My dog's rear smells bad and has white worms. What's happening?

Well the bad smell, aside from feces, is anal glands. Dogs have small glands on each side of their anus that secrete a small amount of fishy smelling liquid. Every time they have a bowel movement a small amount it expressed, it marks their territory. Sometimes if they become excited or scared they express them. The glands can also become impacted, infected, and abcess. If the material in the glands becomes too thick for the dog to express themselves it needs to be manually done. Your vet can do this and show you how to do it.

As for the worms, it sounds like tapeworms, which your dog gets from ingesting fleas. An over the counter drug you can use is Drontal. If you cant get that brand name the active drug is praziquantal. Administer it orally once, then again in two weeks. It is definitly a good idea to take a stool sample in to your vet to make sure they are tapeworms, and also that your dog doesnt have any other intestinal parasites such as hook or roundworms. Your vet will need a sample about the size of a nickel less than 24 hours old. Hope this helps