  Sterile neutrinos as source of the mass and flavor mixing of active neutrinos
as well as genesis of the dark matter (DM) and matter-antimatter asymmetry have
gained special interest. Here we study the case of the Standard Model (SM)
extended with three right-handed (RH) neutrinos and a dark sector with two
extra sterile neutrinos, odd under a discrete $Z_2$ symmetry. The RH neutrinos
are responsible for producing the baryon asymmetry via the high-scale
unflavored leptogenesis. They are superheavy and their abundance at the
electroweak broken stage is vanishingly small, so that they have no impact on
the phenomenology at low energies. The two dark neutrinos generate the tiny
mass of two active neutrinos through a mechanism similar to the minimal linear
seesaw, and saturate the relic abundance as freeze-in DM via decay of the heavy
SM bosons. The absence of the dark Majorana mass terms in the dark linear
seesaw is explained by invoking a hidden symmetry, the so-called presymmetry,
and the DM candidate appears in the form of a quasi-Dirac neutrino. The $Z_2$
symmetry is broken in the dark neutrino sector, but exact in the realm of RH
neutrinos. The required coupling weakness for the freeze-in DM neutrino is
related to a very small breach of unitarity in the active neutrino mixing
matrix. We show how phenomenological constraints on the production and decay of
the DM neutrino imply an upper bound around 1 MeV for its mass and unitarity up
to $\mathcal{O}(10^{-7})$ for the mixing matrix.
