sudo apt-get install ssh
Reading package lists... Done
Building dependency tree... Done
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
这个问题的原因是ubuntu的/etc/apt/source.list中的源比较旧了，需要更新一下，更新方法：
$ sudo apt-get -y update
更新完毕之后，在使用apt-get就没有问题了。