  Explicit analytical expressions are derived for the gluon propagator in a
generic linear covariant $R_\xi$ gauge, by a screened massive expansion for the
exact Faddeev-Popov Lagrangian of pure Yang-Mills theory. At one-loop, if the
gauge invariance of the pole structure is enforced, the gluon dressing function
is entirely and uniquely determined, without any free parameter or external
input. The gluon propagator is found finite in the IR for any $\xi$, with a
slight decrease of its limit value when going from the Landau gauge ($\xi=0$)
towards the Feynman gauge ($\xi=1$). An excellent agreement is found with the
lattice in the range $0<\xi<0.5$ where the data are available.
