Adding a new device

A good all-round advice would be to start by looking at recent commits about adding a new device, to see what files where changed and how. Many files try to be as self-explanatory as possible, most of the times just opening them will be enough to understand their function.

A good method is learn by example, so you can do:

grep -lri mt300a target/

The result is minimal list of files required to add a new board:


Browse the source filtered by "add support for" and checkout the diff for newly added device

This is a general map of where most important files are located:

This folder contains files and folders that will be integrated in the firmware’s /etc folder.

These are its subfolders and files:

  • …board.d/ scripts for defining device-specific default hardware, like leds and network interfaces.
  • …hotplug.d/ scripts for defining device-specific actions to be done automatically on hotplugging of devices
  • …init.d/ scripts for defining device-specific actions to be done automatically on boot
  • …uci-defaults/ files for defining device-specific uci configuration defaults
  • … defines what is the led to use for error codes for each board

Note that some of these functions are now done in the DTS for the board.

This folder contains files and folders that will be integrated in the firmware’s /lib folder.

These are its subfolders and files:

  • …<arch_name>.sh human-readable full board name associated to script-safe board name
  • …preinit/ common <arch_name> preinit startup scripts
  • …upgrade/ common <arch_name> upgrade scripts

This folder contains files and folders that will be integrated in the firmware’s /sbin folder, usually common <arch_name> sbin scripts and tools.

Device tree source files, or dts for short.

Certain architectures have the DTS directory deeper down. ARM devices, for example, typically have it located at files-X.yy/arch/arm/boot/dts/

If the DTS or DTSI file is already present in upstream Linux, they will usually not be present in the OpenWrt source. Configuring for the target and running make target/linux/{clean,prepare} will download and patch Linux, allowing the resulting file to be found in the build_dir

Configuration needed to build device-specific flashable images.

Board-specific configuration.

Arch-specific kernel module config file for menuconfig

After edit the files above, you need to touch the makefiles

touch target/linux/*/Makefile

The patches-* subdirectories contain the kernel patches applied for every target.
All patches should be named 'NNN-lowercase_shortname.patch' and sorted into the following categories:

0xx - upstream backports
1xx - code awaiting upstream merge
2xx - kernel build / config / header patches
3xx - architecture specific patches
4xx - mtd related patches (subsystem and drivers)
5xx - filesystem related patches
6xx - generic network patches
7xx - network / phy driver patches
8xx - other drivers
9xx - uncategorized other patches

All patches must be written in a way that they are potentially upstreamable, meaning:

  1. they must contain a proper subject
  2. they must contain a proper commit message explaining what they change
  3. they must contain a valid Signed-off-by line

Test firmware images without writing them to flash by using ramdisk images.

In make menuconfig select Target Images and then you can select the ramdisk option.

This will create an image with kernel + initramfs, that will have initramfs in the name. The resulting image can be loaded in the device through the bootloader's tftp function and should boot to a prompt without relying on flash/filesystem support.

In order to collect relevant data for a port of OpenWrt to the device of interest one wants shell access. Most devices though do not offer a way to get a shell with telnet or ssh.

Abuse Unsanitized User Input

Some router offers ping test or NTP server configuration and may not properly sanitize user input. Try to enter shell script and see if you are lucky. You may need some javascript knowledges to disable client-side input validation.

Starting telnetd
$( /bin/busybox telnetd -l/bin/sh -p23 & )
Obtain the password hash using HTTP or use ''sed'' to delete/change the default password if telnet login is required
$( cp /etc/shadow /www )
$( cp /etc/passwd /www )

Then try to download them to your computer and crack the hash

Downgrade to older firmware

Some router may try to download a firmware file (e.g. TP-Link Archer C2 AC750) from specific private IP at the beginning of booting, which allow user to downgrade to older firmware

Downgrade by Serial access

Serial access may allow you to enter console mode of u-boot for flashing/loading other firmware. Usually soldering is required. See Generic flashing over the Serial port

HTTP Server Vulnerability

Some routers may be running outdated/insecure HTTP server and may be vulnerable to buffer overflow or other attack


With netgear-telnetenable many Netgear devices can be opened up for telnet access. Also see GitHub: insanid/NetgearTelnetEnable. When such means cannot be used, one could try to flash an image build from the sources published by the vendor with telnetd enabled.

With nmrpflash many Netgear devices can be flashed. Devices that are compatible with this tool become effectively unbrickable.

On WikiDevi lots of information can be found, e.g. the FCC ID is very useful when searching for documentation, datasheets and internal photo's (to be able to distinguish used chips without having to open the casing).

Typically one can use the following commands:

dmesg                          # log buffer might be to small, see note 1.
cat /proc/cmdline
cat /proc/cpuinfo
cat /proc/meminfo
cat /proc/devices
ls /sys/devices/platform
cat /proc/mtd
cat /sys/class/mtd/mtd*/offset # Linux 4.1 and newer, see note 2.
ifconfig -a
ls /sys/class/net
brctl show
cat /sys/kernel/debug/gpio     # GPIO information

Note 1: Often the log buffer is to small and the earliest messages may be missing from the information retrieved with dmesg. If one build a stock image from the sources the vendor has published, a larger buffer size can be set within the kernel config.

Note 2:

Another useful tool for getting information for setting LEDs might be gpiodump, a MT7620 GPIOMODE register dumper (RAMIPS).

Because of the limited space, common file transfer utilities such as rsync/curl/ssh/scp/ftp/http/tftp may not be available, a stripped down version/applet may be available from busybox.

Assume the router ip is, and the file to be transfer located at /tmp/important-data.txt.

HTTP by ''httpd'' and ''busybox mount''

If the web interface are served from /www.

mount -o bind /tmp /www

FTP by ''busybox ftpput''


Setup an FTP server. Add an anonymous account with write permission

python -m pyftpdlib -w -p 21
busybox ftpput important-data.txt /tmp/important-data.txt 

netcat by ''busybox nc''

busybox nc -l -p 12345 > important-data.txt 
cat /tmp/important-data.txt | busybox nc 

TFTP by ''busybox tftp''


Setup a tftp server

busybox tftp -p -l /tmp/important-data.txt -r important-data.txt

Copy from terminal

If all of the above tools/applets are unavailable, you may copy from telnet terminal but it may not work for binary file.

base64 would be a common choice to work around this limitation, but many routers lack such a command. You can first escape binary data to screen-safe hexadecimal by piping to busybox hexdump on the router:

hexdump -v -e '/1 "%02x"'

You can then reverse it on the computer with the following command:

xxd -r -p
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  • Last modified: 2024/03/11 16:04
  • by bkil