Rash on dogs rear leg.

My 6 year old male Jack Russell mix has a rash on one of his rear legs.  He's had the rash for about one month.  I have attached a picture of the rash.  

Hello!  This looks sore and inflamed and I can tell that your dog may be licking it, perpetuating a cycle, from the brown staining present on the fur. There is a huge range of differentials, including fleas, mites, allergy or pain in that particular joint and we would strongly advise a trip to the vets for investigation. Best of luck.