It is some time, when I started experimenting with OpenWRT - an operating system for routers. It starts as a school project, when I had to write an article about operating systems for embedded devices.
OpenWRT is Linux based operating system. It is completely open source with a large community. Also it has huge list of supported devices. You can find more information on the official page.
Hardware
OpenWRT supports lot of procesor architectures, so it can run basically on anything. Except few devices which has not enough resources, such as flash storage or RAM.
I chose very popular TP-LINK TL-WR703N. It is very small and cost about 22 USD. It has 4 MB of flash, 32 MB RAM and MIPS architecture CPU at 400 MHz. It also supports 802.11n wireless standard and has one USB host port.
Serial console
If you are working with some embedded device, it is always good idea to have serial console. It is very handy for its management and configuration and really necessary for device rescue, if something goes wrong.
Every TP-Link router what I met has nice unpopulated PCB header with UART, providing serial console. But because this router is really small, it has only little pads with RX and TX (labeled TP_IN and TP_OUT).
You can find lot of tutorials on the Internet about accessing serial console. But basically each of them requires making holes into PCB for soldering pins or some other invasive method. I solved this by gluing 3-pin header with a hot glue on the back side of the Ethernet connector. Then I just connected RX and TX wires and third wire to the ground.
Identify a target
If your device is listed in supported devices, look at the device target. In case of TL-WR703N, target is ar71xx. If you are not sure about right target of your device, please look at this table. Target is based on the router chipset, not only on the processor's architecture. But let's suppose that you have device supported by OpenWRT.
Filesystem
Next you have to decide, which filesystem you want. There are two basic options.
- JFFS2 – writable compressed filesysem. It supports wear leveling and journaling. Compressed with LZMA compression.
- SquashFS – read-only filesystem compressed with LZMA. It is used in conjunction with overlayfs. In the result, there are two filesystems merged into one. This mechanism is called UnionFS. Lower one is read-only SquashFS and upper one is writable JFFS2.
I prefer two layered filesystem. It's writable and any changes can be easily restored into default state by erasing upper layer.
Please note, that SquashFS shouldn't be used on the NAND flash storage. If your router has NAND flash, you have to use another filesystem.
Installation
After determining correct target and identifying your needs, installation is really simple. Fist download image file. In my case it is http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-factory.bin
On most devices (and TL-WR703N isn't exception) you simply can access web interface and upload firmware image into appropriate upgrade form.
First of all, you have to connect to the router. I prefer wired connections. Most devices are configured with some reasonable address. You can read manual or simply guess it.
buben@debian:~$ sudo ifconfig eth0 192.168.1.10 buben@debian:~$ ping 192.168.1.1 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_req=1 ttl=255 time=1.48 ms 64 bytes from 192.168.1.1: icmp_req=2 ttl=255 time=3.09 ms 64 bytes from 192.168.1.1: icmp_req=3 ttl=255 time=1.40 ms 64 bytes from 192.168.1.1: icmp_req=4 ttl=255 time=2.02 ms
In my case, I had to use crossover cable. If you can't ping a device, make sure that this is not an issue.
I bought my TP-Link on ebay and it came with Chinese localization. I didn't have character sets installed for displaying page correctly. But it is not hard to find firmware upload form, even if I can't read basicaly anything.
Main menu in on the left side. The last item should be system management. After clicking on it, another sub-menu appears.
Menu items are nothing more than links. If you move a mouse on the link, most browsers will show you its location. Third menu item should be, what I'm looking for (mouse cursor isn't captured on the screenshot).
Before you upload an image file to the form, it's not bad idea to rename it to some short name. Also the file should have .bin
suffix. In my case a file name length have to be up to 64 characters long and .bin
suffix is also required.
buben@debian:~$ mv openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-factory.bin openwrt.bin
Next select image file and submit it.
Firmware upgrade is most critical step in this process. Don't unplug a power source for your router. If you will do so, you will brick it. Brick can be restored, but I have no experience with that.
After successful flashing, you can access a serial console a see nice welcome message.
BusyBox v1.19.4 (2013-03-14 11:28:31 UTC) built-in shell (ash)
Enter 'help' for a list of built-in commands.
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
ATTITUDE ADJUSTMENT (12.09, r36088)
-----------------------------------------------------
* 1/4 oz Vodka Pour all ingredients into mixing
* 1/4 oz Gin tin with ice, strain into glass.
* 1/4 oz Amaretto
* 1/4 oz Triple sec
* 1/4 oz Peach schnapps
* 1/4 oz Sour mix
* 1 splash Cranberry juice
-----------------------------------------------------
root@OpenWrt:/#
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