it is as easy as:

dd if=/dev/cdrom of=~/cdrom_image.iso

in my case the dvd-drive was sr0 not cdrom.

dd if=/dev/sr0 of=/mnt/sda4/DVD/filename.iso

# to identify your dvd-drive you can use:

inxi -d; # inxi = Command line system information script for console and IRC

# show all drives
Drives:    HDD Total Size: 256.1GB (60.5% used) ID-1: /dev/sda model: SanDisk_SDSSDHP2 size: 256.1GB
           Optical: /dev/sr0 model: N/A dev-links: cdrom,cdrw,dvd,dvdrw
           Features: speed: 62x multisession: yes audio: yes dvd: yes rw: cd-r,cd-rw,dvd-r,dvd-ram

# actually not correct, the drive can also do dvd+r

you can test-mount your filename.iso like this and check if everything is on it:

sudo bash;
mkdir /mnt/cdrom/;
mount -t iso9660 -r -o loop /mnt/sda4/DVD/filename.iso /mnt/cdrom/