Part of the problem is our dog is a rescue dog and he's not a puppy, he's just over 3 years old. He was never socialized very well, he was a working dog that didn't do his job properly, so he was neglected much of the time.
We got him 6 months ago, and he was great when we got him. We live in a marina, and in the summer, there are lots of people around and he was good with everyone and with other dogs. However, in the winter we are very isolated and he doesn't see many people, and I think he's expanding his territory and becoming more protective or something.
All the research I've done suggests that this behavior is very much part of the breed, and the vet sees nothing physically wrong with him.
Normally he is not aggressive at all, he's sweet and loving, but when he hears a noise at the door he starts barking and runs for it. Yesterday my dad walked in and the dog started barking when he heard the door open and ran to it ready to attack, but stopped when he saw it was my dad, who he loves. If it weren't my dad, the dog probably would have attacked.
When he does attack or bark aggressivly at a stranger, he immediately gets yelled at, smacked, and either locked in a room or tied up outside, so he knows he's getting punished.
My thinking on the Citronella or shock collar is to distract him and stop him when I can't reach him. If his bad behavior is met with an upleasant blast in the face each time, or a shock, wouldn't that deter him? I know it would deter me.