I've been lunged at a few times, I found that having a firm grip on your dogs collar allows you to use him as leverage: pull back hard on your dog as you pivot at the hips, lift your foot up and kick the attacking mutt under the chin as he lunges. I've done it twice and stunned the attacker into retreat. It even surprised my dog so much he also stands down.

If you separate fighting dogs, seize the hind legs and pull hard. They will let go with their jaws and turn on you. Let you foot be convincing…

I carry pepper spray (dog repellent), used it once, it works, you will get a mouthful yourself and you need time to pull it out.