Can I fix this leaking tub drain myself or is it pro time?

The pressure from plunging is likely what finished off a failing joint in that old plumbing. When you've got a mix of cast iron and PVC, those transition points are usually where things go south. If it’s leaking where the trap hits the vertical line, you probably blew out a seal or cracked a brittle section of galvanized pipe that was alredy paper-thin from decades of corrosion. Plungers are great for simple clogs, but they put alot of stress on old, weakened fittings.

Since you have access from the basement, this is acutally the best-case scenario for a DIY fix. You should probably just cut out that old metal P-trap and the drain assembly and replace the whole section with PVC. Trying to patch a leak on rusted cast iron or galvanized pipe is a losing battle. You’ll want to use a **shielded Fernco coupling**—the kind with the stainless steel sleeve—to connect your new PVC to the existing cast iron stack. It’s a much more reliable connection than just a basic rubber boot, which can sag over time. For the clog itself, make sure you're snaking through the **overflow plate** on the tub, not the drain hole. The drain hole has a crossbar that stops most augers, and you have to make a sharp 90-degree turn that usually just results in the snake getting stuck. Going through the overflow gives you a straight shot down into the P-trap where the hair and soap scum actually collect. And since you’re rebuilding it, do yourself a favor and use slip joints for the P-trap instead of gluing everything solid. If it ever clogs this badly again, you can just unscrew the trap in the basement, dump it into a bucket, and clear the line without fighting an auger through the tub. It’s a beginner-friendly project as long as you have a hacksaw or a reciprocating saw to cut the old metal. Just make sure the pipes are supported properly with some strap hangers so the weight of the water doesn't stress the new joints.

That shielded Fernco thing sounds like a solid plan, but I’m a bit nervous about cutting into the old metal. How do you actualy tell the difference between cast iron and galvanized pipe? Everything under my tub just looks like a rusted mess of grey and brown.

Also, for those slip joints on the P-trap, do I still need to use that purple primer or glue? I’ve seen people use it on other PVC projects, but if they’re meant to be unscrewed for cleaning, I'm guessing I just hand-tighten them? I’d hate to get it all put together and have it drip because I didn't tighten a plastic nut enough.

Give the pipe a quick scratch with a screwdriver. If it shows up shiny silver like a nickel, it’s galvanized. Cast iron is thicker and usually stays dark or dull grey.

On the slip joints, absolutely no glue or primer! Those rely on a compression washer and a nut to seal. If you glue them, you’re locking it forever, wich makes future clogs a nightmare. Just hand-tighten the nuts and give them a tiny nudge with channel locks. Make sure the beveled side of the washer is facing into the fitting, or it’ll leak for sure. It’s an easy mistake to make when you’re starting out.