To get rid of a smelly drain pour around 1/4 cup of baking soda down the smelly drain along with some vinegar until it's bubbling and let it sit for around 10 minutes and then heat up a pot of water until it's boiling.
Once the water is boiling pour the boiling water down the drain to wash the vinegar and baking soda solution out and your drains should no longer smell.
If it still does then repeat the procedure until the smell is gone.