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 安装完虚拟机之后，如果想装ssh软件进行连接，发现几乎所有的包都装不成功。
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
将以下的内容添加到/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
然后就能安装成功了，如果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可以了