Is hachnasas orchim (welcoming guests) a mitzvah (either of biblical or rabbinic origin)? If not why is it considered more important than greeting the divine presence (bereshis 18:3 Rashi)? Does it have technical mitzvah parameters - for example is it still 'welcoming guests' if it is a relative? if you don't own the home itself? for a non-jew? for a minor? etc.