Setting up bucket's name placed domain-style (bucket.s3.amazonaws.com) with Rails and Paperclip

Paperclip doc about url options: You can choose to have the bucket's name placed domain-style (bucket.s3.amazonaws.com) or path-style (s3.amazonaws.com/bucket). How would look like the setup to actually have bucket's name placed domain-style? I can't force paperclip to generate urls like bucket.s3.amazonaws.com instead of s3.amazonaws.com/bucket.