  $\alpha$-RuCl$_3$ is drawing much attention as a promising candidate Kitaev
quantum spin liquid. However, despite intensive research efforts, controversy
remains about the form of the basic interactions governing the physics of this
material. Even the sign of the Kitaev interaction (the bond-dependent
anisotropic interaction responsible for Kitaev physics) is still under debate,
with conflicting results from theoretical and experimental studies. The
significance of the symmetric off-diagonal exchange interaction (referred to as
the $\Gamma$ term) is another contentious question. Here, we present resonant
elastic x-ray scattering data that provides unambiguous experimental
constraints to the two leading terms in the magnetic interaction Hamiltonian.
We show that the Kitaev interaction ($K$) is ferromagnetic, and that the
$\Gamma$ term is antiferromagnetic and comparable in size to the Kitaev
interaction. Our findings also provide a natural explanation for the large
anisotropy of the magnetic susceptibility in $\alpha$-RuCl$_3$ as arising from
the large $\Gamma$ term. We therefore provide a crucial foundation for
understanding the interactions underpinning the exotic magnetic behaviours
observed in $\alpha$-RuCl$_3$.
