"Wild" camping is not permitted. In places where there are no businesses dependent on touristic trade and provided you don't light fires, break fences or trample crops and especially don't foul watercourses, grazing land etc etc... Falcon, as usual, is correct. Cooking fires will exceed local tolerance; otherwise discrete camping well away from settlements and outside National Parks and similar is feasible but you may wish to consider how relaxed a nights sleep you are hoping for. That said I have encountered many a Paysanne who has let me camp in a corner, sometimes for €, sometimes for labour, but I have no personal experience of the Camino in France.
Bon chance