Template:Fw checksum cli
You can calculate the checksum of a firmware image file using md5sum. This command should work on most Linux distros or you can upload the firmware into the router and calculate the checksum via a Command line interfaces.
When you select a firmware image file for upload via the System -> Firmware page, the file gets uploaded into /tmp/ directory with the name firmware.img. So the command to calculate the checksum should look like this: