Building Your Own Live CD

You've probably heard of Knoppix, the Debian-based distribution that squeezes 2GB of applications on a single standalone CD. It's been used as a Linux demonstration tool, a rescue disk and even as a Debian installer. It's inspired a small raft of related projects, ranging from CDs containing Knoppix, plus or minus a few extra packages, to complete re-architectures of the system.

I recently set out to produce a live CD for a product demonstration. I started by taking the Knoppix CD apart to see how it ticked, and I ended up with a Makefile and a few ancillary files that are clearly Knoppix-inspired but have little derived code. This is what I learned.

If you put the Knoppix CD in a CD-ROM drive and mount it, you soon notice that it doesn't look much like an ordinary Linux installation. There are a few graphic files and a free music track, but no init, no /dev and no /bin. The magic is in the big file called /KNOPPIX/KNOPPIX, an ISO9660 filesystem image compressed for the cloop device.

The standard loop device in the kernel allows you to access a file in some filesystem as if it were a device; requests for blocks of the device are mapped to requests for blocks in the underlying file. Because you can mount the device, this effectively means you can create images of filesystems and access them as if they were real hardware disks. If you downloaded Knoppix from the Net, you have an ISO9660 image that can be loop mounted to look at its contents:

# mkdir /tmp/knoppix-cd
# mount -o loop -r \
$HOME/KNOPPIX_V3.3-2003-09-24-EN.iso /tmp/knoppix-cd

The cloop compressed loop device takes this a step further. In this adaptation of the loop device, each block is compressed with gzip and transparently decompressed when it's accessed. /KNOPPIX/KNOPPIX is an image for this device that is mounted during startup—this is how Knoppix gets 2GB onto a 650MB CD.

You don't need to install cloop in your usual kernel if you simply want to look around the inner filesystem. Install the cloop-utils package and use extract_compressed_fs, as shown below. You need about 2GB of free space in /var/tmp or wherever you decide to put the image:

# mkdir /tmp/knoppix-cloop
# extract_compressed_fs \
   /tmp/knoppix-cd/KNOPPIX/KNOPPIX \
    >/var/tmp/KNOPPIX-cloop
# mount -o loop /var/tmp/KNOPPIX-cloop \
   /tmp/knoppix-cloop
# find /tmp/knoppix-cloop -print

You can look, but you can't touch—the ISO9660 filesystem is read-only. To modify the distribution, you first need to copy both filesystem images to ordinary directories:

# mkdir $HOME/my-knoppix-tree  \
    $HOME/my-knoppix-cd-tree
# tar -C /tmp/knoppix-cloop -cf - . | \
    tar -C $HOME/my-knoppix-tree -xvpf -
# tar -C /tmp/knoppix-cd -cf - . | \
    tar -C $HOME/my-knoppix-cd-tree -xvpf -
# umount /tmp/knoppix-cd /tmp/knoppix-cloop

Now, you can hack away to your heart's content. The most convenient way to do this is to change root into the Knoppix inner tree using the chroot command:

# mount -t proc none $HOME/my-knoppix-tree/proc
# cp /etc/resolv.conf \
$HOME/my-knoppix-tree/etc/resolv.conf
# chroot $HOME/my-knoppix-tree /bin/sh

From here, you can use all the usual Debian package management commands (dpkg, apt-get and so on) to install or delete whatever you like. When you're done, exit the chroot and unmount proc, unless you want your development system's process list immortalised on CD. Then, use create_compressed_tree and mkisofs to create the inner and outer images:

# mkisofs -L -R -l -V "KNOPPIX ISO9660" -v \
-allow-multidot $HOME/my-knoppix-tree | \
create_compressed_fs - 65536 > \
$HOME/my-knoppix-cd/KNOPPIX/KNOPPIX

# mkisofs -l -r -J -V "KNOPPIX with local stuff" \
-hide-rr-moved -v -b KNOPPIX/boot-en.img \
-c KNOPPIX/boot.cat -o knoppix.iso \
$HOME/my-knoppix-cd

Finally, burn knoppix.iso to a CD-ROM and boot it. If you prefer, you can test without burning by using Bochs or VMware.

This simple approach starts to break down, however, when you want more extensive customizations. For example, if you want X to start a particular window manager but don't want to use all of GNOME or KDE, you have to edit the script yourself. This isn't hard to do, but it means that you've essentially forked Knoppix. When a new Knoppix version comes out, you'll have to do it again. In addition, if you intend to sell your Knoppix-based CD commercially, you need to remain compliant with the licenses of all the software you distribute, which means knowing exactly what's on it. The Knoppix version I looked at contained some files that weren't from Debian packages, and sometimes they weren't even free software.

So, is there some other place we could start? Happily, yes. Between the efforts of Progeny, which donated its installer to the Debian Project; Klaus Knopper, the author of Knoppix and the creator of the cloop device; and other Debian developers who are working on adding his custom code into the main Debian repository—today we can put together a passable live CD system from scratch using only Debian packages. The rest of this article describes how.