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This paper explores how to generate a minimized Linux target from a standard distribution. Most minimalization tools either create from scratch: a new distribution dedicated to a special functionality (ex: DSL, Leaf, OpenPBX, LTSP, ...) or reduce an existing distribution by skimming installed packages (OpenSuSE/KIWI, Ubuntu/JeOS, RedHat/ELDS). This paper describes a mixed approach, where you start from a standard running distribution (Ubuntu, OpenSuSE, ...) and extract the minimum set of binaries required for your target.The advantage of this host/target approach, is that it does not impose you a special distribution, provide a simple method to keep in sync your development host and embedded target. On the other hand you should understand in deep the dependencies of selected binaries and your boot process.

How look a minimized Linux ?

A minimal Linux is a skimmed down operating system that only support required applications, This being said and independently of the number of  applications, Linux needs at 4 elements:

• a booting process
• a kernel with necessary modules to support your hardware
• a root file system with basic (/etc, /dev,..)
• at least one application (apache, sshd, ...)

How small can it be ? this obviously depend on the application you want to embedded. Linux minimal footprint (kernel + minimal root filesystem) is around 6MB, if you add some minimal applications like sshd+httpd you end up with a final distribution below 8MB. On RAM side, when starting a TinyLinux with 32M of Ram 12M remain free, which set our ram footprint ~20M on which 3M are used for our root filesystem in memory.

MyTinyLinux script automates minimization of Linux and generate an ISO cd image of your target.

MkLinux is a very simple script that  takes one config file as input, this files describes which binary commands you want in your target, as well as config and/or application files. The script will search for you shared libraries dependencies, will build a minimal root directory from its generic template and will finally build a bootable ISO CD image.

1) Chose binaries you want in your target OS. Obviously those binaries must be installed on your Linux host, if you miss one (ex: busybox, lighthttp, tcpdump, ...) get it from your standard package management tool (ex: zypper, apt-get, ...) Let's say that you only want "sshd" you config in your target:

2) Define kernel module to support your target hardware:  by default outside of Ethernet interface  and SATA drivers most distributions include everything you need inside their core kernel, as we run only in memory and thus do not need disk, adding your NIC driver is probably enough. If you intend to run under VirtualBox you need Intel e100 driver, for vmWare pcnet32, .... af_packet module is needed for dhcp, etc ....

3) Basic basic utilities needed to operate Linux: almost independently of the application you want to run, you will need for "extra" tools. Like "ifconfig" to choose target's  IP address, modeprobe to load your NIC kernel module, ... this without forgetting some basic shell commands capabilities, "vi" or equivalent to edit configuration file directly target, and often some debug tools like "tcpdump, netstat, ....". A special note for Swiss knife "busybox" , it  provides most of required simple tools, indeed it is so painful to build an embedded Linux without it, that MkTinyLinux defaults its inclusion. Note that needed utilities may depend on chosen target hardware or virtualization environment.

4) Files/directories that cannot be detected automatically: MkTinyLinux try to discover dependencies automatically, while this works well for sharedlib, unfortunately they is no way to find that "ssd" need "/etc/ssh", that you "/lib/libnss_files.so.2" is requirer to enable login/password, that your target need a French keyboard, ...

Download MkTinyLinux

• configuration samples (OpenSuSE-tiny.conf)    (sshd-only-ubuntu.conf)
• iso sample (8MB)         (azerty-dhcp.iso)              (qwerty-dhcp.iso)

Generate a target with MkTinyLinux script

Dependencies

MkTinyLinux is a simple "bash" script and has almost no dependencies, nevertheless you may have to add "busybox" and "mkisofs" command that are not defaulted inside typical distributions. Nevertheless they are available witjout requirering compilation, inside most of standard repositories. More rarely you may have to add some package to get supplementary kernel modules to support you hardware.

• OpenSuSE:  zypper install cdrkit-cdrtools-compat busybox
• Debian:         apt-get install genisoimage busybox

Command line options

 mkTinyLinux config=??? target=??? [destdir=. template=generic tmpdir=/tmp root=no busybox=no verbose=1]

• config     your configuration file
• target      fix target runtime environment [openvz,iso,xen]
• destdir    final distribution directory (.iso|.tgz) directory [default=.]
• template root-tree template [default=Templates/Generic] it is used as foundation for your final distribution, you may create here dedicated /etc/init.d scripts.
• tmpdir     temporary directory use to built your distribution [default=tmp], for debug purpose tempry files are not deleted automatically 
• root          it is almost impossible to run MkTinyLinux without being super user, this option cancel the assertion made on root user.
• busybox Swiss knife is almost mandatory to built embedded Linux [default=yes], allow you may change default behavior, when ever busybox is not appropriated. 
• verbose  Provide some guidance at MkTinyLinux actions.

Quickstart MkTinyLinux

Download MkTinyLinux

With configuration samples and root file system template from (here)

Something is going wrong?

1) Try a "chroot" on temporary directory

The simplest way is to use a virtualization environment like VirtualBox that is available for free for Linux & Windows. This being said if you fail to boot your ISO cdrom image, the first thing is to check your distribution with a simple "chroot" command. As mkTinyLinux script does not remove automatically temporary files you can use them "/tmp" directly.

2) try to boot with virtualbox 

the next step is too check your init process. Unfortunately you cannot do this with "chroot" command, and you need a real virtualization tool as VirtualBox, KVM, QEMU, vmWare, etc. You only need to define boot cdrom through its ISO image, VirtualBox will warn you that you forgot to define a hard disk, just ignore the advice and move forward. Usually what's happen is that your init process fail to execute some commands from /etc/init.d or from what ever is your initial boot process [ex: /etc/evend.d for ubuntu, /etc/inittab for OpenSuSE, ...] If everything works you should arrive to Linux boot prompt.(see VirtualBox snapshot here after).