using dd command on ubuntu 16 to create iso from dvd,Sourav Bhattacharya,RHCE Faculty,Kolkata 09748184075

First check the block size and volume size using this command

isoinfo -d -i /dev/cdrom | grep -i -E 'block size|volume size'

then

dd if=/dev/cdrom of=/home/sourav/centos.iso bs=block size found from previous command 

 count=volume size found from previous command

source://https://www.thomas-krenn.com/en/wiki/Create_an_ISO_Image_from_a_source_CD_or_DVD_under_Linux