Ubuntu 16.04- Package has no installation candidate解决方式，Ubuntu正确安装openssl-server的方法

　　Ubuntu 17.04 安装完虚拟机以后，若是想装ssh软件进行链接，发现几乎全部的包都装不成功。

root@zengxijin-va:/home/zengxijin# 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

　　报的报错都是Package ‘xxx’ has no installation candidate。 
解决办法： 
（1）备份/etc/apt/sources.list

sudo cp -i /etc/apt/sources.list /etc/apt/sources.list.backup

（2）更新repos 

将如下的内容添加到/etc/apt/sources.list（或者将原来的内容清空再添加下面的源）

deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse 

　(3)更新：

sudo apt-get update

　(4)安装

sudo apt-get install openssh-server

　　而后就能安装成功了，若是遇到版本依赖的冲突，先按照提示安装对应的依赖版本：如

apt install openssh-client=1:6.6p1-2ubuntu2.10

　　再正常安装对应的软件包就能够了