I've used a variety of cameras, from an old Pentax 35mm P&S film camera to a Canon Elph to a dSLR with a very long 500mm lens on it. I've taken photos from inside and outside the terminal, on the jetway, walking across the tarmac, behind a fence or on the plane. I've been told to stop in some cases and have chosen to ignore or cooperate based on knowing what the actual rules are in the country I'm in (pretty much no one can stop you in most cases in the USA) and whether I have the shot(s) I want or not.

Do not be intimidated by someone pretending to know the rules. If you don't stand up for your rights you don't have any.

This older thread can offer a bit of advice as well as some links to resources for various countries.