Peanuts, are technically legumes. But from a nutritional standpoint, they're closer to nuts, which is why they're put in the latter category.
Like most nuts, peanuts are also low in carbs, so yes, they are ketogenic. Keep in mind, though, that peanuts are also delicious and addictive; you can easily overeat on them without even noticing and taking in more carbs than planned.
So do track your carb intake when munching on these tasty nuts, and try to keep you intake to a handful a day. Same goes true for peanut butter. On a low-carb diet, go for the unsweetened, natural kind. These usually have around 4 grams net carbs per two tablespoons.