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.
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.
Remove from heat; let stand 1 hour.
Prep: 10 min.; Soak: 8 hr.; Cook: 6 hr., 15 min.; Stand: 1 hr. You can find dried peanuts in the produce section of your local supermarket or at a farmers market. Store cooked peanuts in the refrigerator up to three days. If you prefer your peanuts warm, heat them in the microwave, covered, at 80% power for 3 to 5 minutes, stirring once.