Creating a Complete Distribution on CD

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If you need a CD distribution that is more flexible and customizable than the ones readily available, create your own.

We all know about the possibilities for installing Linux on a hard drive. Sometimes, however, this option isn't good enough. Say, for example, you simply want to give someone a Linux CD that he can use without installing anything. Another scenario would be if you work for a school and want to teach your students how to use Linux on a PC, the same PC that will be used later in the day for a Windows 2000 class. In this article, the scenario is an enterprise that wants its customers to complete an evaluation at the end of a course. The goal is to insert a CD in the CD-ROM drive, boot and be redirected to some Web site where the evaluation can be completed.

Make It Yourself or Download It?

One might wonder why you should create your own Linux bootable CD when so many solutions are available for downloading on the Net. Before we start, let's look at some of the best bootable Linux CDs.

  1. Minilinux on CD. One of the easiest ways to create your own Linux bootable CD is to take the image of a Minilinux diskette and burn it on a disk. This is a nice solution if you don't like floppy disks, but in truth, this isn't really a complete Linux bootable CD. It's simply a floppy copied to a CD. For this reason we don't choose this solution; it isn't flexible enough.

  2. SuSE Live Evaluation. Starting with SuSE version 7.3, this Linux distributor has created a Live Evaluation CD. This is a complete Linux distribution on a disk; simply insert it in the drive and boot it up to see a complete running Linux. There are, however, some drawbacks, the major one being some files have to be copied to your hard drive before you can use it.

  3. Knoppix. Knoppix is a solution that looks a lot like the SuSE Live Evaluation. It is a nice solution based on Debian Linux, and it includes everything you need. The only drawback is you do need a DHCP server to activate the network card with a fixed IP address.

These all are viable options, but they are too much standardized and leave little room for flexibility. Therefore, you might decide to make your own bootable Linux CD. When you create your own CD, you can customize it and include only the things you need.

Preparation

A good Linux bootable CD starts with a sound preparation. I took a new 20GB hard drive and installed Red Hat 9.0 on a 4GB partition. After that, I installed Red Hat 7.1 on a 2GB partition, and on the third, 650MB partition, I put a minimal installation of Red Hat 7.1. This 650MB partition has to be the master for the CD; once it is complete you simply burn it to a CD and you're ready. Of course, you can use any distribution you like. I wanted to work with Red Hat 7.1 because I am familiar with it. I installed Red Hat 9.0 as "working space", because I also wanted access to the most recent distribution, just in case.

If you are using a multi-Linux system, you have to make a boot menu to be able to boot them all. In LILO, you can do so by adding the following entries to lilo.conf:

boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
message=/boot/message
linear
image=/boot/vmlinuz-2.4.2-2
        label=completeRH71
        read-only
        root=/dev/hda6
image=/hda1/boot/vmlinuz-2.4.18-3
        label=RH73
        read-only
        root=/dev/hda1
image=/hda7/boot/vmlinuz
        label=cd-image
        read-only
        append=?root=/dev/hda7?

Once you complete the installation of the Linux images, it is very easy to modify the configuration of the CD. Try it out to see if it works; if it doesn't, you simply try again. Burn the CD only when you are satisfied with the configuration.

The Challenges

In creating a Linux distribution that can boot from a CD-ROM and doesn't need anything else, you are likely to encounter some challenges. The main problem is the root filesystem is read-only, but some files have to be created and/or modified. This stage concerns files in /dev, in /var and eventually in the user's home directory. The next challenge is to turn off everything you do not need, especially commands that try to create a file somewhere.

Regarding the files in /dev that need to be changed, since kernel 2.2 you can use devfs to access them. This kernel driver creates a virtual filesystem similar to the /proc filesystem in memory, where one file for each device your system needs is created. The /dev/ files exist in RAM only, so there's no problem in changing attributes of the devices. There is, however, a disadvantage: changes to /dev/, including symlinks for your mouse and CD-ROM, are lost. As a solution to this problem you can use the script rc.devfs, which comes with devfs. Any changes you make to /dev can be saved by using this script; simply type /etc/rc.d/rc.devfs save /etc/sysconfig , and your changes are recorded. In short, all you have to do is compile your kernel with devfs support and record changes you make to /dev using rc.devfs, and you are able to use and modify all necessary devices on the read-only filesystem.

For the files in directories including /var, /tmp and eventually the user's home directories, we need another solution. To make it possible to change and create files in these directories later on, you have to create them in an early stage of the boot process. You can do this by including some code at the beginning of rc.sysinit. In the example below, we use a special file called rc.iso for this step.

#rc.iso. Must be included at the beginning of rc.sysinit.
#
#create /var
echo Creating /var ?
mke2fs -q -I 1024 /dev/ram1 4096
mount /dev/ram1 /var -o defaults,rw
cp -a /lib/var /
#restore devfs settings, if any. Needs proc
mount -t proc /proc /proc
/etc/rc.d/rc.devfs restore /etc/sysconfig
umount /proc

In the listing, two different things happen. First, a RAM drive is created and mounted on /var. Next, any settings saved to rc.devfs are restored to /etc/sysconfig so you can use them the next time you boot your system.

When you've done that, you can go on to the next important step: disabling the read/write remounting of your root filesystem and all lines associated with it. Simply look for the command mount -n -o remount,rw / in /etc/rc.d/rc.sysinit. At the same time, you can disable all lines that perform a check of your filesystems, because it doesn't make sense to check check a read-only filesystem automatically when you are booting your system.

Before you go on and burn the first trial version of your own distribution, you have to make some arrangements to create a symbolic link to a /tmp directory and to an /etc/mtab directory. The latter one is needed so the system can work with mounted filesystems. In addition, you need to make a template for /var. All of these steps are in a script, included below. Be aware, however, that you need to do this from the complete Linux distribution to the distribution you are creating on the reserved partition. In the listing below, the Linux partition that serves as the model for the CD to be created later is mounted on the directory /mnt/hda7. You need to execute this script only once.

#!/bin/sh
# make arrangements for /tmp
# delete tmp on hda7 if it exists
rm -fR /mnt/hda7/tmp
# create a symbolic link for /tmp to /var/tmp which will be writable when the cd is used
ln -s var/tmp /mnt/hda7/tmp
###
# create /proc/mounts on the cd so that you can make a link
touch /mnt/hda7/proc/mounts
# remove mtab if it exists
rm /mnt/hda7/etc/mtab
# recreate mtab as a link to /proc/mounts on the cd
ln -s /proc/mounts /test/etc/mtab
###
# create a template for /var/lib in /lib
mv /mnt/hda7/var/lib /mnt/hda7/lib/var-lib
mv /mnt/hda7/var /mnt/hda7/lib
mkdir /mnt/hda7/var
ln -s /lib/var-lib /mnt/hda7/lib/var/lib
rm -fR /mnt/hda7/lib/var/catman
rm -fR /mnt/hda7/var/log/httpd
rm -f /mnt/hda7/lib/var/log/samba/*
# recreate all files in /var/log as new empty files
for I in `find /mnt/hda7/lib/var/log -type f`; do 
      cat /dev/null > $i;
done
rm `find /mnt/hda7/lib/var/lock -type f`
rm `find /mnt/hda7/lib/var/run -type f`

Take a moment here to test if everything works so far. Save any changes you made and reboot from the CD image partition of your hard drive. You probably will see a lot of errors on your next reboot, but don't worry about them. Think of them as the fine-tuning that needs to be done afterwards. For now, the only important thing is that you end up at a login prompt. You also may encounter some serious error messages that end in something like this:

touch: creating '/var/lock/subsys/xfs': No such file or directory ]
touch: creating '/var/lock/subsys/local': No such file or directory
mkdir: cannot create directory '/var/root': File exists
mkdir: cannot create directory '/var/temp': File exists
ln: '/var/temp/tmp': File exists
mkdir: cannot create directory '/var/log': File exists
INIT: Id "1" respawning too fast: disabled for 5 minutes
INIT: Id "9" respawning too fast: disabled for 5 minutes
INIT: no more processes left in this runlevel

If you receive something like this, you probably to activate the devfs filesystem. Be sure that you included the line /sbin/devfsd /dev at the beginning of rc.sysinit. If you didn't your devices won't behave the way they should, and you can't do anything with your system. You say you just checked that devfsd is started and you still have problems? Be sure your RAM drives are initialized. If they are, you should see the line /dev/ram1 on /var type ext2 (rw) when using mount. If you don't see any RAM device, make sure that support for RAM drives is available in your current kernel configuration.

If you have been successful to this point, it isn't a bad idea to burn it all to a CD. You never know what might happen later, so it is a good idea to be have something stable to fall back on if you can't see what you did later in the process. Here's the procedure in short:

  • Get boot.img form a Red Hat installation CD.

  • Mount boot.img through loopback by typing

    mount boot.img /somedir -o loop -t vfat
    

  • Remove everything from the mounted boot.img file except for ldlinux.sys and syslinux.cfg.

  • Copy the kernel image from your test partition to boot.img.

  • Edit syslinux.cfg so it contains the following, in which bzImage should refer to the kernel image file you use and /dev/cdrom should refer to the device file of your CD-ROM device.

    default Linux
    label Linux
    kernel bzImage
    append root=/dev/hdc
    

  • Umount boot.img with umount /somedir.

  • Copy boot.img to the partition that is the master for the CD.

  • Change directories to the location where you want to store the image, and make sure you have enough free space.

  • Create the image with

    mkisofs -R -b boot.img -c boot.catalog -o mydistro.iso /
    

  • Write the image to a CD.

______________________

Comments

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great article! if your

Anonymous's picture

great article!
if your wanting to remaster ubuntu or other debian based distributions check out this guide:
http://www.cyberpunkcafe.com/page.php?15

thanks anyhow!

creating a live cd

uk's picture

nice article, i'm trying to create one based on this. but i've some doubts.
1. i didn't find any devfs in /etc/rc.d/rc.devfs( thn wht did u mean by tht)
2.i installed a minimum of fedora core but not gotgraphics. how could i enable it, on live cd

Re: Creating a Complete Distribution on CD

kurtm's picture

Would it be possible to keep in mind that non-x86 Linux ports exist?

I realize it's really beyond the scope of this article to cover the different boot formats needed for non-x86 architectures, but you could at least mention that the procedure is different for them.

The Linux community is quick to point at how many platforms Linux runs on, but then assumes everyone's machine is x86.

Re: Creating a Complete Distribution on CD

Anonymous's picture

Try the Gentoo LiveCD. I haven't personally tried it, but it claims to work on several different types.

Gentoo

Re: Creating a Complete Distribution on CD

Anonymous's picture

Smaller than Knoppix and Morphix: DamnsmallLinux and LinuxBBC

bootcd automates the process

Anonymous's picture

At least in the Debian distro, there is a set of scripts in a package called "bootcd" that automates the creation of a bootable live CD.

The scripts basically copy the installed system to a working directory, compress the directory, and generate a compressed ISO (using the Rockridge compressed filesystem extensions in the Linux kernel). I've personally gotten a 1.6GB system onto a 700MB CD in this way. The scripts even handle creating ram disks to hold /dev, /var, /etc, /tmp, /root, and /home. You can tweak what is and isn't included on the CD and what is and isn't loaded into the ram disks through the config file.

I use a 10GB partition for the install and to allow room for authoring the CD. 1.6GB of installed software + roughly 1.6GB for the "working directory" copy it creates + roughly 1-1.4GB for the compressed directory + 700MB for the resulting ISO == about 5-6GB (with some room to spare). Once the script finished creating the ISO, it automatically cleans up after itself by removing the working and compressed directories. By default it removes the ISO as well if it is configured to immediately burn a CD. I turned this off in the config file so I can burn multiple copies.

I've been impressed so far.

As since I'm doing this in Debian, configuring exactly what packages I want on the CD is simple with apt-get. Do a very simple install, "apt-get install" the software you want, "apt-get remove" the software you don't, and then start making ISOs.

Descriptions, sources, and packages (for Debian) can be found at
http://packages.qa.debian.org/b/bootcd.html
http://packages.debian.org/stable/utils/bootcd.html

Re: Creating a Complete Distribution on CD

Anonymous's picture

This is not a good article, it really doesn't prove the point of why you should make your own bootable; makes erroneous comments in options like knoppix, doesn't cover very important topics like how to discover where the cd rom is, it just forces you to have a distribution customized for a particular configuration... nevertheless you can see some of the "behind the scenes" live cds preparation

Re: Creating a Complete Distribution on CD

Anonymous's picture

Also check out Timo's Rescue Cd Creation Set on sourceforge:

http://sourceforge.net/projects/rescuecd/

The iso is a great pre-built rescue disk based on debian, but you can also download the source and custom tweak it however you like.

More than 650 MB?

Anonymous's picture

How do you get more than 650 MB? I believe that Knoppix is using compression and allows one to have considerably more software than one could have in just a 650MB partition. Interesting article.

what about morphix

Anonymous's picture

If you want a prepackaged distro that allows you to add
exactly what you want, try
http://www.morphix.net/

Re: what about morphix

Anonymous's picture

http://www.morphix.net/ is a web site designer, what you want is
Morphix.org

Re: Creating a Complete Distribution on CD

Anonymous's picture

About Knoppix author wrote:

3. Knoppix. Knoppix is a solution that looks a lot like the SuSE Live Evaluation. It is a nice solution based on Debian Linux, and it includes everything you need. The only drawback is you do need a DHCP server to activate the network card with a fixed IP address.

No, that is not true. In Knoppix you can activate the network card with a fixed IP address! You do not need DHCP server for that.

1. Command prompt : netcardconfig
2. KDE Menu: KNOPPIX-Network/Internet-Network card configuration

Re: Creating a Complete Distribution on CD

Anonymous's picture

Morphix seems to be another good choice.
its originally based upon knoppix but is designed as a framework
for adding just the components you need.

http://www.morphix.net/

Re: Creating a Complete Distribution on CD

Anonymous's picture

The web address is http://www.morphix.org.

The Mandrake method

Anonymous's picture

1)Install Mandrake 9.2

2)Add a urpmi medium for contrib
See http://plf.zarb.org/~nanarodon for help on doing this (or 'urpmi urpmi.setup; urpmi.setup)

3)Install mklivecd
# urpmi mklivecd

4)Make your Live CD

# mklivecd livecd.iso

Of course, there are many things you might want to change, and that is why mklivecd has a lot of options (allowing you to build the image from a chrooted installation, specifying which kernel to run, which bootsplash screen to use etc).

What does this method give over the method in the article?
-cloop support (like Knoppix), so you can have filesystems of up to about 2GB
-important places in /etc and /var writeable, as well as /home (in memory)
-it's trvial to use
-It's quicker to do than remastering Knoppix

There are some things that don't work in the version in 9.2 contrib (like local printing, although remote printing works fine), but most of those are fixed in CVS. Also, be sure to use the updated kernels for Mandrake 9.2, you need cloop-1.02 for reliable use of a live CD, which was merged into the official kernels for 2.4.22-18mdk.

One CD has already been distributed en masse, and there are a few more coming.

Unfortuntely the web site (actually, minicd's website at http://minicd.berlios.de) hasn't been updated, but the mailing list is relatively active, and CVS has a few commits a week.

Re: Creating a Complete Distribution on CD

Anonymous's picture

Don't forget the excellent and improving Dyne::Bolic -- not just a general distribution on CD, but specializing in audio/video editing and streaming tools. Great "brickout" style game, too. :)

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