In D&D 5e, the barbarian's Rage is somewhat different from its 3.5e counterpart, but the thing that most confused me is the need to be actively part of mayhem and destruction at every round to keep being angry (emphasis mine):
Your rage lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven't attacked a hostile creature since your last turn or taken damage since then. You can also end your rage on your turn as a bonus action.
Can a barbarian (with or without a shield) punch/stomp/bite/headbutt itself to take damage and maintain their rage?
To me, it seems its possible by RAW, but I'm not sure about the rules intent on whether an unmissable attack (I refuse to think that you can miss when trying to hit yourself if you are not heavily drunk) with the lowest melee damage possible from a Barbarian can keep you raging for the full duration.
Best Answer
Yes, a Barbarian can self-inflict damage to keep their Rage going.
It shows this right in the rules text you quoted:
There is no qualifier to who or what caused the damage, only that you took any.
Keep in mind that, no matter how much you might refuse to believe it, there is no RAW to suggest you can self inflict damage for free in any way. The only allowed mechanics to attack yourself are by making an actual roll, which means you're sacrificing an attack and you could fail to "hit" (read: meaningfully hurt) yourself. It's not that easy to hurt yourself.
(Much easier to set yourself on fire.)