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+Linux is now one of the most widespread operating system for embedded devices due
+to its openess as well as the wide variety of platforms it can run on. Many
+manufacturer actually use it in firmware you can find on many devices: DVB-T
+decoders, routers, print servers, DVD players ... Most of the time the stock
+firmware is not really open to the consumer, even if it uses open source software.
+
+You might be interested in running a Linux based firmware for your router for
+various reasons: extending the use of a network protocol (such as IPv6), having
+new features, new piece of software inside, or for security reasons. A fully
+open-source firmware is de-facto needed for such applications, since you want to
+be free to use this or that version of a particular reason, be able to correct a
+particular bug. Few manufacturers do ship their routers with a Sample Development Kit,
+that would allow you to create your own and custom firmware and most of the time,
+when they do, you will most likely not be able to complete the firmware creation process.
+
+This is one of the reasons why OpenWrt and other firmware exists: providing a
+version independent, and tools independent firmware, that can be run on various
+platforms, known to be running Linux originally.
+
+\subsection{Which Operating System does this device run?}
+
+There is a lot of methods to ensure your device is running Linux. Some of them do
+need your router to be unscrewed and open, some can be done by probing the device
+using its external network interfaces.
+
+\subsubsection{Operating System fingerprinting and port scanning}
+
+A large bunch of tools over the Internet exists in order to let you do OS
+fingerprinting, we will show here an example using \textbf{nmap}:
+
+\begin{Verbatim}
+nmap -P0 -O <IP address>
+Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:05 CET
+Interesting ports on 192.168.2.1:
+Not shown: 1693 closed ports
+PORT STATE SERVICE
+22/tcp open ssh
+23/tcp open telnet
+53/tcp open domain
+80/tcp open http
+MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys)
+Device type: broadband router
+Running: Linksys embedded
+OS details: Linksys WRT54GS v4 running OpenWrt w/Linux kernel 2.4.30
+Network Distance: 1 hop
+\end{Verbatim}
+
+nmap is able to report whether your device uses a Linux TCP/IP stack, and if so,
+will show you which Linux kernel version is probably runs. This report is quite
+reliable and it can make the distinction between BSD and Linux TCP/IP stacks and others.
+
+Using the same tool, you can also do port scanning and service version discovery.
+For instance, the following command will report which IP-based services are running
+on the device, and which version of the service is being used:
+
+\begin{verbatim}
+nmap -P0 -sV <IP address>
+Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:06 CET
+Interesting ports on 192.168.2.1:
+Not shown: 1693 closed ports
+PORT STATE SERVICE VERSION
+22/tcp open ssh Dropbear sshd 0.48 (protocol 2.0)
+23/tcp open telnet Busybox telnetd
+53/tcp open domain ISC Bind dnsmasq-2.35
+80/tcp open http OpenWrt BusyBox httpd
+MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys)
+Service Info: Device: WAP
+\end{verbatim}
+
+The web server version, if identified, can be determining in knowing the Operating
+System. For instance, the \textbf{BOA} web server is typical from devices running
+an open-source Unix or Unix-like.
+
+\subsubsection{Wireless Communications Fingerprinting}
+
+Although this method is not really known and widespread, using a wireless scanner
+to discover which OS your router or Access Point run can be used. We do not have
+a clear example of how this could be achieved, but you will have to monitor raw
+802.11 frames and compare them to a very similar device running a Linux based firmware.
+
+\subsubsection{Web server security exploits}
+
+The Linksys WRT54G was originally hacked by using a "ping bug" discovered in the
+web interface. This tip has not been fixed for months by Linksys, allowing people
+to enable the "boot\_wait" helper process via the web interface. Many web servers
+used in firmwares are open source web server, thus allowing the code to be audited
+to find an exploit. Once you know the web server version that runs on your device,
+by using \textbf{nmap -sV} or so, you might be interested in using exploits to reach
+shell access on your device.
+
+\subsubsection{Native Telnet/SSH access}
+
+Some firmwares might have restricted or unrestricted Telnet/SSH access, if so,
+try to log in with the web interface login/password and see if you can type in
+some commands. This is actually the case for some Broadcom BCM963xx based firmwares
+such as the one in Neuf/Cegetel ISP routers, Club-Internet ISP CI-Box and many
+others. Some commands, like \textbf{cat} might be left here and be used to
+determine the Linux kernel version.
+
+\subsubsection{Analysing a binary firmware image}
+
+You are very likely to find a firmware binary image on the manufacturer website,
+even if your device runs a proprietary operating system. If so, you can download
+it and use an hexadecimal editor to find printable words such as \textbf{vmlinux},
+\textbf{linux}, \textbf{ramdisk}, \textbf{mtd} and others.
+
+Some Unix tools like \textbf{hexdump} or \textbf{strings} can be used to analyse
+the firmware. Below there is an example with a binary firmware found other the Internet:
+
+\begin{verbatim}
+hexdump -C <binary image.extension> | less (more)
+00000000 46 49 52 45 32 2e 35 2e 30 00 00 00 00 00 00 00 |FIRE2.5.0.......|
+00000010 00 00 00 00 31 2e 30 2e 30 00 00 00 00 00 00 00 |....1.0.0.......|
+00000020 00 00 00 00 00 00 00 38 00 43 36 29 00 0a e6 dc |.......8.C6)..??|
+00000030 54 49 44 45 92 89 54 66 1f 8b 08 08 f8 10 68 42 |TIDE..Tf....?.hB|
+00000040 02 03 72 61 6d 64 69 73 6b 00 ec 7d 09 bc d5 d3 |..ramdisk.?}.???|
+00000050 da ff f3 9b f7 39 7b ef 73 f6 19 3b 53 67 ea 44 |???.?9{?s?.;Sg?D|
+\end{verbatim}
+
+Scroll over the firmware to find printable words that can be significant.
+
+\subsubsection{Amount of flash memory}
+
+Linux can hardly fit in a 2MB flash device, once you have opened the device and
+located the flash chip, try to find its characteristics on the Internet. If
+your flash chip is a 2MB or less device, your device is most likely to run a
+proprietary OS such as WindRiver VxWorks, or a custom manufacturer OS like Zyxel ZynOS.
+
+OpenWrt does not currently run on devices which have 2MB or less of flash memory.
+This limitation will probably not be worked around since those devices are most
+of the time micro-routers, or Wireless Access Points, which are not the main
+OpenWrt target.
+
+\subsubsection{Pluging a serial port}
+
+By using a serial port and a level shifter, you may reach the console that is being shown by the device
+for debugging or flashing purposes. By analysing the output of this device, you can
+easily notice if the device uses a Linux kernel or something different.
+
+\subsection{Finding and using the manufacturer SDK}
+
+Once you are sure your device run a Linux based firmware, you will be able to start
+hacking on it. If the manufacturer respected the GPL, it will have released a Sample
+Development Kit with the device.
+
+\subsubsection{GPL violations}
+
+Some manufacturers do release a Linux based binary firmware, with no sources at all.
+The first step before doing anything is to read the license coming with your device,
+then write them about this lack of Open Source code. If the manufacturer answers
+you they do not have to release a SDK containing Open Source software, then we
+recommend you get in touch with the gpl-violations.org community.
+
+You will find below a sample letter that can be sent to the manufacturer:
+
+\begin{verse}
+Miss, Mister,
+
+I am using a <device name>, and I cannot find neither on your website nor on the
+CD-ROM the open source software used to build or modify the firmware.
+
+In conformance to the GPL license, you have to release the following sources:
+
+\begin{itemize}
+\item complete toolchain that made the kernel and applications be compiled (gcc, binutils, libc)
+\item tools to build a custom firmware (mksquashfs, mkcramfs ...)
+\item kernel sources with patches to make it run on this specific hardware, this does not include binary drivers
+\end{itemize}
+
+Thank you very much in advance for your answer.
+
+Best regards, <your name>
+\end{verse}
+
+\subsubsection{Using the SDK}
+
+Once the SDK is available, you are most likely not to be able to build a complete
+or functional firmware using it, but parts of it, like only the kernel, or only
+the root filesystem. Most manufacturers do not really care releasing a tool that
+do work every time you uncompress and use it.
+
+You should anyway be able to use the following components:
+
+\begin{itemize}
+\item kernel sources with more or less functional patches for your hardware
+\item binary drivers linked or to be linked with the shipped kernel version
+\item packages of the toolchain used to compile the whole firmware: gcc, binutils, libc or uClibc
+\item binary tools to create a valid firmware image
+\end{itemize}
+
+Your work can be divided into the following tasks:
+
+\begin{itemize}
+\item create a clean patch of the hardware specific part of the linux kernel
+\item spot potential kernel GPL violations especially on netfilter and USB stack stuff
+\item make the binary drivers work, until there are open source drivers
+\item use standard a GNU toolchain to make working executables
+\item understand and write open source tools to generate a valid firmware image
+\end{itemize}
+
+\subsubsection{Creating a hardware specific kernel patch}
+
+Most of the time, the kernel source that comes along with the SDK is not really
+clean, and is not a standard Linux version, it also has architecture specific
+fixes backported from the \textbf{CVS} or the \textbf{git} repository of the
+kernel development trees. Anyway, some parts can be easily isolated and used as
+a good start to make a vanilla kernel work your hardware.
+
+Some directories are very likely to have local modifications needed to make your
+hardware be recognized and used under Linux. First of all, you need to find out
+the linux kernel version that is used by your hardware, this can be found by
+editing the \textbf{linux/Makefile} file.
+
+\begin{verbatim}
+head -5 linux-2.x.x/Makefile
+VERSION = 2
+PATCHLEVEL = x
+SUBLEVEL = y
+EXTRAVERSION = z
+NAME=A fancy name
+\end{verbatim}
+
+So now, you know that you have to download a standard kernel tarball at
+\textbf{kernel.org} that matches the version being used by your hardware.
+
+Then you can create a \textbf{diff} file between the two trees, especially for the
+following directories:
+
+\begin{verbatim}
+diff -urN linux-2.x.x/arch/<sub architecture> linux-2.x.x-modified/arch/<sub architecture> > 01-architecture.patch
+diff -urN linux-2.x.x/include/ linux-2.x.x-modified/include > 02-includes.patch
+diff -urN linux-2.x.x/drivers/ linux-2.x.x-modified/drivers > 03-drivers.patch
+\end{verbatim}
+
+This will constitute a basic set of three patches that are very likely to contain
+any needed modifications that has been made to the stock Linux kernel to run on
+your specific device. Of course, the content produced by the \textbf{diff -urN}
+may not always be relevant, so that you have to clean up those patches to only
+let the "must have" code into them.
+
+The first patch will contain all the code that is needed by the board to be
+initialized at startup, as well as processor detection and other boot time
+specific fixes.
+
+The second patch will contain all useful definitions for that board: addresses,
+kernel granularity, redefinitions, processor family and features ...
+
+The third patch may contain drivers for: serial console, ethernet NIC, wireless
+NIC, USB NIC ... Most of the time this patch contains nothing else than "glue"
+code that has been added to make the binary driver work with the Linux kernel.
+This code might not be useful if you plan on writing drivers from scratch for
+this hardware.
+
+\subsubsection{Using the device bootloader}
+
+The bootloader is the first program that is started right after your device has
+been powered on. This program, can be more or less sophisticated, some do let you
+do network booting, USB mass storage booting ... The bootloader is device and
+architecture specific, some bootloaders were designed to be universal such as
+RedBoot or U-Boot so that you can meet those loaders on totally different
+platforms and expect them to behave the same way.
+
+If your device runs a proprietary operating system, you are very likely to deal
+with a proprietary boot loader as well. This may not always be a limitation,
+some proprietary bootloaders can even have source code available (i.e : Broadcom CFE).
+
+According to the bootloader features, hacking on the device will be more or less
+easier. It is very probable that the bootloader, even exotic and rare, has a
+documentation somewhere over the Internet. In order to know what will be possible
+with your bootloader and the way you are going to hack the device, look over the
+following features :
+
+\begin{itemize}
+\item does the bootloader allow net booting via bootp/DHCP/NFS or tftp
+\item does the bootloader accept loading ELF binaries ?
+\item does the bootloader have a kernel/firmware size limitation ?
+\item does the bootloader expect a firmware format to be loaded with ?
+\item are the loaded files executed from RAM or flash ?
+\end{itemize}
+
+Net booting is something very convenient, because you will only have to set up network
+booting servers on your development station, and keep the original firmware on the device
+till you are sure you can replace it. This also prevents your device from being flashed,
+and potentially bricked every time you want to test a modification on the kernel/filesystem.
+
+If your device needs to be flashed every time you load a firmware, the bootlader might
+only accept a specific firmware format to be loaded, so that you will have to
+understand the firmware format as well.
+
+\subsubsection{Making binary drivers work}
+
+As we have explained before, manufacturers do release binary drivers in their GPL
+tarball. When those drivers are statically linked into the kernel, they become GPL
+as well, fortunately or unfortunately, most of the drivers are not statically linked.
+This anyway lets you a chance to dynamically link the driver with the current kernel
+version, and try to make them work together.
+
+This is one of the most tricky and grey part of the fully open source projects.
+Some drivers require few modifications to be working with your custom kernel,
+because they worked with an earlier kernel, and few modifications have been made
+to the kernel in-between those versions. This is for instance the case with the
+binary driver of the Broadcom BCM43xx Wireless Chipsets, where only few differences
+were made to the network interface structures.
+
+Some general principles can be applied no matter which kernel version is used in
+order to make binary drivers work with your custom kernel:
+
+\begin{itemize}
+\item turn on kernel debugging features such as:
+\begin{itemize}
+\item CONFIG\_DEBUG\_KERNEL
+\item CONFIG\_DETECT\_SOFTLOCKUP
+\item CONFIG\_DEBUG\_KOBJECT
+\item CONFIG\_KALLSYMS
+\item CONFIG\_KALLSYMS\_ALL
+\end{itemize}
+\item link binary drivers when possible to the current kernel version
+\item try to load those binary drivers
+\item catch the lockups and understand them
+\end{itemize}
+
+Most of the time, loading binary drivers will fail, and generate a kernel oops.
+You can know the last symbol the binary drivers attempted to use, and see in the
+kernel headers file, if you do not have to move some structures field before or
+after that symbol in order to keep compatibily with both the binary driver and
+the stock kernel drivers.
+
+\subsubsection{Understanding the firmware format}
+
+You might want to understand the firmware format, even if you are not yet capable
+of running a custom firmware on your device, because this is sometimes a blocking
+part of the flashing process.
+
+A firmware format is most of the time composed of the following fields:
+
+\begin{itemize}
+\item header, containing a firmware version and additional fields: Vendor, Hardware version ...
+\item CRC32 checksum on either the whole file or just part of it
+\item Binary and/or compressed kernel image
+\item Binary and/or compressed root filesystem image
+\item potential garbage
+\end{itemize}
+
+Once you have figured out how the firmware format is partitioned, you will have
+to write your own tool that produces valid firmware binaries. One thing to be very
+careful here is the endianness of either the machine that produces the binary
+firmware and the device that will be flashed using this binary firmware.
+
+\subsubsection{Writing a flash map driver}
+
+The flash map driver has an important role in making your custom firmware work
+because it is responsible of mapping the correct flash regions and associated
+rights to specific parts of the system such as: bootloader, kernel, user filesystem.
+
+Writing your own flash map driver is not really a hard task once you know how your
+firmware image and flash is structured. You will find below a commented example
+that covers the case of the device where the bootloader can pass to the kernel its partition plan.
+
+First of all, you need to make your flash map driver be visible in the kernel
+configuration options, this can be done by editing the file \
+\textbf{linux/drivers/mtd/maps/Kconfig}:
+
+\begin{verbatim}
+config MTD_DEVICE_FLASH
+ tristate "Device Flash device"
+ depends on ARCHITECTURE && DEVICE
+ help
+ Flash memory access on DEVICE boards. Currently only works with
+ Bootloader Foo and Bootloader Bar.
+\end{verbatim}
+
+Then add your source file to the \textbf{linux/drivers/mtd/maps/Makefile}, so
+that it will be compiled along with the kernel.
+
+\begin{verbatim}
+obj-\$(CONFIG_MTD_DEVICE_FLASH) += device-flash.o
+\end{verbatim}
+
+You can then write the kernel driver itself, by creating a
+\textbf{linux/drivers/mtd/maps/device-flash.c} C source file.
+
+\begin{verbatim}
+// Includes that are required for the flash map driver to know of the prototypes:
+#include <asm/io.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mtd/map.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/partitions.h>
+#include <linux/vmalloc.h>
+
+// Put some flash map definitions here:
+#define WINDOW_ADDR 0x1FC00000 /* Real address of the flash */
+#define WINDOW_SIZE 0x400000 /* Size of flash */
+#define BUSWIDTH 2 /* Buswidth */
+
+static void __exit device_mtd_cleanup(void);
+
+static struct mtd_info *device_mtd_info;
+
+static struct map_info devicd_map = {
+ .name = "device",
+ .size = WINDOW_SIZE,
+ .bankwidth = BUSWIDTH,
+ .phys = WINDOW_ADDR,
+};
+
+static int __init device_mtd_init(void)
+{
+ // Display that we found a flash map device
+ printk("device: 0x\%08x at 0x\%08x\n", WINDOW_SIZE, WINDOW_ADDR);
+ // Remap the device address to a kernel address
+ device_map.virt = ioremap(WINDOW_ADDR, WINDOW_SIZE);
+
+ // If impossible to remap, exit with the EIO error
+ if (!device_map.virt) {
+ printk("device: Failed to ioremap\n");
+ return -EIO;
+ }
+
+ // Initialize the device map
+ simple_map_init(&device_map);
+
+ /* MTD informations are closely linked to the flash map device
+ you might also use "jedec_probe" "amd_probe" or "intel_probe" */
+ device_mtd_info = do_map_probe("cfi_probe", &device_map);
+
+ if (device_mtd_info) {
+ device_mtd_info->owner = THIS_MODULE;
+
+ int parsed_nr_parts = 0;
+
+ // We try here to use the partition schema provided by the bootloader specific code
+ if (parsed_nr_parts == 0) {
+ int ret = parse_bootloader_partitions(device_mtd_info, &parsed_parts, 0);
+ if (ret > 0) {
+ part_type = "BootLoader";
+ parsed_nr_parts = ret;
+ }
+ }
+
+ add_mtd_partitions(devicd_mtd_info, parsed_parts, parsed_nr_parts);
+
+ return 0;
+ }
+ iounmap(device_map.virt);
+
+ return -ENXIO;
+}
+
+// This function will make the driver clean up the MTD device mapping
+static void __exit device_mtd_cleanup(void)
+{
+ // If we found a MTD device before
+ if (device_mtd_info) {
+ // Delete every partitions
+ del_mtd_partitions(device_mtd_info);
+ // Delete the associated map
+ map_destroy(device_mtd_info);
+ }
+
+ // If the virtual address is already in use
+ if (device_map.virt) {
+ // Unmap the physical address to a kernel space address
+ iounmap(device_map.virt);
+ // Reset the structure field
+ device_map.virt = 0;
+ }
+}
+
+
+// Macros that indicate which function is called on loading/unloading the module
+module_init(device_mtd_init);
+module_exit(device_mtd_cleanup);
+
+
+// Macros defining license and author, parameters can be defined here too.
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Me, myself and I <memyselfandi@domain.tld");
+\end{verbatim}
+
+\subsection{Adding your target in OpenWrt}
+
+Once you spotted the key changes that were made to the Linux kernel
+to support your target, you will want to create a target in OpenWrt
+for your hardware. This can be useful to benefit from the toolchain
+that OpenWrt builds as well as the resulting user-space and kernel
+configuration options.
+
+Provided that your target is already known to OpenWrt, it will be
+as simple as creating a \texttt{target/linux/board} directory
+where you will be creating the following directories and files.
+
+Here for example, is a \texttt{target/linux/board/Makefile}:
+
+\begin{Verbatim}[frame=single,numbers=left]
+#
+# Copyright (C) 2009 OpenWrt.org
+#
+# This is free software, licensed under the GNU General Public License v2.
+# See /LICENSE for more information.
+#
+include $(TOPDIR)/rules.mk
+
+ARCH:=mips
+BOARD:=board
+BOARDNAME:=Eval board
+FEATURES:=squashfs jffs2 pci usb
+
+LINUX_VERSION:=2.6.27.10
+
+include $(INCLUDE_DIR)/target.mk
+
+DEFAULT_PACKAGES += hostapd-mini
+
+define Target/Description
+ Build firmware images for Evaluation board
+endef
+
+$(eval $(call BuildTarget))
+\end{Verbatim}
+
+\begin{itemize}
+ \item \texttt{ARCH} \\
+ The name of the architecture known by Linux and uClibc
+ \item \texttt{BOARD} \\
+ The name of your board that will be used as a package and build directory identifier
+ \item \texttt{BOARDNAME} \\
+ Expanded name that will appear in menuconfig
+ \item \texttt{FEATURES} \\
+ Set of features to build filesystem images, USB, PCI, VIDEO kernel support
+ \item \texttt{LINUX\_VERSION} \\
+ Linux kernel version to use for this target
+ \item \texttt{DEFAULT\_PACKAGES} \\
+ Set of packages to be built by default
+\end{itemize}
+
+A partial kernel configuration which is either named \texttt{config-default} or which matches the kernel version \texttt{config-2.6.x} should be present in \texttt{target/linux/board/}.
+This kernel configuration will only contain the relevant symbols to support your target and can be changed using \texttt{make kernel\_menuconfig}.
+
+To patch the kernel sources with the patches required to support your hardware, you will have to drop them in \texttt{patches} or in \texttt{patches-2.6.x} if there are specific
+changes between kernel versions. Additionnaly, if you want to avoid creating a patch that will create files, you can put those files into \texttt{files} or \texttt{files-2.6.x}
+with the same directory structure that the kernel uses (e.g: drivers/mtd/maps, arch/mips ..).
+
+The build system will require you to create a \texttt{target/linux/board/image/Makefile}:
+
+\begin{Verbatim}[frame=single,numbers=left]
+#
+# Copyright (C) 2009 OpenWrt.org
+#
+# This is free software, licensed under the GNU General Public License v2.
+# See /LICENSE for more information.
+#
+include $(TOPDIR)/rules.mk
+include $(INCLUDE_DIR)/image.mk
+
+define Image/BuildKernel
+ cp $(KDIR)/vmlinux.elf $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.elf
+ gzip -9 -c $(KDIR)/vmlinux > $(KDIR)/vmlinux.bin.gz
+ $(STAGING_DIR_HOST)/bin/lzma e $(KDIR)/vmlinux $(KDIR)/vmlinux.bin.l7
+ dd if=$(KDIR)/vmlinux.bin.l7 of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma bs=65536 conv=sync
+ dd if=$(KDIR)/vmlinux.bin.gz of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.gz bs=65536 conv=sync
+endef
+
+define Image/Build/squashfs
+ $(call prepare_generic_squashfs,$(KDIR)/root.squashfs)
+endef
+
+define Image/Build
+ $(call Image/Build/$(1))
+ dd if=$(KDIR)/root.$(1) of=$(BIN_DIR)/openwrt-$(BOARD)-root.$(1) bs=128k conv=sync
+
+ -$(STAGING_DIR_HOST)/bin/mkfwimage \
+ -B XS2 -v XS2.ar2316.OpenWrt \
+ -k $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma \
+ -r $(BIN_DIR)/openwrt-$(BOARD)-root.$(1) \
+ -o $(BIN_DIR)/openwrt-$(BOARD)-ubnt2-$(1).bin
+endef
+
+$(eval $(call BuildImage))
+
+\end{Verbatim}
+
+\begin{itemize}
+ \item \texttt{Image/BuildKernel} \\
+ This template defines changes to be made to the ELF kernel file
+ \item \texttt{Image/Build} \\
+ This template defines the final changes to apply to the rootfs and kernel, either combined or separated
+ firmware creation tools can be called here as well.
+\end{itemize}