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They don't even have to be hardcoded, they just have to ignore anything you specify or not give you any option to specify your own. As long as a device manufacturer can push updates to your device (even by IP address) they can regularly update their chosen DNS servers when needed. Honestly though, for many devices I doubt they'd even bother. Companies seem to have little trouble taking the position that if your device is more than a few years old you're insane for expecting them to still support it and you should have already thrown it away and bought another one.