VirtualBox内刚刚安装完Debian9系统，也无法设置共享文件夹。解决的方法就是安装VirtualBox客户端增强包。
1、若直接安装客户端增强包会得到如下提示：
root@debian:/opt# ./VBoxLinuxAdditions.run
Verifying archive integrity… All good.
Uncompressing VirtualBox 5.1.34 Guest Additions for Linux………..
VirtualBox Guest Additions installer
Removing installed version 5.1.34 of VirtualBox Guest Additions…
Copying additional installer modules …
Installing additional modules …
vboxadd.sh: Starting the VirtualBox Guest Additions.
Failed to set up service vboxadd, please check the log file
/var/log/VBoxGuestAdditions.log for details.
root@debian:/opt#
不能成功安装
2、备份你的源文件，以免出错后可以恢复。
3、更新Debian9系统安装源文件/etc/apt/sources.list内容如下：
deb http://mirrors.163.com/debian/ stretch main non-free contrib
deb http://mirrors.163.com/debian/ stretch-updates main non-free contrib
deb http://mirrors.163.com/debian/ stretch-backports main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch-backports main non-free contrib
deb http://mirrors.163.com/debian-security/ stretch/updates main non-free contrib
deb-src http://mirrors.163.com/debian-security/ stretch/updates main non-free contrib
4、更新163源之后，更新源缓存
root@debian:~# apt update
5、查看自己的debian的内核版本，命令如下：
root@debian:~# uname -r
6、安装增强包需要的运行环境，执行指令如下：
root@debian:~# apt install build-essential
#安装编译程序必须的软件工具包组build-essential
root@debian:~# apt install linux-headers-`uname -r`
#安装与内核相同版本号的内核头文件
安装完成后，重新启动虚拟机即可安装客户端增强包了。
