No, don't use a damp rag. You don't want the fur to be damp or wet. That will just attract flies to lay eggs there. You need to use a flea comb or some other item to scrap off any eggs you find stuck on the fur.
What do you mean by dirty legs? Are they dry urine stained, wet or damp urine stained, or dirty with dried poop? Does your rabbit have some sort of medical condition or other reason, that is making it this way? Normally rabbits will be very clean. They are fastidious groomers. Usually the only reason a rabbit will have dirty fur is if there is a health problem, an environment that isn't staying clean enough(pee accidents outside litter box, litter box needing more cleaning, etc), or is an unneutered male spraying territory.