Bear44 actually is a dog expert. Literally.
As far as the way my dog would act if someone aggressively came in the house, I only say that I'm confident in what he would do because I have seen it first hand and it caught me by surprise.
That is most likely territorial behavior. The real test would be to remove him from familiar ground and set up an "attack" against you while he is off leash. The reason for him to be off leash is to give him the option to retreat.
A dog that is territorial of the house, could very well be good home protection. I just think it's important to understand the behavior so you don't get unpleasantly surprised when you are counting on him.
I've seen it to many times. I even had a LEO retire his patrol dog when he saw that the dog was just mean and not actually interested in protecting the officer at all.

