1. Place Cajun seasoning in a large stock pot; add peanuts and water to cover. Soak peanuts at least 8 hours or up to 24 hours. (You may need to weigh down peanuts with a large plate or lid to ensure that they are fully submerged.) Drain and rinse.
2. Place peanuts, salt, and crab boil in stock pot with 4 1/2 qt. water; bring to a boil over high heat. Cover, reduce heat to medium-low, and cook 6 hours or until peanuts are tender, adding water as needed to keep peanuts covered; stir occasionally.
3. Remove from heat; let stand 1 hour.