What to do with an attacking dog, if you don’t have a gun:
Wheelbarrow: Grab the dog’s hind legs, and hang on TIGHT. Slowly back up, circling slightly. The dog will have to move his front legs sideways to keep from falling, and will most likely release whatever he has unless it’s light enough to drag. I imagine this would also work well to get the dog into a position where somebody else could safely shoot it without hitting the victim.
Breakstick: A breakstick is a smooth, flat piece of wood, about a foot long and two inches wide. Anything about that wide can be inserted behind the teeth, between the soft gums. Just twist and the dogs mouth opens. Works best in conjunction with method #2...
Lips: Understanding that if you let go you’re going to be bitten, place your hand over the bridge of the dog’s muzzle, fingers on one side and thumb on the other, as close to it’s eyes as possible. Push the lips on either side into the dogs mouth and under whatever teeth are not full, and squeeze as hard as you can. The dog will release.