This tool can, so far, given a serial port connected to a device with brnboot/amazonboot, dump its flash into a file.
It has been tested with
- Arcadyan ARV4518PW http://wiki.openwrt.org/toh/arv4518pw (as distributed by the spanish isp ya.com)
- Arcadyan ARV7506PW11 (tested by Jan-Philipp Litza and einsiedlerkrebs)
- SMC SMC7904WBRA (as distributed by the spanish isp ya.com)
About brnboot: http://wiki.openwrt.org/doc/techref/bootloader/brnboot
The serial port in these two devices is hidden inside their case. If opened, there's a 5x2 header. Going horizontally from the marked pin, which is 1: 2.RX, 3.TX, 5.GND, 6.+3.3v. UART is at 115200bps.
Some devices might have a different pinout, but I have yet to encounter any such hardware.
To dump the whole flash of my AR4518PW into some file, I do: ./brntool.py --read=AR4518PW_whole.dump --addr=0xB0000000 --verbose --size=0x400000
And then turn it on.
A successful flash block read will output '.' while a botched one (a byte or more gets lost in the serial port) will output '!' and retry. Even so, unless in a hurry, I'd recommend to at least dump twice and compare the dumps, just to be on the safe side.
See the "erase flash" menu on the menus brnboot provides through the serial port in order to figure out the flash layout in these devices. I cannot guarantee that none of the devices using brnboot will just start erasing the whole flash by just selecting the option, so act at your own risk.
Thanks to Jan-Philipp Litza for patches and confirming ARV7506PW11 works.
Thanks to einsiedlerkrebs for more testing and reminding me about Python 3.