我尝试使用Xshell连接VirtualBox中安装的windows和Linux（Ubuntu 16.04）。但最近它们无法互相通信。我关闭了防火墙，使用了“仅主机”模式、NAT和Bridge模式。仍然不起作用。有人建议安装ssh，我发现openssh-client已经安装，但openssh-server尚未安装，所以我尝试通过命令行安装openssh-server
sudo apt-get install openssh-server，
问题如下：**
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
** 我尝试使用“sudo apt-get update”，但出现了另一个问题，如下所示”
**
sudo apt-get update
Err:1 http://dl.google.com/linux/chrome/deb stable InRelease Temporary failure resolving 'dl.google.com'
Err:2 http://ca.archive.ubuntu.com/ubuntu xenial InRelease
Temporary failure resolving 'ca.archive.ubuntu.com' >
Reading package lists...Done>
W: Failed to fetch
http://ca.archive.ubuntu.com/ubuntu/dists/xenial/InRelease Temporary
failure resolving 'ca.archive.ubuntu.com'
W: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/InRelease
Temporary failure resolving 'dl.google.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.**
我更改了源，例如主源或 Canda 源。但不起作用。我非常感谢那些愿意提供帮助的人。非常感谢！