  The Good-Walker mechanism for diffraction is shown to provide a link between
total and diffractive structure functions and to be relevant for QCD
calculations at small x_{Bj}. For Deep-Inelastic scattering on a small-size
target (cf. an onium) the r\^ ole of Good-Walker ``diffractive eigenstates'' is
played by the QCD dipoles appearing in the $1/N_C$ limit of QCD. Hard
diffraction is thus related to the QCD tripe-dipole vertex which has been
recently identified (and calculated) as being a conformal invariant correlator
and/or a closed-string amplitude. An extension to hard diffraction at HERA via
$k_T-$factorisation of the proton vertices leads to interesting phenomenology.
