The 32-year-old has scored 406 goals in 394 appearances since joining Madrid from Man United in 2009.
The attacker's 42 goals during the 2016-17 campaign helped the Spanish side land their second successive Champions League crown and wrestle the La Liga title away from Barcelona.
Ronaldo has been accused of defrauding the Spanish tax office of almost £13m over four counts of tax fraud relating to image rights, however, and despite being backed by Real Madrid, the attacker is said to want out of the Bernabeu this summer.
Paris Saint-Germain, Bayern Munich and Chelsea have all been credited with an interest, but according to The Sun, Ronaldo is only interested in a return to Man United due to his special relationship with the club.
Ronaldo, who is currently with the Portugal squad at the Confederations Cup, scored 118 times in 292 appearances for Man United between 2003 and 2009 - winning three Premier League titles and one Champions League in the process.