SAT SRI AKAL,
Rasna means the bani of SSGS which is been mediated by Waheguru ( god) itself and
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/new-to-sikhism/29471-questions-from-non-sikhs-to-sikhs.html
that Rasna is given to Holy Guru Sahib and the message is been delivered to masses.
Hopeful it answer you're queries.
Regards
Gurveen Singh