  We propose a gauged $U(1)_{B-L}$ version of the light Dirac neutrino portal
dark matter. The $U(1)_{B-L}$ symmetry provides a UV completion by naturally
accommodating three right handed neutrinos from anomaly cancellation
requirements which, in combination with the left handed neutrinos, form the
sub-eV Dirac neutrinos after electroweak symmetry breaking. The particle
content and the gauge charges are chosen in such a way that light neutrinos
remain purely Dirac and dark matter, a gauge singlet Dirac fermion, remain
stable. We consider both thermal and non-thermal production possibilities of
dark matter and correlate the corresponding parameter space with the one within
reach of future cosmic microwave background (CMB) experiments sensitive to
enhanced relativistic degrees of freedom $\Delta N_{\rm eff}$. The interplay of
dark matter, CMB, structure formation and other terrestrial constraints keep
the scenario very predictive leading the $U(1)_{B-L}$ parameter space into
tight corners.
