Creating a Complete Distribution on CD

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.PreparationA 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 ChallengesIn 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

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

Cleaning UpWhen your first trial CD is working, it's time to do some
cleaning. Basically, there are two locations where some major
cleaning up of the boot process can be done. First, you have to
take care of /etc/rc.d/rc.sysinit. In this file, which always is
executed on system boot, a lot of things are taken care of and
probably a lot of things can be skipped for your specific
configuration. The most important things to look at all are the
instances where files are created. Besides that, you can disable
the lines where services are activated that you don't need. Think,
for example, of services such as isapnp setup and probably many
more.Next, you have to clean the boot-up of your runlevel. Let's
first do a quick refresher of how services are activated when
entering a runlevel. On Red Hat, you find all the general scripts
used to start services in /etc/rc.d/init.d. A script called smb,
for instance, can be used to start Samba services. If you want this
script to be executed when entering runlevel 3, you have to create
a symbolic link that starts with S followed by a number to
determine the exact moment when the script should be executed. By
default, many of these links probably start services you don't
need. You could encounter, amongst others, the link S60lpd in
/etc/rc.d/rc3.d, which dictates that the line printer dæmon
is started every time you enter runlevel 3. In order to clean the
startup procedure of services you don't need, simply remove all of
these links.Automatic Login and Starting XReturning to our initial example, say you want to use your
Linux distribution for customers who have to complete an evaluation
without providing a user name and password. By changing a simple
line in /etc/inittab, you can log in your users automatically. You
can use any account you like, because it is a read-only
filesystem--you even could use root if you wanted. In order to log
in to the system automatically, change

1:2345:respawn:/sbin/mingetty tty1

to

1:2345:respawn:/usr/bin/open -c 1 -w -- /bin/login -f username

Don't forget to remove the password for the user you are
here. You probably don't want to oblige your users to fill in
anything before they can start using your CD.Before you can give the CD to an innocent user, you need to
take one more step, making it possible to start the X Window
System. It is rather easy to make this happen; simply give your
default user a writable home directory. At an earlier stage you
created a read/write accessible /var director, so this is a nice
location in which to create the home directory. After that, X is
happy to be able to create its temporary files, and everything
works the way it should.ResourcesDiskless-HOWTO-3Root
Over NFS Clients & Server HowtoLinux
Devfs FAQFor Knoppix and the source code used for Knoppix automatic
hardware detection, go to
www.knoppix.org.Sander van Vugt lives in the
Netherlands. He works for Azlan Network Training (part of the
Techdata Group) as a trainer and consultant, and he has written
several books and articles about Linux.

email: sander.van.vugt@azlan.com