 Some forks have mid-fork eyelets on the inside as well as outside to provide mounting of some specialized carriers such as the Tubus Duo.For bicycles that do not have these eyelets or "braze-ons", various alternatives exist.Some of these come in the form of "U-Bolts", clamps, or some sort of clamp-on eyelet (see Tubus Aluminum Alloy Fork Mounting Kit) 
When installing carriers, a drop of thread locking compound is advised to ensure that the screws do not loosened from vibrations under load.