Shodan.. pěkné :)

In other words, if your browser’s user agent string is “xmlset_roodkcableoj28840ybtide” (no quotes), you can access the web interface without any authentication and view/change the device settings (a DI-524UP is shown, as I don’t have a DIR-100 and the DI-524UP uses the same firmware)

It’s said in the Russian forum post:
Try to read the string backwards.

xmlset_roodkcableoj28840ybtide