First, put the CD/DVD you want to make image into your optical drive, do not mount that CD/DVD.

If it was mounted automatically, unmount it with this command:

# umount /[mount]/[directory]

Then, do this command to create your ISO:

# dd if=/dev/cdrom of=/tmp/[isoname].iso

• if=/dev/cdrom is the source
• of=/tmp/[isoname].iso is the filename of the ISO you create

# dd if= of= bs= skip=< > seek=< > conv= count=< >

for bs (byte size), usually some power of 2, not less than 512 bytes, but can be any reasonable number (512, 1024, 2048, 4096, 8192, 16384)