The VirtualBox Guest Additions consist of device drivers and system applications that optimize the operating system for better performance and usability. i will show you in article how to install Guest additions in Fedora28/28 en Centos 7.

A1. Change to root user

su -  

A2. Make sure that you are running latest kernel

Update virtual machine kernel and reboot

## Fedora 29/28/27/26/25/24/23/22 ##
dnf update 

##  CentOS/RHEL 7/6/ ##
yum update 

reboot

A3. Mount VirtualBox Guest Additions

Click Devices > Install Guest Additions… on VirtualBox

Mount VirtualBox Guest Additions device

mount -r /dev/cdrom /media/ 

A4. Install following packages

On CentOS/Red Hat (RHEL) 7/6/5, EPEL repo is needed

## CentOS 7 and Red Hat (RHEL) 7 ##
rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

<## CentOS 6 and Red Hat (RHEL) 6 ##
rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
 

Install following packages

dnf install gcc kernel-devel kernel-headers dkms make bzip2 perl

## Fedora 21/ and Older, CentOS/RHEL 7/6/5 ##
yum install gcc kernel-devel kernel-headers dkms make bzip2 perl

A5. Install Guest Additions
cd  /media/ 
#For 32-bit and 64-bit systems run following<
./VBoxLinuxAdditions.run


Output looks like following:
[root@Centosradar ~]# mount  /dev/cdrom   /media/
mount: /dev/sr0 is write-protected, mounting read-only

[root@Centosradar ~]# cd  /media/
[root@Centosradar media]# ./VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.2.22 Guest Additions for Linux........
VirtualBox Guest Additions installer
Copying additional installer modules ...
Installing additional modules ...
VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel modules.  This may take a while.
VirtualBox Guest Additions: Starting.

A6. Reboot guest system

reboot

Then VirtualBox Guest Additions install is ready.

i have now more options for full screen mode