  We perform a systematic analysis of standard model extensions with an
additional anomaly-free gauge $U(1)$ symmetry, to generate Dirac neutrino
masses at one loop. Under such symmetry, standard model fields could either
transform or be invariant, corresponding to an active $U(1)_X$ or a dark
$U(1)_D$ symmetry, respectively. Having an anomaly-free symmetry imposes
nontrivial conditions to the number and charges of the unavoidable new states.
We perform an intensive scan, looking for non-anomalous solutions for given
number of extra chiral fermions. In particular, we concentrate on solutions
giving rise to scotogenic neutrino masses via the effective Dirac mass
operator. We study the cases where the Dirac mass operator with dimension 5 or
6, is mediated by Dirac or Majorana states, and corresponds to an active
$U(1)_X$ or a dark $U(1)_D$ symmetry. Finally, we comment on the solutions
featuring no massless chiral fermions.
