Not known (currently)
We don't know if Durmstrang has "houses". No mention has been made in any of the canon writings; books, films, games, Top Trumps cards, twitters, interview or any of the other material released in the Harry Potter universe.
We do know that the school itself is relatively small (just four storeys high) so there's a very real possibility that all of the students are simply roomed in adjoining dormitories.
For the record, we also don't know whether Beauxbatons has any houses either. Apparently it's rather larger so there's more of a possibility but it's more of a British tradition than a French one, so it's really anyone's guess at this point in time. JKR isn't saying and the only "houses" we see are those from fanfic.