The kan’on 漢音 for 奴 is do, but the go’on is nu, identical to the usually reconstructed MS nu for 奴. The d-~n- alternation is standard when there is an opposition btw kan’on and go’on readings involving original MS /n/, for example 男性 dansei ‘male’ vs 男体 nantai ‘male body’. South Coblin has a detailed study of this, looking not just at the phenomenon in Japanese kan’on but at Tibetan and intra-Sinitic Buddhistic readings. In modern Japanese, kan’on readings are vastly less marked, and almost always used in neologized kango 漢音. Go’on readings have a strong association with Buddhism.