Quantum Chromodynamics, the microscopic theory of strong interaction, is asymptotically free. As a consequence, perturbation theory is a useful tool to study high energy phenomena. In the opposite regime, the infrared, QCD is believed to be non-perturbative. However, last decade lattice simulations have showed that the coupling constant is finite in the infrared and not so big. That makes us think that some features of infrared QCD could be understood using perturbation theory. Moreover, the completely gauge-fixed Lagrangian is not known in the infrared.