I agree with SallyInIndiana. Well, partially anyway.
First off, what kind of dog is it? If it is a "high prey drive" dog, like a Jack Russell or Rat Terrier, I doubt you will be able to train it out of him. Dogs that were bred for the express purpose of killing small animals will kill small animals no matter what. Now, somtimes a mix of a high prey drive dog with a non high will be able to be trained to leave them alone.
The next thing, how old is this dog? Is it a puppy? if it is a puppy, then it will be much easier to train than an older dog. If it is much more than 2 or 3 years, again, it's probably been chasing small animals it's whole life and will not respond to training.
Did the dog eat the chickens? If he ate it, then there's no hope. It is true, once a dog tastes an animal, he will always see it as food.
If the dog is a non high prey drive (especially retrievers.Most of them will not attack and kill because they were bred to "wait" for the signal. It's not true for all of the dogs in those breeds, but its like as not that the won't kill), is a younger dog, and DID NOT eat the chickens, you need to reassert yourself as "alpha" dog. It will sound mean, but it is something that I use, other rescues use, and trainers use. Get the dog around the chickens. Make sure he can't actually get them. When he lunges or makes a move, grab him and put him to the ground. If he does not roll immediately on his back, put him on his back. Hold him to the ground by the throat. BUT DO NOT, under any circumstance, hurt him. Be very gentle, it is easy to accidentally hurt him when you do that. Make sure he can breathe, but let him know you are the boss. Let him up, if he lunges again, repeat the process. Once he will not lunge, put him in a dog crate/cage where the chickens are. Hopefully he will get used to them.
If your dog is well trained, you can also train him to view the chickens as a "no touch" object. Most dogs when they come to me are older, and training them that well is not feasible. Depending on your budget, you can hire a licensed trainer to work with him. They can be very expensive though.
So how do you keep a dog from chasing/killing your chickens? My dog killed two of mine. :-( I'm so upset. I don't want to have to build a large run/pen as we have so much land they can roam on. Suggestions?