Are you just traveling in Europe or further afield as I wouldn't have thought you'd have needed a jerrycan of spare fuel in most of Europe.
The leak is probably the rubber seal in the cap has perished. You can buy new ones for a couple of euros on ebay. I've got about 10 cans in various sizes - 20. 10 and 5L and replace the seals periodically. The seals in the older, ex military, ones seem to have lasted for ever but the newer smaller civilian market cans seem to need new seals every few years.
I've no idea about the French ban on full jerrycans - hopefully someone else will comment in due course, but I have traveled through France a number of times with (5L) jerrycans fixed to the back of my bike and never had a problem. They have been empty though as I've not needed the extra range in France and 10L of fuel weight at the back of the bike does change the handling. What I do use is a small plastic 2L bottle fixed to the rack to store a small amount of emergency fuel. It's used if I run out just short of a fuel station (quite often!) and I top up my stove fuel bottle from it as well.
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