Abstract: The invention relates to a telescopic shock absorber for an aircraft undercarriage, the shock absorber comprising a strut (1) in which a rod (2) is mounted to slide, the shock absorber including controlled retraction means (41 . . . 43) for retracting the rod into the strut to a retracted position. The shock absorber includes positive retention means (27, 29) for retaining the rod in the strut and adapted to retain the rod in the retracted position when the rod is moved thereto by the retraction means, the positive retention means being arranged, at least in the event of a severe landing of the aircraft, to allow the rod to be pushed into the strut beyond the retracted position.