How to train a dog?

Do you bring your dog out often like daily. If not I think your dog is scared or excited when his is out. He may not have seen stranger often that is why he barked at stranger whenever he is out.

If you are out on a walk and he does it...Maybe try and bring some treats with you and when you see someone getting close, stop and get his attention with the treat, it will take a little time and patience...

Dogs bark for several different reasons. When they are angry, they bark. When they are afraid, they bark. When they want to attract attention, they bark. Barking at passingby strangers, it could be just telling people they are there, or they are afraid of strangers. Let him meet strangers more often and soon he will get used to it, I think. Give him a treat if he did not bark when he see strangers.