Plumbing Question

It's called an air admittance valve. They are used extensively in both the RV and home industry when it's impractical to run a vent to the roof. An example in a home would be under an kitchen island sink. They are not "cheating". It is a building code approved vent system. However, they do fail from time to time. Just purchase one from any big box store. They are threaded in. It is not normally necessary to use any type of thread sealer because the pressure in the system is very slight. However, I'd personally use Teflon tape on the threads as mentioned above.

FWIW - I had one fail in my brand new fiver under the bathroom sink and it was a very easy fix. Replacement took about five minutes.