The 24-year-old has been without a club since leaving Blackpool and is on trial with the Millers hoping to earn a contract until the end of the season.
Neil Warnock has offered Ranger a lifeline as the boss searches for firepower in a bid to kick-start the great escape in the Championship.
In Warnock's first three games at the helm, the Millers have failed to score and are six points adrift from safety, four worse than when he started.
Ranger has startling rap sheet in a controversial career that has taken in spells at Newcastle, Swindon, Sheffield Wednesday, Barnsley and Blackpool.
He was pictured with a replica gun in 2011, has two convictions for criminal damage after also spending time in a young offenders' institute for his involvement in a robbery. He has been cleared of a rape charge in 2014 and also an assault charge.
Ranger went on strike from Blackpool last season but the club still enforced a contract extension in the summer, though the striker never returned to Bloomfield Road and his exit was finally confirmed at the beginning of the month.
His last game was November 2014, ironically against the Millers.
If he does earn a new deal he will be the seventh out-of-contract player to sign for the club following mid-season arrivals of Stephen Kelly, Leon Best, Luciano Becchio, Jerome Thomas, Paddy Kenny and Lloyd Doyley.
