We often disregard the laws governing wild camping, so here is something to quickly give you an idea of how to enjoy nights in the great outdoors legally!
According to the French government's official website, it is forbidden to camp:
In woods, forests and parks, designated as nature reserves, and protected natural heritage sites.
On roads and public highways.
On sea shores.
Within 200 meters of any water mains.
Within 500 meters of a listed or inscribed historical monument.
Certainly all these points strongly reduce the possibilities! But do not be discouraged, you can simply speak to the locals pleasantly, ask the owners of the coveted spots for permission to put up your tent. It's an opportunity to meet some great people.
And note also the important nuance between camping and bivouacking: while wild camping is prohibited in all these places, bivouacking is often tolerated (this is the case in some national parks and in the mountains).
Bivouacking means staying for the night and setting off again the next morning. However, it is best to make enquiries beforehand with any park rangers, at refuges or the town hall! The main condition of course is to respect nature and to leave places as clean as when you arrived.