In a medium bowl, whisk together the kosher salt, brown sugar, paprika, black pepper, garlic powder, onion powder, mustard powder, and cayenne pepper until evenly blended and lump-free.
Use immediately or transfer the rub to an airtight container. Store in a cool, dark place for up to 6 months.
To use, pat pork dry and apply about 2 tbsp rub per pound, coating all sides and pressing to adhere. Refrigerate 30 minutes to overnight before cooking.