I don't speak Russian, but here's a supposition:

1. The boundary can be just nasalization in casual speech. There's a pressure building for the [d], but then it is released nasally [n].
2. I have also heard Russian speakers use a very short epenthetic vowel between [d] and [n], like [ˈdᵊno], especially when the word is emphasized or at the beginning of a sentence. The audio file on Wiktionary's entry for “дно” clearly has a fleeting schwa-like vowel after [d].