Advice from a Mini Aussie, Mystical Muse, Mistress of the Sofa
 

First, we're going to implore Athena, goddess of wisdom, to send a magical owl down.  The owl will whisper into your dog's ear and let him know which humans are pure and upright, and which are evil. First, we'll need to write a nice prayer...

Wait, you don't believe me? That's just MYTHOLOGY? Well, guess what.  Ok, sit down.

It's a myth that dogs act aggressive to strangers out of a duty to protect.

Yup.

Here's the deal. A long time ago, you humans decided, "hey! dogs have pointy teeth! Pointy teeth can keep other humans away from my stuff!"  Then, you realized that dogs show those pointy teeth when they're scared. Humans decided to breed guardian dogs to have a genetic propensity (yeah, I read Webster's) to be afraid of new people.

Your dog is fearful.
 
I know.  He ACTS like a big tough protector. But really, dogs that bark, lunge, or snap at strangers are most likely afraid. The problem isn't that your dog doesn't know who the bad guys are. The problem is that your dog is scared.

Ok, go get a drink, come back, and we'll talk about the good news.
 

Nice to see you again. The good news is we can reframe strangers for your dog. We can teach him that new people = snacks and play and fun! Then he won't be scared of them any more, and he won't bark and growl. There's even a fantastic little book on this called Cautious Canine that can help you out.

If you're really worried about your safety, the best plan is to invest in some sort of home security option - not to expect your dog to go after an intruder. 

So, once again, probably NOT the answer you wanted to hear from me. But guess what? I'm gonna tell it like it is anyway. Cause despite all the silly stuff you humans do - like breeding dogs LITERALLY TO BE AFRAID OF YOU - I love you guys anyway, and I want what's best for you.

May you never meet a stranger,