PLUMBING QUESTION: sewage smell coming up from drain?

that is usually a venting issue. if a drain is not properly vented, then it will suck air from the next fixture, which empties the p trap (the curvy thing) and then ges can come in. unfortunately that means checking for vents. if they come straight up, you can see them sticking out your roof, but if they join the main stack you will need to pull walls apart to make sure they are there. check local code to see how many feet from each toilet, sink, and tub they need to be. for sinks, you can use a cheater vent, but not for toilets.