From Birnam Designs Wiki

In Linux, creating a CDROM image is really easy:

dd if=/dev/cdrom of=/tmp/cdimg1.iso

if=/dev/cdrom -- (input file) read from /dev/cdrom (use /dev/ location, not /mnt/ location, to get raw data)

of=/tmp/cdimg1.iso -- (output file) write to FILE cdimg1.iso, i.e. create an ISO image