Your dog's anus is not where the odor is coming from. The odor is coming from your dog's anal sacs. http://www.marvistavet.com/html/body_anal_sacs.html
Anal sacs are located on either side of your dog's anus. His anal sacs empty their contents into his rectal area by way of a small connecting duct. Each sac fills up with a thick, foul- smelling, brownish fluid which normally is expelled when your dog has a bowel movement. The odor produced by these sacs helps your dog "mark his territory" and may be responsible for much of what is described as the characteristic "anus smell". Problems arise when a dog's anal sacs cannot easily empty their contents and they become infected or overly full causing the dog discomfort and pain. When this happens dogs often develop an offensive odor and try to relieve their discomfort through biting, licking, and scooting along the ground.
Check with your vet regarding anal expressions. Some dogs need help with this. Look for signs like your dog dragging its bottom along the floor. Most dogs can empty these glands voluntarily for scent marking or in self-defense, like a skunk might; however, domesticated dogs have largely lost their ability to empty these sacs voluntarily. Walking around and normal defecation usually serves to empty the glands but some dogs become unable to empty their glands on their own at all. The sacs become impacted and uncomfortable. Dogs with impacted anal sacs usually scoot their rear on the ground in an attempt to empty the glands. Some dogs will lick their anal area and other dogs will chase their tails.
A rag or tissue is held up to the anus and both sides of the anal area are squeezed. If the secretion is very pasty, this method may be inadequate to empty your dog's sacs. A lubricated gloved finger is inserted in the anus and the sac is squeezed between thumb and forefinger into a tissue held externally. The full anal gland feels like a grape. The emptying procedure is repeated on the opposite side. If the sacs have been emptied adequately, the scooting should resolve in a couple of days.
Most dogs are not too concerned about having their anal sacs expressed but some dogs resent it even though they are trying hard to express them on their own by scooting and rubbing on the rug or ground. It probably is uncomfortable for the pet to express these sacs when they are very full or difficult to express. If you want to try this at home it is often possible to do so. The anal sacs are located at about the 4 o'clock and 8 o'clock positions if you imagine the area around the rectum to be a clock-face. It is usually possible to feel them under the skin at these points when they are full. In some dogs the sacs can be pretty far to the side of the rectum but most are about 1/4 to 1/2 inch to the side. If the glands are pressed against each other by pinching the rectal area together they will usually express. It is hard to get them as empty as the vet can by doing this rectally but most pets tolerate it better so it can be done more frequently. Don't push so hard that you rupture an anal sac, though. That leads to significant problems. If you succeed, you should see an exudate exuding from the ducts. It can vary from liquid to a thick paste and may be gray, tan, brown, or black and be normal. Blood or other colored exudates may indicate a problem. If this is a persistant problem it may be best to have your vet remove the sacs, especially in a dog that resents having them expressed.
If an impacted sac doesn't get emptied then an abscess can form and rupture out through the skin. This is a painful, messy, and pretty smelly condition often mistaken for rectal bleeding. If an anal sac abscess forms, it must be properly treated by your veterinarian. Antibiotics will be needed. How often the anal sacs should be emptied is a highly individual situation. The best recommendation is to let the pet tell you when the sacs are full. If the pet starts scooting again, it is time to bring him in.
If your dog's anal sacs seem to require emptying all the time then to avoid the expense of having the sacs emptied, you can learn to empty them yourself at home but most people feel it is well worth having someone else perform this service. A non-invasive technique that helps some patients is a change to a high fiber diet. This will produce a bulkier stool that may be more effective in emptying the sac as it passes by. If the sacs need to be emptied every few weeks or more, you may opt to have the sacs permanently removed. This procedure is complicated by many local nerves controlling fecal continence, the fact that any change in the local musculature of the anal sphincter area can affect fecal continence, and the fact that with chronic anal sac problems anatomy is distorted. Draining tracts can develop after surgery if the gland is not completely removed. Still, despite these pitfalls, anal sac removal is considered a relatively simple surgery by most board certified surgery specialists. If this procedure is to be done we generally recommend that a specialist (a veterinarian with extensive experience with anal sacculectomy) perform it.
If scooting continues for more than a few days after sac emptying, the sacs should be re-checked. For some individuals, it takes several sac emptyings in a row before the sacs stay emptied. If the sacs are empty and scooting is persisting, another cause (such as itchy skin or lower back pain) should be pursued.
Anal sac disease can be caused either by an infection or simply a physical obstruction. Bacteria which are abundant around the anal area may invade the anal sac where they produce an inflammation and swelling of the inside lining of the sac and its emptying duct. The swelling causes a narrowing of the diameter of the duct and as a result the fluid can not flow out into the rectum as easily and consequently is retained. The longer the fluid remains in the anal sac the more likely it is to become thicker and paste like. Eventually the consistency of the anal sac material may reach a toothpaste like consistency. At this point it is almost impossible for the sac to empty itself naturally and help is required to open up and empty the plugged sac.
In order for your veterinarian to help relieve the dog's discomfort he will put on a lubricated rubber glove and with one finger inserted into the rectum and another finger on the outside he will attempt to squeeze the contents through the plugged or narrowed duct. If the anal sac material is very thick or the duct very narrow manual squeezing may not be successful and additional treatment may be necessary.
In cases where manual pressure does not completely empty your dog's anal sac your veterinarian will most likely recommend that the anal sac be catheterized and flushed out with a solution that softens the pasty material and wash it away. After a thorough flushing he may infuse an antibiotic cortisone paste which helps treat both infection and inflammation. It is often necessary for this flushing process to be repeated after several weeks.
Besides infection another cause of anal sac disease is a poor quality diet that leads to the production of soft feces---soft bowel movements. Soft feces usually cannot exert the necessary pressure for emptying the sacs. Poor muscle tone, especially in obese dogs, can predispose an animal to anal sac disease. Both of these can be prevented through exercise and dietary management.
If this condition is left untreated the condition may develop into an anal sac abscess which results in the drainage of a bloody pussy fluid through a hole in the skin. An anal gland abscess usually requires surgery but is easily avoidable with early treatment.
In order to prevent anal gland problems some pets, especially the very small ones, will need their anal sacs emptied every few months by their veterinarian. Pets that have frequent anal sac problems may need to have these sacs surgically removed if medical treatment does not provide a solution.
I hope this information is helpful for you.