Just a heads up, your dog could be impacted. This isn't dangerous, and the problem is easy to fix. Your vet, or some groomers can help you out. This is a pretty common thing among dogs. My current dog doesn't have it, but my two other dogs did.
If you dog is licking its butt excessively, or dragging its butt across the carpet excessively, or when he goes to the bathroom, he goes, and then tries to go another 3 or 4 times after that but nothing happens, these are sings that there COULD be impaction.
To get really gross, essentially what the vet has to do in order to release secretions that may be caught in the anal glands, he would have to stick his finger up there and squeeze the stuff out.
Yum.. time for breakfast.