domain-aliasing

I have a client that has a good amount of domain alias' and wants them all redirected to the one main domain on the site. They also want to know which of the domain alias' is doing the redirecting. I have that part down but I want to optimize the code to the best most proper way it should be and to eliminate the amount of code I have to write. I am wanting to know if there is a way to pass to the RewriteRule url the domain alias that was used. This is what I have now. I am looking for the domain alias that is being hit and then passing that alias to the url. Then in google analytics I can see