How do we defend against a dog?

Use your hands to close his month.

EDIT: After you details, I'd say to study dog whispering. There are people who can stop any dog by saying "STOP" in a certain tone of voice and body posture. But they are trained to do so and so can you.

If you're already under the dog or under attack, attack with all your force. Use stones and all weapons you can reach. Attack with punches or stones in the head or neck to kill him (he's a danger to you and others but I wouldn't consider this possibility too soon) or attack the legs. Legs are pretty fragile in a dog and can stop him from chasing you.

Not much you can do..if its a trained attack/guard dog..just cover your head and neck keep fingers in a fist and call fro help

if its a smaller/not trained dog lift it up -most dogs drop their grip here if not slam it onto a fence/railing on its spine..gouging its eyes makes it even more frenzied but might be worth it in the end.

Cover your throat most predators go for the most vulnerable spot on every animal which is the throat because its connected to the Nevis system, the respiratory system and the circulatory system plus very important bones like your vertebra

dont run from them

tell them to stop and tell the owner to stop the dog

Crush its windpipe