Mackie was in and out of the team last season as Rangers were relegated from the Premier League and his situation at Loftus Road shows little sign of improvement as Hoops manager Harry Redknapp targets new additions to his attack.
With Redknapp linked with moves for Gary Hooper, Andros Townsend and Charlie Austin, Mackie has informed the west London club that he wants to leave.
Nottingham Forest are interested in signing the 27-year-old and have had a bid rejected by Rangers because it did not meet their valuation of the player.
Forest boss Billy Davies has long admired Mackie having tried to sign him during his first spell in charge at the City Ground when the Scotland international was a Plymouth player.
Despite the addition of Jamie Paterson this summer, Davies remains keen to add further attacking flair to his side and Mackie fits the bill due to his ability to play both through the middle or on either flank.