How do you train a dog to bark when someone knocks on the door?

Sorry! It's instinct. They either will or not.You might experiment with a friend playfully attempting to hurt you (You yell ouch when making believe hurt).But this may not work either as dogs have instincts about fears and friendships.Worth a try.......Also contact dog trainers and see if they have had requests along this line. Some dogs just won't bark. Some don't until certain ages.

do yourself a favor and train him to bark on command and NOT at the door. Get him excited and tell him a key word that you want to use every time you want him to bark.(speak or bark) whatever word you decide on use it consistantly. Every time you use the word and he makes any attempt to make a sound (even just a grunt at first ) reward him with his favorit LOW CALORIE snack. (I say this because at first you will be giving him alot of snacks) and give him lots of praise. When he starts to bark consistantly at the key work then start alternating snack and praise rewards. Eventually you will be able to cut out the snack and just say good boy and he will be tickled that he made you happy. If you teach him to bark at the door you will come to regret it.

go to petsmart and get a bark collar. tough love but that come with the dog and you can either deal with it or train him, which i think is very difficult if you dont get a bark collar because it is like getting the dog to pee without lefting up his leg but barking at the door isn't that important and they won't care. i actually like when my dog barks because he let's me know who is coming but 56% of the other time i would rather him not. ps teaching to bark on command only works on puppies usually.