What is the name and purpose of the ridged cylinder often seen carried clipped to the rear waist band of WWII German army uniforms?

by Gnome_de_Plume

Google is failing me on this. Is it a can to keep general stuff in, or is it for specific stuff (e.g., ammunition?). Or is it a cooking pot? A canteen? For a radio? Not an earth-shaking question, just something I've wondered about. I don't recall seeing other WWII soldiers having a comparable piece of gear.

Picture 1. Picture 2.

yoinkmasta107

It's a gas mask canister.

Also, this probably would have been better suited for /r/whatisthisthing.