One word:
MEGAFOLDERS.
In California, the one really nice aspect of our weapons laws is the legal concealed carry of big folding knives, so long as they aren't switchblades.
See also:
http://www.ninehundred.com/~equalccw/knifelaw.html
At the bottom of the page is a selection of some good legal knives.
Dogs have thin "patches" on the top of their heads, to either side of a strong central ridge. In other words, you can often whack a big aggressive dog over the head with a baseball bat, to no effect, but ram the butt of a mini-maglight straight down and to either side of that ridge, and you'll punch through the bone and puree the brain. You can do the same thing with the point of a knife, especially from a reverse grip, BUT you'd best have a strong lock and a grip that resists a "slip up" type accident. A Camillus Maxx with the clip removed and carried in a 1911 magazine holster is near-perfect for this but shorter blades can work OK too. Forward-grip holds can work OK too, and if the dog is right up there in your face do NOT try a shift from forward grip to reverse. That said, I consider the reverse grip the better choice for dogs and most other animals.
Dogs that attack low are possibly professionally trained, and if trained (police/military) are wary of such overhead moves. Untrained dogs will often attack HIGH, jumping for your throat. That leaves their throats/chest wide open...barehanded, flat-out strangling 'em works but if there's more than one, a knife comes in very handy.
Now, what about dogs bounding towards you that are actually friendly? Happens all the time. Pulling a roscoe or a big knife can then be embarassing
.
I pay attention to the tongue. If it's flapping and waving in the breeze, he "looks happy" to our eyes. What's really going on is it's being used for cooling, BUT it also means he doesn't plan on biting anyone or anything! If he's nervous or planning a fight, he retracts the tongue to protect it from his own teeth. This is actually a better indicator than the tail...some dogs will wag their tails because THEY'RE HAPPY THEY'RE GONNA CHEW ON YOU!