1Linux is now one of the most widespread operating system for embedded devices due 2to its openess as well as the wide variety of platforms it can run on. Many 3manufacturer actually use it in firmware you can find on many devices: DVB-T 4decoders, routers, print servers, DVD players ... Most of the time the stock 5firmware is not really open to the consumer, even if it uses open source software. 6 7You might be interested in running a Linux based firmware for your router for 8various reasons: extending the use of a network protocol (such as IPv6), having 9new features, new piece of software inside, or for security reasons. A fully 10open-source firmware is de-facto needed for such applications, since you want to 11be free to use this or that version of a particular reason, be able to correct a 12particular bug. Few manufacturers do ship their routers with a Sample Development Kit, 13that would allow you to create your own and custom firmware and most of the time, 14when they do, you will most likely not be able to complete the firmware creation process. 15 16This is one of the reasons why OpenWrt and other firmware exists: providing a 17version independent, and tools independent firmware, that can be run on various 18platforms, known to be running Linux originally. 19 20\subsection{Which Operating System does this device run?} 21 22There is a lot of methods to ensure your device is running Linux. Some of them do 23need your router to be unscrewed and open, some can be done by probing the device 24using its external network interfaces. 25 26\subsubsection{Operating System fingerprinting and port scanning} 27 28A large bunch of tools over the Internet exists in order to let you do OS 29fingerprinting, we will show here an example using \textbf{nmap}: 30 31\begin{Verbatim} 32nmap -P0 -O <IP address> 33Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:05 CET 34Interesting ports on 192.168.2.1: 35Not shown: 1693 closed ports 36PORT STATE SERVICE 3722/tcp open ssh 3823/tcp open telnet 3953/tcp open domain 4080/tcp open http 41MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys) 42Device type: broadband router 43Running: Linksys embedded 44OS details: Linksys WRT54GS v4 running OpenWrt w/Linux kernel 2.4.30 45Network Distance: 1 hop 46\end{Verbatim} 47 48nmap is able to report whether your device uses a Linux TCP/IP stack, and if so, 49will show you which Linux kernel version is probably runs. This report is quite 50reliable and it can make the distinction between BSD and Linux TCP/IP stacks and others. 51 52Using the same tool, you can also do port scanning and service version discovery. 53For instance, the following command will report which IP-based services are running 54on the device, and which version of the service is being used: 55 56\begin{verbatim} 57nmap -P0 -sV <IP address> 58Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:06 CET 59Interesting ports on 192.168.2.1: 60Not shown: 1693 closed ports 61PORT STATE SERVICE VERSION 6222/tcp open ssh Dropbear sshd 0.48 (protocol 2.0) 6323/tcp open telnet Busybox telnetd 6453/tcp open domain ISC Bind dnsmasq-2.35 6580/tcp open http OpenWrt BusyBox httpd 66MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys) 67Service Info: Device: WAP 68\end{verbatim} 69 70The web server version, if identified, can be determining in knowing the Operating 71System. For instance, the \textbf{BOA} web server is typical from devices running 72an open-source Unix or Unix-like. 73 74\subsubsection{Wireless Communications Fingerprinting} 75 76Although this method is not really known and widespread, using a wireless scanner 77to discover which OS your router or Access Point run can be used. We do not have 78a clear example of how this could be achieved, but you will have to monitor raw 79802.11 frames and compare them to a very similar device running a Linux based firmware. 80 81\subsubsection{Web server security exploits} 82 83The Linksys WRT54G was originally hacked by using a "ping bug" discovered in the 84web interface. This tip has not been fixed for months by Linksys, allowing people 85to enable the "boot\_wait" helper process via the web interface. Many web servers 86used in firmwares are open source web server, thus allowing the code to be audited 87to find an exploit. Once you know the web server version that runs on your device, 88by using \textbf{nmap -sV} or so, you might be interested in using exploits to reach 89shell access on your device. 90 91\subsubsection{Native Telnet/SSH access} 92 93Some firmwares might have restricted or unrestricted Telnet/SSH access, if so, 94try to log in with the web interface login/password and see if you can type in 95some commands. This is actually the case for some Broadcom BCM963xx based firmwares 96such as the one in Neuf/Cegetel ISP routers, Club-Internet ISP CI-Box and many 97others. Some commands, like \textbf{cat} might be left here and be used to 98determine the Linux kernel version. 99 100\subsubsection{Analysing a binary firmware image} 101 102You are very likely to find a firmware binary image on the manufacturer website, 103even if your device runs a proprietary operating system. If so, you can download 104it and use an hexadecimal editor to find printable words such as \textbf{vmlinux}, 105\textbf{linux}, \textbf{ramdisk}, \textbf{mtd} and others. 106 107Some Unix tools like \textbf{hexdump} or \textbf{strings} can be used to analyse 108the firmware. Below there is an example with a binary firmware found other the Internet: 109 110\begin{verbatim} 111hexdump -C <binary image.extension> | less (more) 11200000000 46 49 52 45 32 2e 35 2e 30 00 00 00 00 00 00 00 |FIRE2.5.0.......| 11300000010 00 00 00 00 31 2e 30 2e 30 00 00 00 00 00 00 00 |....1.0.0.......| 11400000020 00 00 00 00 00 00 00 38 00 43 36 29 00 0a e6 dc |.......8.C6)..??| 11500000030 54 49 44 45 92 89 54 66 1f 8b 08 08 f8 10 68 42 |TIDE..Tf....?.hB| 11600000040 02 03 72 61 6d 64 69 73 6b 00 ec 7d 09 bc d5 d3 |..ramdisk.?}.???| 11700000050 da ff f3 9b f7 39 7b ef 73 f6 19 3b 53 67 ea 44 |???.?9{?s?.;Sg?D| 118\end{verbatim} 119 120Scroll over the firmware to find printable words that can be significant. 121 122\subsubsection{Amount of flash memory} 123 124Linux can hardly fit in a 2MB flash device, once you have opened the device and 125located the flash chip, try to find its characteristics on the Internet. If 126your flash chip is a 2MB or less device, your device is most likely to run a 127proprietary OS such as WindRiver VxWorks, or a custom manufacturer OS like Zyxel ZynOS. 128 129OpenWrt does not currently run on devices which have 2MB or less of flash memory. 130This limitation will probably not be worked around since those devices are most 131of the time micro-routers, or Wireless Access Points, which are not the main 132OpenWrt target. 133 134\subsubsection{Pluging a serial port} 135 136By using a serial port and a level shifter, you may reach the console that is being shown by the device 137for debugging or flashing purposes. By analysing the output of this device, you can 138easily notice if the device uses a Linux kernel or something different. 139 140\subsection{Finding and using the manufacturer SDK} 141 142Once you are sure your device run a Linux based firmware, you will be able to start 143hacking on it. If the manufacturer respected the GPL, it will have released a Sample 144Development Kit with the device. 145 146\subsubsection{GPL violations} 147 148Some manufacturers do release a Linux based binary firmware, with no sources at all. 149The first step before doing anything is to read the license coming with your device, 150then write them about this lack of Open Source code. If the manufacturer answers 151you they do not have to release a SDK containing Open Source software, then we 152recommend you get in touch with the gpl-violations.org community. 153 154You will find below a sample letter that can be sent to the manufacturer: 155 156\begin{verse} 157Miss, Mister, 158 159I am using a <device name>, and I cannot find neither on your website nor on the 160CD-ROM the open source software used to build or modify the firmware. 161 162In conformance to the GPL license, you have to release the following sources: 163 164\begin{itemize} 165\item complete toolchain that made the kernel and applications be compiled (gcc, binutils, libc) 166\item tools to build a custom firmware (mksquashfs, mkcramfs ...) 167\item kernel sources with patches to make it run on this specific hardware, this does not include binary drivers 168\end{itemize} 169 170Thank you very much in advance for your answer. 171 172Best regards, <your name> 173\end{verse} 174 175\subsubsection{Using the SDK} 176 177Once the SDK is available, you are most likely not to be able to build a complete 178or functional firmware using it, but parts of it, like only the kernel, or only 179the root filesystem. Most manufacturers do not really care releasing a tool that 180do work every time you uncompress and use it. 181 182You should anyway be able to use the following components: 183 184\begin{itemize} 185\item kernel sources with more or less functional patches for your hardware 186\item binary drivers linked or to be linked with the shipped kernel version 187\item packages of the toolchain used to compile the whole firmware: gcc, binutils, libc or uClibc 188\item binary tools to create a valid firmware image 189\end{itemize} 190 191Your work can be divided into the following tasks: 192 193\begin{itemize} 194\item create a clean patch of the hardware specific part of the linux kernel 195\item spot potential kernel GPL violations especially on netfilter and USB stack stuff 196\item make the binary drivers work, until there are open source drivers 197\item use standard a GNU toolchain to make working executables 198\item understand and write open source tools to generate a valid firmware image 199\end{itemize} 200 201\subsubsection{Creating a hardware specific kernel patch} 202 203Most of the time, the kernel source that comes along with the SDK is not really 204clean, and is not a standard Linux version, it also has architecture specific 205fixes backported from the \textbf{CVS} or the \textbf{git} repository of the 206kernel development trees. Anyway, some parts can be easily isolated and used as 207a good start to make a vanilla kernel work your hardware. 208 209Some directories are very likely to have local modifications needed to make your 210hardware be recognized and used under Linux. First of all, you need to find out 211the linux kernel version that is used by your hardware, this can be found by 212editing the \textbf{linux/Makefile} file. 213 214\begin{verbatim} 215head -5 linux-2.x.x/Makefile 216VERSION = 2 217PATCHLEVEL = x 218SUBLEVEL = y 219EXTRAVERSION = z 220NAME=A fancy name 221\end{verbatim} 222 223So now, you know that you have to download a standard kernel tarball at 224\textbf{kernel.org} that matches the version being used by your hardware. 225 226Then you can create a \textbf{diff} file between the two trees, especially for the 227following directories: 228 229\begin{verbatim} 230diff -urN linux-2.x.x/arch/<sub architecture> linux-2.x.x-modified/arch/<sub architecture> > 01-architecture.patch 231diff -urN linux-2.x.x/include/ linux-2.x.x-modified/include > 02-includes.patch 232diff -urN linux-2.x.x/drivers/ linux-2.x.x-modified/drivers > 03-drivers.patch 233\end{verbatim} 234 235This will constitute a basic set of three patches that are very likely to contain 236any needed modifications that has been made to the stock Linux kernel to run on 237your specific device. Of course, the content produced by the \textbf{diff -urN} 238may not always be relevant, so that you have to clean up those patches to only 239let the "must have" code into them. 240 241The first patch will contain all the code that is needed by the board to be 242initialized at startup, as well as processor detection and other boot time 243specific fixes. 244 245The second patch will contain all useful definitions for that board: addresses, 246kernel granularity, redefinitions, processor family and features ... 247 248The third patch may contain drivers for: serial console, ethernet NIC, wireless 249NIC, USB NIC ... Most of the time this patch contains nothing else than "glue" 250code that has been added to make the binary driver work with the Linux kernel. 251This code might not be useful if you plan on writing drivers from scratch for 252this hardware. 253 254\subsubsection{Using the device bootloader} 255 256The bootloader is the first program that is started right after your device has 257been powered on. This program, can be more or less sophisticated, some do let you 258do network booting, USB mass storage booting ... The bootloader is device and 259architecture specific, some bootloaders were designed to be universal such as 260RedBoot or U-Boot so that you can meet those loaders on totally different 261platforms and expect them to behave the same way. 262 263If your device runs a proprietary operating system, you are very likely to deal 264with a proprietary boot loader as well. This may not always be a limitation, 265some proprietary bootloaders can even have source code available (i.e : Broadcom CFE). 266 267According to the bootloader features, hacking on the device will be more or less 268easier. It is very probable that the bootloader, even exotic and rare, has a 269documentation somewhere over the Internet. In order to know what will be possible 270with your bootloader and the way you are going to hack the device, look over the 271following features : 272 273\begin{itemize} 274\item does the bootloader allow net booting via bootp/DHCP/NFS or tftp 275\item does the bootloader accept loading ELF binaries ? 276\item does the bootloader have a kernel/firmware size limitation ? 277\item does the bootloader expect a firmware format to be loaded with ? 278\item are the loaded files executed from RAM or flash ? 279\end{itemize} 280 281Net booting is something very convenient, because you will only have to set up network 282booting servers on your development station, and keep the original firmware on the device 283till you are sure you can replace it. This also prevents your device from being flashed, 284and potentially bricked every time you want to test a modification on the kernel/filesystem. 285 286If your device needs to be flashed every time you load a firmware, the bootlader might 287only accept a specific firmware format to be loaded, so that you will have to 288understand the firmware format as well. 289 290\subsubsection{Making binary drivers work} 291 292As we have explained before, manufacturers do release binary drivers in their GPL 293tarball. When those drivers are statically linked into the kernel, they become GPL 294as well, fortunately or unfortunately, most of the drivers are not statically linked. 295This anyway lets you a chance to dynamically link the driver with the current kernel 296version, and try to make them work together. 297 298This is one of the most tricky and grey part of the fully open source projects. 299Some drivers require few modifications to be working with your custom kernel, 300because they worked with an earlier kernel, and few modifications have been made 301to the kernel in-between those versions. This is for instance the case with the 302binary driver of the Broadcom BCM43xx Wireless Chipsets, where only few differences 303were made to the network interface structures. 304 305Some general principles can be applied no matter which kernel version is used in 306order to make binary drivers work with your custom kernel: 307 308\begin{itemize} 309\item turn on kernel debugging features such as: 310\begin{itemize} 311\item CONFIG\_DEBUG\_KERNEL 312\item CONFIG\_DETECT\_SOFTLOCKUP 313\item CONFIG\_DEBUG\_KOBJECT 314\item CONFIG\_KALLSYMS 315\item CONFIG\_KALLSYMS\_ALL 316\end{itemize} 317\item link binary drivers when possible to the current kernel version 318\item try to load those binary drivers 319\item catch the lockups and understand them 320\end{itemize} 321 322Most of the time, loading binary drivers will fail, and generate a kernel oops. 323You can know the last symbol the binary drivers attempted to use, and see in the 324kernel headers file, if you do not have to move some structures field before or 325after that symbol in order to keep compatibily with both the binary driver and 326the stock kernel drivers. 327 328\subsubsection{Understanding the firmware format} 329 330You might want to understand the firmware format, even if you are not yet capable 331of running a custom firmware on your device, because this is sometimes a blocking 332part of the flashing process. 333 334A firmware format is most of the time composed of the following fields: 335 336\begin{itemize} 337\item header, containing a firmware version and additional fields: Vendor, Hardware version ... 338\item CRC32 checksum on either the whole file or just part of it 339\item Binary and/or compressed kernel image 340\item Binary and/or compressed root filesystem image 341\item potential garbage 342\end{itemize} 343 344Once you have figured out how the firmware format is partitioned, you will have 345to write your own tool that produces valid firmware binaries. One thing to be very 346careful here is the endianness of either the machine that produces the binary 347firmware and the device that will be flashed using this binary firmware. 348 349\subsubsection{Writing a flash map driver} 350 351The flash map driver has an important role in making your custom firmware work 352because it is responsible of mapping the correct flash regions and associated 353rights to specific parts of the system such as: bootloader, kernel, user filesystem. 354 355Writing your own flash map driver is not really a hard task once you know how your 356firmware image and flash is structured. You will find below a commented example 357that covers the case of the device where the bootloader can pass to the kernel its partition plan. 358 359First of all, you need to make your flash map driver be visible in the kernel 360configuration options, this can be done by editing the file \ 361\textbf{linux/drivers/mtd/maps/Kconfig}: 362 363\begin{verbatim} 364config MTD_DEVICE_FLASH 365 tristate "Device Flash device" 366 depends on ARCHITECTURE && DEVICE 367 help 368 Flash memory access on DEVICE boards. Currently only works with 369 Bootloader Foo and Bootloader Bar. 370\end{verbatim} 371 372Then add your source file to the \textbf{linux/drivers/mtd/maps/Makefile}, so 373that it will be compiled along with the kernel. 374 375\begin{verbatim} 376obj-\$(CONFIG_MTD_DEVICE_FLASH) += device-flash.o 377\end{verbatim} 378 379You can then write the kernel driver itself, by creating a 380\textbf{linux/drivers/mtd/maps/device-flash.c} C source file. 381 382\begin{verbatim} 383// Includes that are required for the flash map driver to know of the prototypes: 384#include <asm/io.h> 385#include <linux/init.h> 386#include <linux/kernel.h> 387#include <linux/mtd/map.h> 388#include <linux/mtd/mtd.h> 389#include <linux/mtd/partitions.h> 390#include <linux/vmalloc.h> 391 392// Put some flash map definitions here: 393#define WINDOW_ADDR 0x1FC00000 /* Real address of the flash */ 394#define WINDOW_SIZE 0x400000 /* Size of flash */ 395#define BUSWIDTH 2 /* Buswidth */ 396 397static void __exit device_mtd_cleanup(void); 398 399static struct mtd_info *device_mtd_info; 400 401static struct map_info devicd_map = { 402 .name = "device", 403 .size = WINDOW_SIZE, 404 .bankwidth = BUSWIDTH, 405 .phys = WINDOW_ADDR, 406}; 407 408static int __init device_mtd_init(void) 409{ 410 // Display that we found a flash map device 411 printk("device: 0x\%08x at 0x\%08x\n", WINDOW_SIZE, WINDOW_ADDR); 412 // Remap the device address to a kernel address 413 device_map.virt = ioremap(WINDOW_ADDR, WINDOW_SIZE); 414 415 // If impossible to remap, exit with the EIO error 416 if (!device_map.virt) { 417 printk("device: Failed to ioremap\n"); 418 return -EIO; 419 } 420 421 // Initialize the device map 422 simple_map_init(&device_map); 423 424 /* MTD informations are closely linked to the flash map device 425 you might also use "jedec_probe" "amd_probe" or "intel_probe" */ 426 device_mtd_info = do_map_probe("cfi_probe", &device_map); 427 428 if (device_mtd_info) { 429 device_mtd_info->owner = THIS_MODULE; 430 431 int parsed_nr_parts = 0; 432 433 // We try here to use the partition schema provided by the bootloader specific code 434 if (parsed_nr_parts == 0) { 435 int ret = parse_bootloader_partitions(device_mtd_info, &parsed_parts, 0); 436 if (ret > 0) { 437 part_type = "BootLoader"; 438 parsed_nr_parts = ret; 439 } 440 } 441 442 add_mtd_partitions(devicd_mtd_info, parsed_parts, parsed_nr_parts); 443 444 return 0; 445 } 446 iounmap(device_map.virt); 447 448 return -ENXIO; 449} 450 451// This function will make the driver clean up the MTD device mapping 452static void __exit device_mtd_cleanup(void) 453{ 454 // If we found a MTD device before 455 if (device_mtd_info) { 456 // Delete every partitions 457 del_mtd_partitions(device_mtd_info); 458 // Delete the associated map 459 map_destroy(device_mtd_info); 460 } 461 462 // If the virtual address is already in use 463 if (device_map.virt) { 464 // Unmap the physical address to a kernel space address 465 iounmap(device_map.virt); 466 // Reset the structure field 467 device_map.virt = 0; 468 } 469} 470 471 472// Macros that indicate which function is called on loading/unloading the module 473module_init(device_mtd_init); 474module_exit(device_mtd_cleanup); 475 476 477// Macros defining license and author, parameters can be defined here too. 478MODULE_LICENSE("GPL"); 479MODULE_AUTHOR("Me, myself and I <memyselfandi@domain.tld"); 480\end{verbatim} 481 482\subsection{Adding your target in OpenWrt} 483 484Once you spotted the key changes that were made to the Linux kernel 485to support your target, you will want to create a target in OpenWrt 486for your hardware. This can be useful to benefit from the toolchain 487that OpenWrt builds as well as the resulting user-space and kernel 488configuration options. 489 490Provided that your target is already known to OpenWrt, it will be 491as simple as creating a \texttt{target/linux/board} directory 492where you will be creating the following directories and files. 493 494Here for example, is a \texttt{target/linux/board/Makefile}: 495 496\begin{Verbatim}[frame=single,numbers=left] 497# 498# Copyright (C) 2009 OpenWrt.org 499# 500# This is free software, licensed under the GNU General Public License v2. 501# See /LICENSE for more information. 502# 503include $(TOPDIR)/rules.mk 504 505ARCH:=mips 506BOARD:=board 507BOARDNAME:=Eval board 508FEATURES:=squashfs jffs2 pci usb 509 510LINUX_VERSION:=2.6.27.10 511 512include $(INCLUDE_DIR)/target.mk 513 514DEFAULT_PACKAGES += hostapd-mini 515 516define Target/Description 517 Build firmware images for Evaluation board 518endef 519 520$(eval $(call BuildTarget)) 521\end{Verbatim} 522 523\begin{itemize} 524 \item \texttt{ARCH} \\ 525 The name of the architecture known by Linux and uClibc 526 \item \texttt{BOARD} \\ 527 The name of your board that will be used as a package and build directory identifier 528 \item \texttt{BOARDNAME} \\ 529 Expanded name that will appear in menuconfig 530 \item \texttt{FEATURES} \\ 531 Set of features to build filesystem images, USB, PCI, VIDEO kernel support 532 \item \texttt{LINUX\_VERSION} \\ 533 Linux kernel version to use for this target 534 \item \texttt{DEFAULT\_PACKAGES} \\ 535 Set of packages to be built by default 536\end{itemize} 537 538A 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/}. 539This kernel configuration will only contain the relevant symbols to support your target and can be changed using \texttt{make kernel\_menuconfig}. 540 541To 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 542changes 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} 543with the same directory structure that the kernel uses (e.g: drivers/mtd/maps, arch/mips ..). 544 545The build system will require you to create a \texttt{target/linux/board/image/Makefile}: 546 547\begin{Verbatim}[frame=single,numbers=left] 548# 549# Copyright (C) 2009 OpenWrt.org 550# 551# This is free software, licensed under the GNU General Public License v2. 552# See /LICENSE for more information. 553# 554include $(TOPDIR)/rules.mk 555include $(INCLUDE_DIR)/image.mk 556 557define Image/BuildKernel 558 cp $(KDIR)/vmlinux.elf $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.elf 559 gzip -9n -c $(KDIR)/vmlinux > $(KDIR)/vmlinux.bin.gz 560 $(STAGING_DIR_HOST)/bin/lzma e $(KDIR)/vmlinux $(KDIR)/vmlinux.bin.l7 561 dd if=$(KDIR)/vmlinux.bin.l7 of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma bs=65536 conv=sync 562 dd if=$(KDIR)/vmlinux.bin.gz of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.gz bs=65536 conv=sync 563endef 564 565define Image/Build/squashfs 566 $(call prepare_generic_squashfs,$(KDIR)/root.squashfs) 567endef 568 569define Image/Build 570 $(call Image/Build/$(1)) 571 dd if=$(KDIR)/root.$(1) of=$(BIN_DIR)/openwrt-$(BOARD)-root.$(1) bs=128k conv=sync 572 573 -$(STAGING_DIR_HOST)/bin/mkfwimage \ 574 -B XS2 -v XS2.ar2316.OpenWrt \ 575 -k $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma \ 576 -r $(BIN_DIR)/openwrt-$(BOARD)-root.$(1) \ 577 -o $(BIN_DIR)/openwrt-$(BOARD)-ubnt2-$(1).bin 578endef 579 580$(eval $(call BuildImage)) 581 582\end{Verbatim} 583 584\begin{itemize} 585 \item \texttt{Image/BuildKernel} \\ 586 This template defines changes to be made to the ELF kernel file 587 \item \texttt{Image/Build} \\ 588 This template defines the final changes to apply to the rootfs and kernel, either combined or separated 589 firmware creation tools can be called here as well. 590\end{itemize} 591