Sum

Sum files (.sum) are files stored on the root of a USB storage device are used by the default firmware to check the integrity of a software update that is being applied via the USB storage device. These files may be hidden on some operating systems.

The USB storage device must be recognised by the default firmware. As a rule, the first FAT volume on the 1st external USB storage device will attempt to be mounted by the OS on boot. This sometimes fails due to the specifics of the USB device (try several).

It is evaluated by scripting inside of '/openpeak/tango/run' and does the following.

 # update device from a usb key, if specific files exist
  SUM0=`cat /mnt/.sum`
  SUM1=`cat /mnt/opupdt.tgz /openpeak/tango/.usb-key | \
     md5sum | sed -e 's/  .*//'`

These 2 checksums are compared and should the checksum prove valid it will then run opupdt.run from the USB storage device.

This update method is 'ab'used by scripts such as the Telnet enabling hack to trick the default OS into running arbitrary commands contained in the opupdt.run shell script.

As long as you have a valid sum file (made up of /openpeak/tango/.usb-key and opupdt.tgz) you can modify opupdt.run to run whatever commands on the default firmware you wish.

All these commands are run on the device as root (so yes, incorrect commands in opupdt.run can easily ruin your day).