Joy of Joys, almost.
The 6 nations in their entirety are available on France2, which is one of the channels available on Kabel Deutschland as part of their their basic digital package.
This is a mere 3 euro a month extra if you already have analogue as part of your rent agreement. If you have SKY (DE) over cable (as many do to get the premiership and champions league football) you already have the box and smartcard needed so a phonecall with your bank details and you are all set.
The only bummer is that its in French and not original BBC/ RTÉ commentary.
Still, an enthuastic french commentator is better than a half arsed attempt by some DSF muppet that knows more about olympic handball than rugby.
I have copied the info from the France2 website just to confirm that all games are available. http://sport.france2.fr/rugby/tournoi-six-nations/index-fr.php?page=guidetv