An exterior cover can’t keep the whole pack dry in heavy rain. Water can run down your back and soak into the pack that way. Things that really need to stay dry

An exterior cover can’t keep the whole pack dry in heavy rain. Water can run down your back and soak into the pack that way. Things that really need to stay dry, need a sealed bag. But you don’t need a liner for the whole pack, either. Cookware and stoves can get wet, and the tent is probably wet anyway if it is raining. Typically only the food, sleeping bag, and clothes really need to stay dry, so you can just seal those each in their own bag. I kept those things dry for a month in the Pacific Northwest using just regular kitchen trash bags.