Infrared QCD coupling at $T = 0$ and $T>Tc$

We calculate the polarization operator of a valence gluon, which propagates in the confining background in the form of a color‐octet bound state. The result leads to the infrared freezing (i.e. finiteness) of the running strong coupling in the vacuum, as well as in the quark‐gluon plasma up to the temperature of dimensional reduction. The momentum scale defining the onset of freezing is found both analytically and numerically. The nonperturbative contribution to the thrust variable, originating from the freezing, makes the value of this variable closer to the experimental one.