A live CD or live DVD is a complete bootable Linux operating system loaded from a CD or DVD. Although there are a lots of live Linux CDs, for seemingly every taste and purpose, it might still be useful on occasion to build your own. This guide details the steps to build a bootable live CD/DVD based on Debian "bullseye".

Step 1 – Installing the necessary software

These are the software packages you need to install on your Debian system:

```bash
apt-get install xorriso live-build syslinux squashfs-tools isolinux
```
Step 2 – Create a basic filesystem

Start by creating a new work directory, and bring in a basic Debian filesystem using debootstrap. Depending on your network connection, it will take some time downloading all the necessary packages:

```bash
# mkdir ~/livework && cd ~/livework
# debootstrap --arch=amd64 bullseye chroot http://deb.debian.org/debian
```
The new filesystem was created in ~/livework/chroot directory. It is time to chroot into the new filesystem and finish the installation.


Step 3 – chroot
```bash
# cd ~/livework
# LANG=C.UTF-8 chroot chroot /bin/bash
# mount none -t proc /proc && \
  mount none -t sysfs /sys && \
  mount none -t devpts /dev/pts
# export LC_ALL=C
# export PS1="\e[01;31m(live):\W \$ \e[00m"
```

In chroot you need to bring in a Linux kernel and the necessary livecd packages. You can also set up a root password:

```bash
(live):/ $ apt-get install dialog dbus
(live):/ $ dbus-uuidgen > /var/lib/dbus/machine-id
(live):/ $ apt-get install linux-image-amd64 live-boot
(live):/ $ apt-get install vim ssh wget curl net-tools iputils-ping isc-dhcp-client less binutils parted fdisk
(live):/ $ echo "liveCD" > /etc/hostname
(live):/ $ passwd
```
Add hostname to hosts file
```bash
(live):/ $ cat > /etc/hosts << EOF
127.0.0.1 localhost
127.0.1.1 $(cat /etc/hostname)

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
EOF
```

This is a very basic Debian system. On top of it you can install packages such as desktop environment (apt-get install lxde), a web browser (apt-get install iceweasel) etc. When you are done, cleanup apt caches and exit chroot.
```bash
(live):/ $ apt-get clean && apt-get autoclean && apt-get autoremove
(live):/ $ rm /var/lib/dbus/machine-id && rm -rf /tmp/*
(live):/ $ umount /proc /sys /dev/pts
(live):/ $ exit
```
Step 4 – ISOLINUX

The CD/DVD image is set up using ISOLINUX. Start by creating a new directory, binary, containing the Linux kernel, a compressed copy of chroot, and isolinux executables:

```bash
# cd ~/livework
# mkdir -p binary/live && mkdir -p binary/isolinux
# cp chroot/boot/vmlinuz-*-amd64 binary/live/vmlinuz
# cp chroot/boot/initrd.img-*-amd64 binary/live/initrd
# mksquashfs chroot binary/live/filesystem.squashfs -comp xz -e boot
# cp /usr/lib/ISOLINUX/isolinux.bin binary/isolinux/.
# cp /usr/lib/syslinux/modules/bios/* binary/isolinux/.
# touch binary/isolinux/DEBIAN_IMAGE
```
Next, an isolinux config file is created:
```bash
# cat <<'EOF' > binary/isolinux/isolinux.cfg

ui menu.c32
prompt 0
menu title Boot Menu
timeout 300
label live-amd64
	menu label ^Live (amd64)
	menu default
	linux /live/vmlinuz
 	append initrd=/live/initrd boot=live persistence quiet

label live-amd64-failsafe
	menu label ^Live (amd64 failsafe)
	linux /live/vmlinuz
	append initrd=/live/initrd boot=live persistence config memtest noapic noapm nodma nomce nolapic nomodeset nosmp nosplash vga=normal

endtext
EOF
```
Step 5 – Building the iso image

I use GNU xorriso to build the final iso image. It creates an isohybrid image that can be transferred to a USB stick using dd command.

```bash
# cd ~/livework
# xorriso -as mkisofs -r -J -joliet-long -l -cache-inodes \
-isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin -partition_offset 16 \
-A "Debian Live"  -b isolinux/isolinux.bin -c \
isolinux/boot.cat -no-emul-boot -boot-load-size 4 \
-boot-info-table -o bullseye-livecd.iso binary
```

Conclusion

The iso image in this example has a size of 203MB. It is a basic Debian system as created by debootstrap, with only the necessary livecd executables. From here it will grow as more packages are added and the image is personalized.

I have decided to document my steps in case anyone might find them useful. Please let me know if you run into problems, or if you have any questions or suggestions. I use these steps to build small network appliances, servers, and rescue disks, nothing important. I’ve never went as far as to build a full distribution.

If you are considering it for a more serious project, better try live-build. Debian team uses live-build to build the official Debian CDs. The tool is very powerful and highly configurable, and it goes well beyond what I’ve covered in this example.