How to make an ISO image in linux

If you want to make an iso file from CD or DVD goto the following steps.
1) First place the disk in the cd/dvd drive. Donot mount it.
2) If it auto mounts then unmount it.
3) Type the following commands

To make an iso image from a dvd

$dd if=/dev/dvd of=dvd.iso

To make an iso image from cd
$dd if=/dev/cdrom of=cd.iso

To make an iso image from an scsi cdrom use command

$dd if=/dev/scd0 of=cd.iso

4) To make an ISO from files on your hard drive, create a directory which holds the files you want and use the mkisofs command.

$mkisofs -o /tmp/cd.iso /tmp/dir/

The result is a file cd.iso which contains all the files in directory /tmp/dir

If you get a message error mkisofs not found ,it means the program mkisofs is not present in the system ( not installed). So install program called mkisofs.
for debian type this as root

$apt-get install mkisofs