VirtualBox内刚刚安装完CentOS6.9和7系统，无法调整屏幕的分辨率，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@CentOS 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...
vboxadd.sh: Stopping VirtualBox 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.
不能成功安装
2、备份你的原镜像文件，以免出错后可以恢复。
3、下载新的CentOS-Base.repo 到/etc/yum.repos.d/目录
4、CentOS系统更换软件安装源
CentOS 6
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
5、更新阿里的源之后，更新源缓存
yum clean all
yum makecache
6、安装增强包需要的运行环境，执行指令如下：
[root@CentOS ~]# yum install kernel.x86_64
[root@CentOS ~]# yum install kernel-headers.x86_64
[root@CentOS ~]# yum install kernel-devel.x86_64
[root@CentOS ~]# yum install gcc.x86_64
安装完成后，重新启动虚拟机即可安装客户端增强包了。