Odor is a symptom, not a diagnosis.
Drainage systems use water-filled traps and venting to help keep sewer gases from entering occupied spaces. An odor near a drain can be associated with a dry trap, drainage or venting conditions, buildup, or other plumbing issues.
Start with where the odor is strongest, whether the fixture is rarely used, whether you hear gurgling, and whether drains are slow or backing up. Because odor alone cannot identify the cause, persistent or strong sewer-gas odors deserve appropriate evaluation.
If an odor is strong, persistent, associated with illness, or you suspect a gas leak rather than a drain odor, leave the area as appropriate and contact the relevant utility or emergency service. Do not rely on an online drain guide to identify an unknown gas.
What information helps?
Note which fixtures are affected, what happens immediately before the symptom, whether another fixture reacts at the same time, and whether the problem has happened before. Those details can help a drain professional choose an appropriate next step without asking you to diagnose the system yourself.