Because the domestic water supply has a higher pressure than what's in your boiler, if there were a leak, it would tend to overpressurize the boiler, not leak water into the tank. But, the tank itself could be ready to die and the rust you are seeing is from the liner. Or, there may be a steel nipple on the supply somewhere. Is the hot water rusty everywhere in the house, or only at a single or a few places? What are your supply lines - if galvanized, you may be ready for a repipe! The hot tends to go first.