get update

       sudo apt-get upgrade

命令能找到资源并更新成功。然后再次使用

   sudo apt-get install openssh-server