The 24-year-old joined Hannover from Manchester United in 2010 and still has two years left on his contract with the club.
But Hannover are keen for Zieler to sign a new deal in order to remove a reported €7million release clause in his current deal.
“We are keen on signing a new contract with Ron-Robert Zieler,” Dufner told Sport-BILD weekly.
“No club in this business is happy to have players under contract that are based on release clauses. Of course, we are also not happy about such circumstances.”