With no prior knowledge of this, I would say
Wer ist das?
MY question is related to what I read here
It's a page from the Italian version of Wikipedia and it talks about the "vasistas", a name which is apparently used in Romance languages to identify the small opening in a door made to see who's knocking (by the way, I've never heard this noun).
The section on its etymology says that the term originates from German: when German soldiers heard some knocking at the door, they said "Was ist das?" and then French soldiers (who were knocking) identified the "vasistas" as the opening (I know, it sounds silly).
So I ask: is "was ist das?" a proper way to ask who is it at the door?