In this paper we present Kalman duality principle for a class of linear Differential-Algebraic Equations (DAE) with arbitrary index and time-varying coefficients. We apply it to an ill-posed minimax control problem with DAE constraint and derive a corresponding dual control problem. It turns out that the dual problem is ill-posed as well and so classical optimality conditions are not applicable in the general case. We construct a minimizing sequence û for the dual problem applying Tikhonov method. Finally we represent û in the feedback form using Riccati equation on a subspace which corresponds to the differential part of the DAE. © 2013 Springer Science+Business Media New York.