Foul smells like rotten eggs (hydrogen sulfide) can enter the home if dry P-traps allow gases to bypass the vent system.
Here’s a detailed and engaging write-up on how to unclog a vent pipe, written to be both practical and interesting for a homeowner or DIY enthusiast. how to unclog vent pipe
If you live in a freezer, the clog might be ice. DO NOT use boiling water (it can crack cast iron). DO NOT chip at it with a screwdriver (you’ll puncture the pipe). Instead, pour a bucket of hot (not boiling) water mixed with a cup of rock salt. The salt lowers the freezing point. Wait 20 minutes. Repeat. It’s slow, but it won’t break your pipe. Foul smells like rotten eggs (hydrogen sulfide) can