The following result is obtained by our Internet Research Algorithm:
The answer to your question is in Valmiki's Ramayana itself
Following were the words of Kaykeyi -
पुरा देव असुरे युद्धे पित्रा ते मम राघव |
रक्षितेन वरौ दत्तौ सशल्येन महा रणे || 2-18-32
Oh, Rama! During a battle and a great war between celestials and demons long ago, I protected your father who was pierced by darts. Then, he gave me two boons."
तत्र मे याचितः राजा भरतस्य अभिषेचनम् |
गमनम् दण्डक अरण्ये तव च अद्य एव राघव || 2-18-33
"Oh, Rama! According to those boons, I asked the king for the coronation of Bharata and for your going to Dandaka forest today itself."
In Ramayana, Dhankana Aranya was described as a vast forest passage begins immediately from the south of Yamuna spread from Bundelkhand region to Krishna river.