Page 321 - Hacking Roomba

302 Part III — More Complex Interfacing

The peripherals supported by OpenWrt are constrained by the type of hardware it’s running
on. Obviously if the wireless router has no USB interface, it cannot support USB devices. Most
peripherals that both work on Linux and have some way to be hooked up to a router work in
OpenWrt. These include devices like IDE drives, flash drives, disk drives, serial adapters, web-
cams, and printers.

How OpenWrt Differs from Regular Linux
Unlike the Linux you may be familiar with, OpenWrt is pared down severely in order to fit in
the 4 MB flash chip. The most striking difference is the lack of permanent disk space. The disk
is really the flash chip, and the base OpenWrt system fills most of that up. OpenWrt also cre-
ates a RAM disk with the available RAM and that offers a good temporary storage location.
Many OpenWrt setups operate entirely in a read-only file system mode where the flash disk
cannot be written to. This is more secure, but makes for a miserable hacking experience.

To store changeable configuration information like network setup and Wi-Fi settings, a special
64 KB section of the flash chip is reserved for what is known as NVRAM (for non-volatile
RAM, even though it’s not RAM any longer). OpenWrt, the bootloader, and the stock
vendor firmware all use NVRAM. This means that firmware upgrades don’t erase your net-
work configuration.

When you have OpenWrt up and running, type nvram show on the command line to see all the
nvram variables.

To provide the command line tools you expect in a Linux system, OpenWrt uses BusyBox
(www.busybox.net/), a single program that combines most of the tiny utilities everyone uses.
So if you were to look in the /bin directory of an OpenWrt machine, you’d see the following:
lrwxrwxrwx 1 root root 7 Mar 27 00:06 ash ->
busybox
-rwxr-xr-x 1 root root 551416 Mar 27 00:06 busybox
lrwxrwxrwx 1 root root 7 Mar 27 00:06 cat ->
busybox
lrwxrwxrwx 1 root root 7 Mar 27 00:06 chmod ->
busybox
lrwxrwxrwx 1 root root 7 Mar 27 00:06 cp ->
busybox
lrwxrwxrwx 1 root root 7 Mar 27 00:06 dd ->
busybox
lrwxrwxrwx 1 root root 7 Mar 27 00:06 df ->
busybox
[...]
Using BusyBox saves an enormous amount of space. Another big space saver is the use of
uCLib (http://uclibc.org/), a C library that is much smaller than the GNU C Library
that is standard with Linux. The standard C library contains hundreds of useful functions like