In reply to swoe:
The other official bivouac site is near the race course by the Rochers St Germain, you follow the access road to the race track and the area of grass is on the left after 500m. No water or facilities and reserved for overnight camping, not long stays officially anyway. Beware of thieves too, best not to leave valuables in a tent.
As for the forest as a whole, "wild camping" is prohibited but if you are discrete and take your tent down in the morning they aren't going to lock you up. There are many modified caves that witness the old habit of Parisian clumbers going there for the weekend, around the Trois Pignons there are several with chimneys even but this is no longer allowed - lighting fires is definitely frowned upon for obvious reasons.