In reply to a few questions on my French Bloggs I thought that I would try and explain the situation as I understand it. bear in mind I'm not a lawyer, and have no 'authority' here that said I would say that two distinct situations present themselves.
Either wild camping is explicitly forbidden (notice-boards, or the mere fact of being in a protected area e.g. nature park or reserve) in which case one must have a very good reason:
-- Explicit authorization of a land owner (insofar as the permission does not contradict the protection of the site/park)
-- Zone expressly dedicated to camping (often with signposts, sometimes with a fee, eg the bivvy in the Ardeche)
-- An emergency (bivouac tolerated, from sunset to sunrise, for one night at the same place, with minimal equipment, if more than one hour from an inhabited zone, road, or the limit of a protected area.
Wild camping is relatively "tolerated" under certain basic conditions:
-- Permission of the owner. Make an effort to find the farmer and ask.
-- No inconvenience to the people of the area, the wildlife and flowers (eg avoid tourist areas or areas having any specific interest).
-- There is a general tolerance for an overnight bivouac (= a night.)
I have been doing this in France for some years, mainly when fishing or climbing. Here are some straight forward tips
-- Always keep quiet, find a quiet place, avoid busy roadsides ...
-- Keep it simple, no tables, chairs, brollies, i.e. not a gypsy camp (no offence intended)
-- Fire: Many areas have specific fire bans, many land owners are extremly sensitive about this. Whilst it is nice to sit around a crackling fire it may be better to use the camping gaz alternative.
-- Find your own place, Don't use somewhere obvious, veiw points, car parks, or places where others are wild camping
-- Leave absolutely no traces of your visit.
-- Lastly if challenged by the owner or 'les flics = Police' Stay polite, plenty of yes sirs no sirs three bag full sirs, and in my experience (only a couple of times) they have been fine about the overnight, when I made clear that I was moving on in the morning.