1. Cheap Off-the-Shelf Tiny Linux By Dzuy Nguyen dzuy@itlogistic.net
COST Linux
Cheap Off-the-Shelf Tiny Linux
By Dzuy Nguyen

2. What it is
Home-brewed tiny Linux distro. More like Linux From Scratch.
Made from readily available hardware (i386) and binaries from your favorite distro.
Customed compiled kernel.
Small storage device: Solid State Disk (SSD) rather than spinning hard drive.

3. Usage: network appliances
Firewall/VPN
Router
DNS server
Load Balancer (LVS)
Proxy server
Any dedicated network service
Other projects:
Carputer
Home automation

4. Advantage Secured. Reliable (no spinning part). Crash proof.
Hardened (no extra software that you don't need).
Easy upgrade: just replace file system images.
Flexible: separation of root and application file system.
Did I mention low COST?

5. Ingredients
Kernel: (custom) compiled as module as much as you can to keep kernel small.
Boot strap initrd to pre-load module and prepare memory as disk.
/: Busy box
Login: Tiny login (with PAM)
Logs: can be remote or local (clean with cron).
Other: binaries from your RPM or compiled apps.

6. Compressed file system images
Root FS: for /. Resides in RAM disk (initrd), rw. Size: 4MB file system, ~2MB populated.
Local FS: Other apps and libraries. Mount either from memory or flash disk. (ro). Size: 4-8MB depends on amount of apps.
Sysconfig FS: Resides on flash disk (rw) for permanent storage. Stores system's configurations. Size: 100KB and up.

7. Prepare file system on disk
Create a loopback file system (need loop module):
# dd if=/dev/zero of=rootfs bs=1k count=<size>
Make file system:
# mke2fs -N 2048 rootfs
Mount the file system
# mount -o loop rootfs /mnt
Copy files from target workspace.

8. The root file system
Need: /dev, /proc, /etc, /sbin, /bin, /lib, /mnt, /usr, /var, /home, /tmp
/dev: device files (hd*, ram*, tty*, std*, etc.)
# cp -dpR /dev/hda[0-5] /mnt/dev/
/etc: fstab, passwd, group, inittab, mtab, resolv.conf, sysconfig, etc.
(/usr)/bin and (/usr)/sbin: system utilities to be provided by busy box (and more).
/lib: libc (must have), ld-linux (most popular), libscrypt (busybox), libdl, libnsl, libresolv (and more).

9. Shrink binaries - strip them
Strip -s: remove debug symbols
Strip -g: remove all symbols
Strip libraries and executables only. Do not strip modules

10. Apps (/usr/local) file system
/usr/local/bin, /usr/local/sbin: application binaries
/usr/local/etc: rc scripts
/usr/local/lib: other supporting library
/usr/local/lib/modules: kernel modules
/usr/local/lib/security: PAM modules
/usr/local/lib/iptables: iptables modules (if firewall).

11. Sysconfig file system Store system configuration stuffs
Network configuration files.
modules.conf
Other: syslog, logrotate.conf, sysctl.conf, hosts, passwd, shadow, group (link to /etc)
Other apps' config files can be stored here and called appropriately.

12. Initrd file system Similar to root file system
There must be a /linuxrc executable.
The linuxrc executable can be a script or binary. If script, the shell must be present.
All linked libraries must be present and accessible.
Use linuxrc to pre-load modules and prepare (RAM) file system.

13. Boot loader Choice of your favorite bootloader: GRUB or LILO. GRUB
Advantages: easy to config and don't need to re-run like LILO when kernel is upgraded. Splash screen possible.
Disadvantages: larger than LILO.
LILO
Advantages: small, works on all medium.
Must re-run LILO when kernel changes.

14. Boot process
BIOS -> Boot loader -> kernel -> initrd -> linuxrc -> init
Init is not really needed. Can run system from linuxrc, just make sure it does not exit. Most commercial embedded system does not use init.
Init -> inittab -> rc -> rc.sys
Busybox has its own init and shell (ash).
Modify rc and rc.sys to bring up your system.

15. Hardware
Any system with BIOS that is willing to boot our bootloader or kernel.
Cheap thin clients are available on eBay
Netier XL and SL series
Capio II 2992
Acute Network Technologies
GTC Allwell
IBM NetVista 8363 N2200
Single Board Computers
More (not yet hacked).

16. Solid State Disk
DiskOnChip: manufactured by M-Systems. Available on selected boards only. GRUB must be burned in. Special driver module is needed.
DiskOnModule: available in 40 and 44 pin IDE. Sticks right on your IDE port. BIOS sees it as hard disk.
Compact Flash: cheapest SSD. Some boards have CF interface on board. IDE to CF converter is also available.
DiskOnStick: USB flash disk. Some boards can boot from these. Need to load some USB drivers in order to see as /dev/sd (SCSI) disk.