Casillas is expected to work in the front office after announcing his retirement while with Porto.
He said, "You can't hide that after 25 years at Real Madrid you could return again. It's a dream of mine because I can be useful to a world-famous club because I know it very well.
"We don't know what might be around the corner, but it would be really special to return and be able to transmit those values that they taught me as a kid. That will always be there.
"The old staff and players at the club... I'll never close a door where tomorrow I could be happy and contented again."