What you are likely smelling is your dogs anal gland fluid. Anal glands (there is one on the left and one on the right) are small pockets on the inside of your dog's anus that contain a foul smelling (often smells metalic or fishy) liquid that usually comes out with every bowel movement. The liquid gives your dog it's own scent....it lets other dogs in the area know who he/she is. Sometimes, dogs can have a problem with their anal glands (usually caused by food allergies) and the fluid builds up in the pockets. When this happens, dogs tend to drag their bum on the floor because their bum is irritated from the build up of anal gland fluid. If this is happening with your dog (may be the reason why you are smelling the smell), you can try adding flakes of bran (the stuff used for baking bran muffins, not the bran flake cereal) to his/her food every day to bulk up the poop which will in turn put more pressure on the anal glands during the bowel movement, and hopefully empty the anal glands with each poop. You can try roughly 1-2 tbsp sprinkled over his/her food daily. If this does not resolve the problem, he/she should be seen by the veterinarian to be sure that there is not an infection, and treat any underlying causes (allergies for example).
