  Generating small sterile neutrino masses via the same seesaw mechanism that
suppresses active neutrino masses requires a specific structure in the neutral
fermion mass matrix. We present a model where this structure is enforced by a
new U(1)' gauge symmetry, spontaneously broken at the TeV scale. In order not
to spoil the neutrino structure, the additional fermions necessary for anomaly
cancellations need to carry exotic charges, and turn out to form multicomponent
cold dark matter. The active-sterile mixing then connects the new particles and
the Standard Model---opening a new portal in addition to the usual Higgs- and
kinetic-mixing portals---which leads to dark matter annihilation almost
exclusively into neutrinos.
