dont get me wrong ferrets will bolt rats from out of holes but some time, s a ferret will encounter a big old buck rat or a big female mother rat that will fight rather than shift, some people use smaller bitch type ferrets so they can get into the smalller holes these are the ones most at risk the big dog ferrets will kill a rat no problem.. what i know from experiance is the smell of ferrets alone would not stop a rat investigating any food source or such ,the best way as all ready mentioned would be to kill all invading rats then deter entry i have four traps lying permanently set in tunnel