The 38-year-old turned out for the Spanish giants on 725 occasions, winning five La Liga titles and three Champions League crowns before joining Porto in 2015.
The Spaniard's future as a player is unclear having undergone heart surgery last year, but the veteran goalkeeper has talked up a return to the Bernabeu at some point in the future.
"One day... why not? I consider it to my home, even though I have been away for five years," Casillas said in an interview with DjMaRiio. "The connection is palpable. I was there for 25 years. Of course I'm convinced that one day I'll return to the club."
Casillas has made a further 156 appearances during his time with Porto, but the stopper's current deal is due to expire at the end of June.
The veteran was also capped on 167 occasions by the Spanish national team between 2000 and 2016.